Electrostatic Generator.pdf

A VACUUM TUBE FOR AN ELECTROSTATIC GENERATOR

APPROVED: iJ

Major Professor

Minor Prof e r/sbr

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Director, of the Department- of Physic:

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Dean of the Graduate School

A VACUUM TUBE FOR AN ELECTROSTATIC GENERATOR

THESIS

Presented to the Graduate Council of the North Texas State University in Partial Fulfillment of the Requirements

For the Degree of

MASTER OF SCIENCE

By

John Reginald Pool, B. S Denton, Texas August, 1966

TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS

iv

Chapter I. II. III. IV.

INTRODUCTION

1

CONSTRUCTION OF THE VACUUM TUBE

15

EXPERIMENTAL PROCEDURE AND RESULTS

31

CONCLUSIONS . . . . . Generating Voltmeter Charging System Gas Handling System Corona Columns Vacuum System Vacuum Tubes •

46

APPENDIX

58

BIBLIOGRAPHY

77

in

LIST OF ILLUSTRATIONS Figure 1. 2.

3.

Page A Vacuum Tube with "Inverted Cone" Electrode Configuration

59

(A) Cross Section of Corona Column with Vacuum Tube Installed (B) Large Electrode (C) Small Electrode.

60

Longitudinal Section of Corona Column with Vacuum Tube Installed

61

4.

Cement Applicators

62

5.

Foil Cutting Tool

63

6.

Installing the Positioning Jig

64

7.

C-clamp Holding a Positioning Jig, Insulator, Gasket, Electrode, and Jigging Block Assembling Electrode-Insulator Subassemblies in

65

8.

the Small Jig 9. 10. 11. 12.

66

Large Jig Holding the Vacuum Tube Terminal Potential Versus BB Corona Current Prior to Installation of Vacuum Tubes Terminal Potential Versus Generating Voltmeter Reading Prior to Alteration of the Signal Lead .

67 68 69

Terminal Potential Versus AA Corona Current Prior to Installation of Vacuum Tubes

70

13.

Vacuum Feed-Through Assembly

71

14.

Electrostatic Generator with Vacuum System.

72

15.

Determining the Pressure Differential across an Evacuated Vacuum Tube

73

LV

Figure

Page

16.

A Representative Comparison of the Terminal Potential Versus the BB Corona Currents while Operating with and without Electron Loading. . .74

17.

A Representative Curve of Terminal Potential Versus BB Corona Current as Spontaneous Electron Loading Occurs

18.

75

Terminal Potential Versus Generating Voltmeter Reading after Alteration of the Signal Lead. . . 76

CHAPTER I INTRODUCTION Since R. J. Van de Graaff (18) constructed his first electrostatic generator at Round Hill, in 1931, this machine, with the addition of an ion source and accelerating tube, has become the most popular low energy accelerator ever developed.

This machine with its modifications is used not

only for low energy nuclear research but also for injecting beams of ionized particles into many of the high energy machines.

The basic design of the original machine still

remains, and all Van de Graaff accelerators contain a terminal for storing charge, a charging system to maintain the potential of the terminal, an ion source, and an accelerating tube through which the ions accelerate from the charged terminal to ground potential. From the origin of the machine, two factors prevented the Van de Graaff accelerators from obtaining their maximum theoretical potential.

The first factor was the leakage of

charge from the terminal to the atmosphere.

To alleviate

this problem, the entire machine was placed in an enclosed tank and tested first in a vacuum and secondly under pressure of an electronegative gas.

The second of these two solutions

proved to be the more successful, and all succeeding machines

have accepted this modification.

The gases most commonly

used for this purpose have been sulfur hexafluoride or a mixture of carbon dioxide and nitrogen (10). The second factor which limits the operation of the Van de Graaff as an accelerator is the accelerating tube. The accelerating tube serves a dual purpose in the accelerator; to accelerate the ions extracted from an ion source to an energy corresponding to the potential of the terminal and to focus the beam of ions during the acceleration. Although this machine has been utilized as an accelerator for more than thirty years, no one has designed an accelerating tube which will permit a machine to operate at its maximum theoretical potential.

Much research has gone into

examining this problem, and many theories have been forwarded as to the reasons why the accelerating tubes will not sustain the potential gradients obtainable in the machine when the tube is not in place. The only method of, hopefully, developing an acceptable vacuum tube is to incorporate what little knowledge has been acquired from previous tubes, and to try to develop a better tube by trial and error. There are several existing criteria for building tubes, and any new design should attempt to eliminate as many problems as possible by utilizing the previously gained knowledge. Some of the problems which must be overcome are pumping

speed, shielding, focusing, electrical breakdown, sparking, and electron loading.

Some of the better tubes constructed

thus far possess one or a combination of features, each of which serves to improve performance in one manner, but which may possess accompanying disadvantages. It has been found experimentally that the maximum potential gradient an accelerating tube will sustain increases roughly as the square root of the length of the tube (4); thus very little is gained by using longer tubes.

Faced with

this fact, scientists have constructed the tandem Van de Graaff (1).

This machine utilizes a positive terminal elec-

trically isolated from ground and charged by a charge carrying belt similar to the conventional Van de Graaff, but here the similarity ends.

The ion source normally located in the

terminal is now located at ground potential.

The negative

ions produced by this source accelerate through one of two accelerating tubes to the positive central terminal.

While

drifting through this terminal, the ions are stripped of electrons in a charge exchange chamber and emanate from the terminal with a net positive charge.

They are again accel-

erated to ground by the same, potential through a second accelerating tube.

This machine is an energy multiplying

device since it utilizes one potential to accelerate a charge to twice or more the energy obtainable through a single acceleration.

Besides the advantage of energy multiplication, one may use shorter tubes, and thus eliminate many of the problems associated with longer tubes while obtaining energies never before obtainable using the Van de Graaff.

The previous

maximum potential obtained by a single-ended machine at Los Alamos was 9 MeV (10).

The introduction of the tandem has

increased the maximum to above 20 MeV. The majority of research that has gone into design and construction of accelerating tubes has been performed by the University of Wisconsin, Massachusetts Institute of Technology, Los Alamos Laboratory and the High Voltage Engineering Company (13).

Little is known about the mechanism creating electrical

breakdown which can occur both inside and outside the tube; these breakdowns include sparking, external breakdown, insulator breakdown, pulsed internal breakdown, and possibly the most serious—electron loading. Varying design parameters has reduced most of the problems, but electron loading still remains a great hindrance. Electron loading is a phenomenon in which a negative current, mostly electrons, accelerates toward the high potential end of the accelerating tube and a simultaneous flux of positive ions accelerates toward the low potential end.

The beam of

particles is not focused and completely fills the aperture of the electrodes in the accelerating tube.

Electron loading

is accompanied by X-rays created by the electrons striking

electrodes.

The X-rays serve as an indicator of electron

loading, and one can determine the threshold potential for electron loading by observing the potential at which these X-rays begin (17).

Several theories have been forwarded as

to the cause, but it is generally believed that no one theory can fully explain the problem. combination of factors.

It is caused by a

Needless to say, the problem is not

thoroughly understood. The predominant cause of electron loading is believed by most to be the electron-positive ion exchange (15).

In

this process electrons from ionized particles created by the high potential gradient, or a spark, accelerate up the tube toward the high potential end.

The electrons upon striking

electrodes along the path knock secondary positive ions from the surface of the electrodes and create secondary photons. These positive ions accelerate toward ground potential and upon striking electrodes both the ions and photons release electrons which accelerate toward the high potential end of the tube, thus initiating the process again.

It has been

shown, however, that this process is not self-sustaining. If an electron striking an electrode releases "A" positive ions and "B" photons and in turn each positive ion and photon release "C" and "D" electrons respectively upon striking an electrode, then for a self-sustaining reaction clearly AC + BD > 1.

These coefficients should be energy

dependent; therefore, a threshold potential for electron loading should occur.

It has been shown by experimenting

with electron-positive ion interactions that the coefficient —4 "A" is approximately 10 (15) and "C" is approximately 3 depending upon the cleanliness of the surfaces investigated (19).

The electron-photon interaction has been shown to be

important in the multiplication process, but this problem can be minimized by constructing metallic parts with metals of low atomic number, thus reducing the energy of the bremsstrahlung photons (16). By placing the tube in a magnetic field and bending electrons out of the beam of negative particles which are traversing the tube, it has been found that many negative ions also exist, and that these negative ions play an important role in the above reaction.

It has been found that the

threshold voltage for breakdown does not depend on the electrons striking the upper end of the tube (17). It has been found that the electron loading threshold potential increases and the electron loading current decreases by operating the tube at a higher internal pressure (11, 17). Best results in existing machines have been achieved using —5 pressures as high as 2 or 3 x 10 mm with little interference with the injected ion beam.

When the mean free path is

greater than the electrode separation, there is a high flux of electrons striking electrodes at energies high enough to

produce photons.

As the pressure is increased, the mean free

path is decreased until it is less than the electrode separation.

This greatly decreases the energy of the impinging

electrons, and thus decreases the bremsstrahlung radiation. Another procedure used to eliminate electron loading is back-biasing (7, 12), a process in which two electrodes separated by intermediary electrodes are shorted along the accelerating tube.

This is performed at several intervals

along the tube and serves as a retarding potential for the slowly moving electrons which enter into the loading process. These high-potential and low-potential electrodes establish non-uniform gradients in the local fields and deflect the electrons causing them to leave the beam.

The gradient of

back-biasing electrodes increases toward the high energy end of the tubes in order not to affect the slow moving heavier ions. A few of the other theories which have been proposed for internal discharge are the Malter effect, electrical surges, field emission, and colloidal particles.

The Malter

effect (2) results from a positive charge accumulating on a thin film of insulator, such as pump oil, overlying a conducting electrode.

This positive charge on the surface

lowers the potential barrier at the surface allowing electrons to be pulled from the metal.

Electrical surges in

corona currents down the corona columns can produce very

high instantaneous potential differences between electrodes and can create electrical breakdown within the accelerating tube.

Field emission (9) is created by points which may

appear on metal surfaces caused by exposure to very high electric fields.

Electric fields may pull charged colloids

(4) or a "clump" of material from an electrode, accelerating it toward an electrode of lower potential.

Continual bom-

bardment of localized regions of an electrode by these particles quickly heats the metal and thermionic emission may occur.

This condition can quickly initiate electrical

breakdown.

Clean, oil-free systems, utilizing cold traps

and vapor-ion pumps, are rewarded with higher ultimate potentials than dirty systems. It is important in an accelerating tube that the beara never be allowed to strike the side of the tube.

Shielding

(13) is therefore another important factor that must be incorporated into the design of a tube.

If the beam is

allowed to strike the insulators, a charge may be deposited on them.

This accumulation of charge will cause transverse

displacement and/or defocusing of the beam as it moves down the tube.

Shielding is more necessary near the ion-source

end of the tube where the beam is moving at low velocity and is remaining under the influence of the accumulated charge for a longer period of time.

This accumulation of charge

also creates a path for flash-over which may damage the inner surface and may even crack the electrode.

Accelerating tubes with fine subdivisions of insulators provide better shielding than coarse subdivisions and take advantage of the fact that short insulators can withstand higher gradients than long insulators.

However, there is

little evidence indicating that improvement in shielding is significant when the insulator length is less than 1 inch. Fine subdivisions also increase the probability of leaks because of the greater number of seals that must be made per unit length of tube. When flat electrodes with a circular beam aperture are used, shielding is achieved by constructing the electrode and insulator assembly in a manner so that the ratio of the radial distance "r" from the edge of the aperture to the inside edge of the insulator and the electrode separation distance "s" is approximately 3.

This provides adequate

shielding from charge accumulation on the insulator walls. Research has shown that a shielding ratio, r/s, of 1 is inadequate.

Tubes designed and in use with a shielding

ratio of approximately 2.5 have operated satisfactorily with no beam deflection. To develop a high shielding constant using a small accelerating tube requires that the apertures in the electrodes be small.

These small openings act to capture

back-streaming electrons before they attain much energy. In most tubes the aperture is used simultaneously for

10

evacuating the tubes and for focusing the beams of ions.

If

the aperture is made small, the pumping speed decreases, resulting in a higher tube pressure.

The increased pressure

causes scattering of the beam which strikes the electrodes and creates a non-uniform gradient down the tube and may initiate a violent discharge.

A high pumping speed can be

obtained in tubes with large apertures; however, these tubes allow secondary particles to gain high energies in passing down the tube.

The large openings also result in a lower

electron-loading potential.

Some tubes with small apertures

have been successfully utilized with off-center pump holes through the electrodes (14).

In constructing these tubes,

the pump-out holes in each succeeding electrode are oriented 90°, eliminating the possibility of particles using these holes as apertures for a secondary beam. Many additional modifications of the vacuum tube have been constructed and tested.

These include the introduction

of the cusp-shaped electrode, at the University of Wisconsin, the staggered electrodes, at High Voltage Engineering, and the small aperture diaphragms, at various intervals along the tube at Los Alamos.

Independent studies have also been

carried out at various establishments on the other modifications (5, 7).

A more successful design appears to be one of sev-

eral constructed and tested by Associated Electrical Industries Limited (3).

This design incorporated stainless

11

steel toroidal electrodes, which provided a more uniform electric field throughout the porcelain insulators used in fabricating the tube.

The design incorporated electrodes of

varying aperture diaphragms supported by the toroidal electrodes, forming an "inverted cone" configuration (Figure 1). (All figures are in the appendix, pages 58 to 76).

All

diaphragms with small apertures were provided with additional pump-out holes.

Electron loading and associated X-rays were

immeasurably small up to a potential of 3.7 MeV using the tube possessing the "inverted cone" configuration while installed in an electrostatic generator capable of attciining 5.5 MeV without accelerating tubes.

The upper limit to the

voltage obtainable was established by the porcelain insulators which were punctured through their volume after running at 3.7 MeV for several hours. trical breakdown.

There was also occasional elec-

The "inverted cone" geometry broke the

chain of electron multiplication at the sudden discontinuities in the electrode apertures. Following four years of operation, the tube with "inverted cone" electrode configuration was again tested (8).

The tube withstood 3.85 MeV as compared to 3.7 MeV in

its initial testing. the later tests.

Some electron loading was measured in

A new tube of similar design, but employing

glass insulating walls, has operated up to 4.25 MeV without electron loading.

12 The Physics Department at North Texas State University is presently constructing a tandem Van de Graaff accelerator similar to the small tandem machine, constructed by R. G. Herb at the University of Wisconsin, which incorporated small aperture accelerating tubes that produced little electron loading.

The electrostatic generator, including the

pressure tank, closed gas handling system, charging system, corona column, and generating voltmeter, has been constructed and tested.

This machine at present has operated at an

estimated potential of 2.4 MeV (6) on the central terminal under pressure of sulfur hexafluoride without the presence of the accelerating tubes.

Sparking prevents attaining

higher potentials on the terminal.

Completion of the Van de

Graaff as an accelerator will require the addition of accelerating tubes, a charge exchange mechanism in the central terminal, and an ion source. The purpose of this study has been to construct two accelerating tubes with small beam apertures for the Van de Graaff, modifying the prototype tube designed and tested by Wiley (20) , to design and construct a vacuum system for evacuating the tubes, and to determine the characteristics of the tube under operating conditions while installed in the generator.

CHAPTER BIBLIOGRAPHY 1.

Alvarez, L. W. , "Energy Doubling in D. C. Accelerators," The Review of Scientific Instruments, XXII (1951), 705.

2.

Blewett, J. P., "Electron Loading in Ion Accelerating Tubes II," The Physical Review, LXXXI (.1951) , 305A.

3.

Chick, D. R., Hunt, S. E., Jones, W. M., and Petrie, D. P. R., "A Van de Graaff Accelerator Tube of Very Low Retrograde Electron Current," Nuclear Instruments and Methods, V (1959), 518.

4.

Cranberg, L., "The Initiation of Electrical Breakdown in Vacuum," Journal of Applied Physics, XXIII (1952), 518.

5.

f a n d Henshall, J. B., "Small-Aperture Diaphragms in Ion-Accelerator Tubes," Journal of Applied Physics, XXX (1959), 708.

6.

Daniel, R. E., "Construction and Testing of a Charging System and a Corona Column for an Electrostatic Accelerator," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1962.

7.

Firth, K., and Chick, D. R., "The "Screening1 of Neutral Particles in High Voltage Ion Accelerator Tubes," Journal of Scientific Instruments, XXX (1953) , 167.

8.

Hunt, S. E., Cheetham, F. C., and Evans, W. W., "The Performance and Conditioning of 'Inverted Cone1 Van de Graaff Accelerating Tubes," Nuclear Instruments and Methods, XXI (1963), 101.

9.

Jones, F. L., "Electrical Discharges and the Vacuum Physicist," Vacuum, III (1953), 116.

10.

Livingston, M. S., and Blewett, J. P., Particle Accelerators , New York, McGraw-Hill Book Company, 1962, 30.

11.

Mansfield, W. K., and Fortescue, R. L., "Prebreakdown Conduction Between Electrodes in Continuously Pumped Vacuum Systems," British Journal of Applied Physics, VIII (1957), 73.

13

14 12.

McKibben, J. L., "Control of Current Loading and Sparks in Ion Accelerating Tubes by Back-Biased Diaphragms," Bulletin of American Physical Society, I (1956), 61.

12.

Michael, Irving, "The Development and Performance of a New Electrostatic Accelerator," unpublished doctoral dissertation, Department of Physics, University of Wisconsin, Madison, Wisconsin, 1958.

14.

, Berners, E. D. , Eppling, F. J., Knecht, D. J., and Herb, R. G., "New Electrostatic Accelerator," The Review of Scientific Instruments, XXX (1959), 855.

15.

Trump, J. G. Van de Graaff, R. J., "The Insulation of High Voltages in a Vacuum," Journal of Applied Physics, XVIII (1957), 327.

16.

Turner, C. M., "Ionization Loading of Electrostatic Generators," The Physical Review, XCV (1954) , 599.

17.

, "Electron Loading in Ion Accelerating Tubes I," The Physical Review, LXXXI (1951), 305A.

18.

Van Atta, L. C., Northrop, D. L., Van Atta, C. M. , and Van de Graaff, R. J., "The Design, Operation, and Performance of the Round Hill Electrostatic Generator," The Physical Review, XLIX (1936), 761.

19.

Webster, E. W., Van de Graaff, R. J., and Trump, J. G., "Secondary Electron Emission from Metals under Positive Ion Bombardment in High Extractive Fields," Journal of Applied Physics, XXIII (1952), 264.

20.

Wiley, Ralph, "A Vacuum Tube for an Electrostatic Accelerator," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1963.

CHAPTER II CONSTRUCTION OF THE VACUUM TUBE The vacuum tubes constructed at North Texas State University are a modification of a design used at the University of Wisconsin.

A prototype section of this column

constructed and tested by Wiley (3) was found to possess suitable characteristics although several modifications were suggested.

The tube is comprised of a series of metallic

electrodes separated by flat insulating discs concentrically cemented together to form a vacuum-tight enclosure. The insulating discs were secured from the Alite Division of United States Stoneware Company, Orrville, Ohio. These discs are composed of 96 per cent alumina material, a composition commercially known as Alite 212.

The insulators

measure 3/16 inch in thickness, possess an outer diameter of 1-1/4 inches, and a centered hole 5/8 inch in diameter. The original electrodes (Figures 2B and 2C), locally machined of cold-finished mild steel .0575 inch thick, possess .1875 inch central holes which act to focus the ion beam and to allow evacuation of the tube.

The design incorporates

two different, roughly circular, electrode designs, one large with flat outer edges, the other smaller with rounded outer edges.

The design is such that every third electrode is one

16

of the large type, maintained at a separation of 3/4 inch. Each electrode is designed with four 1/2 inch diameter indentations drilled 1.031 inches radially from the center of the disk and spaced at every 90° along the perimeter of the electrode.

Midway between two of these indentations

and .781 inch from the center of the disk is another hole drilled and tapped to accept a 2-56 set screw.

The purpose

of these different designs will be clarified. The vacuum tube is designed for adaptation to the corona column, which was designed by Gray (2) and constructed by Daniel (1).

This corona column consists of a Lucite tube

about which 27 aluminum corona rings are concentrically attached every 3/4 inch.

This is done by means of four rad-

ially extended aluminum stand-off pins which extend through the wall of the Lucite cylinder as shown in Figure 2A. Spherical brass balls 5/8 inch in diameter are screwed onto the protruding ends of these pins on the inside of the Lucite tube.

Spring-loaded plungers for supporting the vacuum tube

extend radially inward from these brass balls. The indentations drilled into each of the electrodes are designed to be longitudinally aligned along the vacuum tube and to allow clearance for the spring-loaded plungers when the tube is inserted into the corona column.

The

design is such that, when inserted, the plungers make contact with the larger electrodes.

By rotating the tube, these

17 plungers are depressed by the taper between the indentations and the flat on the outer edge of the large electrode.

After

a 45° rotation, each plunger rests on the flat portion of the large electrodes perimeter which holds the vacuum tube in place, as illustrated in Figures 2A and 3. During the interim between Wiley's tests on the prototype tube and the construction of the vacuum tubes, it was noted that spots of oxidation were appearing on the previously constructed mild steel electrodes which were to be used in fabricating the tubes.

To alleviate this problem, the elec-

trodes were nickel plated to a thickness of .00025 inch, which gave the plated electrodes a thickness of .058 inch. A preliminary test was performed to determine if this plating would remain adhered to the surface when localized heating occurred due to a flux of impinging ions.

This was

done in conjunction with tests being performed on a von Ardennes type duo-plasmatron ion source.

First, a test

electrode was placed in a beam of hydrogen ions and later in helium ions at an approximate energy of 30 kilovolts.

The

shiny surface of the electrode was discolored with tints of blue and gray; however, there was no sign of deterioration of the surface.

It was then decided to proceed with the con-

struction which incorporated this design modification. As a continuation of preliminary testing, it was decided to determine the properties of a vinyl-acetate cement and

18

also to determine if it would be more suitable for constructing the tube than the epoxy incorporated in the previous test sections.

Most of the troubles encountered during

original tests had been electrical breakdown in the epoxy. In addition it was difficult to obtain seals which contained no voids and which would be leak tight.

When electrical

breakdowns occurred in the epoxy cement, it was believed that they were initiated in these voids (3).

It was hoped

that the presence of voids in the cement joints could be eliminated by thinning the vinyl cement, which made it less viscous than the highly viscous epoxy. The cement procured for constructing the accelerating tube was a vinyl alcohol-acetate resin which is a thermoplastic.

The solution used is commercially known as T-24-9

and was obtained from the Palmer Products Company, Worchester, Pennsylvania.

The directions for use were taken from the

Union Carbide Technical Bulletin Number 224, Section XV, entitled "Vinyl Alcohol-Acetate Resin Solutions." When properly used, this cement provides an adhesive comparable in shock resistance and strength to soft solder. Proper application of the cement requires that surfaces be cleaned carefully in order to remove all grease and dirt. Toluene or ethanol is suggested for cleaning metal surfaces. After cleaning, the adhesive is applied from solution to all surfaces to be joined.

If it is necessary, the adhesive can

19 be thinned with any of the lower alcohols.

To assure maximum

bonding strength, precautions should be taken to allow complete removal of solvents from the cement.

This can be

accomplished by a long air-dry using thin coats or by baking. When using the cement on porous surfaces a forced dry at 200° to 300° Fahrenheit is satisfactory.

After the cement

is dry, the coated pieces are assembled in a jig and subjected to heat and pressure.

For maximum bonding using a

temperature of 400° Fahrenheit, the cement joint must be subjected to a pressure not less than 100 pounds per square inch.

Higher temperatures may be used to raise the resof-

tening point of the cement a few degrees.

This is caused

by the occurrence of some degree of cross-linking.

As scon

as the above mentioned conditions are reached, the assembly should be cooled to 120° Fahrenheit, and the parts released. When returned from the electroplating firm, the electrodes were found to possess some surface graininess.

This

graininess could possibly produce field emission when the electrodes are installed in the vacuum tube.

Each electrode

was polished on both sides by rubbing the electrode in a circular motion on a flat glass plate over which had been placed a film of water and pumice soap.

The pumice soap

particles acted as a very fine grinding compound to remove the surface irregularities.

The electrodes were rinsed in

distilled water and allowed to dry.

They were then placed

20

in an atmosphere saturated with trichloroethylene vapor which condensed on the cool electrodes.

Being a hydorcarbon

solvent, the condensate washed any remaining traces of oil contamination from the surfaces of the electrodes.

The

electrodes were then ready for the application of cement. Two aluminum cement applicators are used in applying cement to the electrodes and insulators (Figure 4).

The

cylindrical cement applicators are constructed with a raised and hollowed annular ring for holding cement.

The outer

diameter of the ring is 1-1/4 inches and the inner diameter 1 inch.

An alignment probe extends from the center of the

applicator concentric to the annular ring, and it is designed to fit into the central holes in the electrodes and insulators This allows positioning of the applicators as the cement is applied.

The aluminum alignment probe on the electrode

cement applicator is .1875 inch in diameter, and the nylon alignment probe on the insulator cement applicator is .625 inch.

Nylon was used on the latter, because, upon inserting

and rotating an aluminum probe in the central hole of the insulator, it was found that small pieces of aluminum adhered to the inner wall of the central hole.

These pieces of •

aluminum provided a conductive path across the insulator. The actual assembly of the test section closely followed i

the procedure incorporated in earlier test sections.

A

small sample of the vinyl cement was placed in a beaker, then,

21

ethyl alcohol was added until the viscosity was approximately that of 30 weight motor oil.

The vinyl cement was then

spread uniformly on the electrode cement applicator until a large semi-circular mound of cement completely filled the annular ring provided.

With the electrode held horizontally,

the alignment probe on the applicator was inserted through the beam aperture in the electrode from the bottom until the applicator and electrode were flush.

The entire assembly

was inverted, and the applicator was rotated approximately 360° to distribute the cement evenly.

The applicator was

then lifted vertically; if there were accompanying streamers of cement, it was taken as an indication that the cement was too viscous.

If the cement had not formed a perfectly annu-

lar ring with an outer diameter of 1-5/16 inches and an inner diameter of 15/16 inch, the electrode was cleaned in isopropyl and the process was repeated.

Before applying cement to the

next electrode, the applicator was thoroughly cleaned in isopropyl alcohol.

It was found that any attempt to apply

cement without first cleaning the applicator was futile. After applying the cement, the electrode was placed horizontally on a rack inside an oven for baking. The same procedure as above was followed in applying cement to the ceramic insulators, although the applicatorinsulator assembly was not inverted after insertion of the alignment probe through the insulator.

It was then verified

22

that the applied cement had extended beyond the edge of the insulator and had actually lapped slightly over the outer edge.

If the application were not suitable, the insulator

was washed in acetone and allowed to dry before using again. The coated insulators were then placed horizontally on a shelf in the oven.

If a perfectly filled annular ring of

cement was not achieved in either of the above procedures, it was taken as an indication that either the cement was too viscous or that the surfaces were not properly cleaned. In the oven, the insulators and electrodes were heated to a temperature of 150° Celsius, as determined by a thermometer placed partially through a hole in the top of the oven.

The purpose of this heating was to drive the solvents

from the cement leaving it hard and transparent.

Two ports

at the top of the oven were then opened, allowing rapid cooling to room temperature. Four electrodes, two large and two small, and three insulators were prepared by the above procedure in constructing the test section.

By measuring the average thickness

of the electrodes and the insulators, it was determined that by cementing the electrodes directly to the insulators, the test section would be .736 inch in length, .0135 inch less than the required .7500 inch.

It was decided that .003 inch

aluminum gaskets would be placed between each electrode and insulator since no other thickness of aluminum was available.

23 This technique was employed by Wiley (3) and acted not only as a shim for obtaining the proper electrode spacing, but also it minimized the flow of cement into the inner portion of the tube.

This reduced the outgassing of exposed cement

which could present a problem during evacuation.

It was

decided to use the same gasket design previously incorporated, which was a washer shaped aluminum gasket .8125 inch in outer diameter and .639 inch in inner diameter. The foil cutting tool for the gaskets (Figure 5) consisted of three steel concentric tubes, the intermediary having an inner diameter of .639 inch and an outer diameter of .8125 inch.

The inner and outer thin walled tubes made

sliding contact with the intermediary tube.

The bottom ends

of these tubes were tapered toward the intermediary tube to form a cutting edge and the top edge of each tube was knurled for gripping.

The foil gaskets were cut, using a. soft wooden

board or a paper magazine as a backing.

If a gasket became

wrinkled during cutting, it was flattened using a photographic print roller.

After cutting, the foils were degreased

in the trichloroethylene vapor. A gasket-insulator alignment jig constructed to assure a concentric assembly was then placed through the central hole of an electrode located on a jigging block.

A gasket

and insulator were placed over the alignment jig trapping the aluminum gasket between the annular rings of cement on

24 both the electrode and insulator-

Holding the insulator in

place, the alignment jig was then carefully replaced by a second positioning jig (Figure 6) which was designed to hold the above configuration during bonding.

The entire assembly

was placed under pressure in a C-clamp (Figure 7).

Three

subassemblies were prepared in this manner utilizing two small electrodes and one large electrode.

The entire assembly

was hung by means of the handle on the clamps from a rack in the oven where it was heated to 150° Celsius. After cooling to room temperature, the subassemblies were removed from the C-clamp.

Cement was then applied to

the insulators on all three subassemblies and to the electrodes of the two small-electrode subassemblies.

The three

subassemblies were again placed inside the oven supported by the positioning jigs which had just been removed from the central holes.

They were then heated to 150° Celsius and allowed

to cool slowly to room temperature.

At this time 1/4 inch,

2-56 set screws, which had been ground to produce a 70° included point, were installed in the small-electrode subassemblies.

This was done with the point protruding from the

insulator side of the electrode. The electrode subassembly was then placed with the coated insulator upward in the small jig (Figure 8), which was constructed with four rods that fit the indentations in the outer edge of the electrodes, holding them aligned during bonding.

25 The gasket-insulator alignment jig was placed through the central hole of the insulator, and a gasket was dropped in place.

After carefully removing the alignment jig, a

small-electrode subassembly, coated electrode downward, was placed in the jig, orienting the set screws 90° counterclockwise to the set screw hole in the large electrode.

A

second gasket, the second small-electrode subassembly, a third gasket, and the second, coated, large electrode, cemented side downward, were installed.

The assembly was

visually examined to insure that each set screw hole in o each electrode was oriented 90 in the preceding electrode.

counterclockwise to the hole

An aluminum cylinder was then

placed on top of the electrode followed by the upper portion of the jig, which was firmly bolted in place by four nuts on the threaded alignment rods.

The jig was placed in the oven

and heated to 200° Celsius and allowed to cool slowly to room temperature. The test section was removed from the jig and was found to be .007 inch over the .7500 inch required.

It was later

found that these sections were not completely compressed. Visual examination showed all cement fillets to be acceptable, Leak chasing proved the test section to be leak tight, and the test section was installed in Wiley's (3) test system, shorting across two of the insulators using small springs. It was found that each insulator could withstand a potential

26

of 40 kilovolts without breakdown in either the vinyl cement or the insulator.

This test section was in an atmosphere of

150 psig of SFg, with an internal vacuum on the order of 8 or 9 x 10~® mm of Hg. It was then decided to continue construction of the accelerating tube.

The same procedure was used in con- •

structing the tube as that incorporated in building the test section with some modifications.

Twenty-seven three-electrode

subassemblies similar to the test section were built.

In

constructing these, the second large electrode and the cement on the uppermost insulator were excluded from each subassembly, A variety of foil thicknesses .005 inch, .001 inch, .002 inch, and .003 inch was purchased for constructing gaskets. The presences of bubbling in the cement between some insulators and electrodes is not clearly understood.

It is

believed that this bubbling was caused by vigorous agitation during thinning, introducing small bubbles in the cement, and possibly by too rapid a heating rate, during which the outer surface of the cement hardened, trapping the bubbles beneath the surface.

These bubbles, although unsightly, did

not seem to prevent good vacuum seals.

As construction of

the tubes proceeded and more rapid construction methods were used, bubbling became more evident. Some subassemblies were heated to temperatures in excess of 240° Celsius.

The cement on these subassemblies became

27 colored with a spectrum ranging from light yellow to dark brown, depending on the maximum temperature to which they were exposed.

This discoloration did not seem to affect the

vacuum seal; it did, however, appear to make the dark brown cement invulnerable to solvents. A piece was machined to fit the end of the tube, and an insulator was concentrically bonded to this piece.

This end-

piece was designed to allow attachment of the vacuum tube to a vacuum system.

Once the 27 three-electrode pieces were

completed, ground set screws were inserted in the remaining large electrodes.

Both ends of each subassembly were coated

with cement; however, the test section was coated on only one end.

The insulator on the special end-piece was coated with

cement, and all of the subassemblies were heated to 150° Celsius. The large gluing jig was assembled, and the subassemblies placed in the jig beginning with the machined aluminum endpiece.

Gaskets of proper thickness to insure the .7500 inch

separation of the large electrodes were inserted, making sure that the set screws were oriented 90° counterclockwise to the set screw in the preceding electrode.

The tube was

terminated with the original test section, leaving a large electrode at the top of the tube.

The jig was assembled,

placing the two alignment pieces at intermediate positions on the four alignment rods along the jig; the top piece off

28

of the smaller jig was placed on top of the tube, followed by an aluminum cylindrical spacer and the top piece of the large jig (Figure 9). then tightened.

The nuts on the threaded rods were

The tube length was measured and was found

to be longer than the 21 inches required. The entire assembly was placed vertically in a kiln.. The jig extended through the top of the kiln and asbestos was placed around the upper portion of the jig leaving the aluminum spacer and the top of the jig exposed.

A ther-

mometer was forced through the asbestos into the kiln.

Upon

reaching 200° Celsius, the kiln was turned off; the temperature continued to rise to 240° Celsius.

Knowing the pitch

of the threads on the threaded alignment rods, the nuts were tightened while the kiln was hot, incurring very little resistance, until the proper length of the vacuum tube was attained.

The tube was allowed to cool to slightly above

room temperature and removed from the jig.

The jig developed

stresses during heating, making disassembly difficult.

In

driving the end pieces and alignment pieces apart with a rubber mallet, the tube broke in several places and had to be repaired.

The best procedure found for removing the

tube has been first to remove the nuts from each end of the alignment rods and then to separately drive each rod out of the jig, using a 3/8 inch steel rod and a rubber mallet.

29 It was found that several gaskets along the tube had shattered and had been partially forced from between the electrodes and insulators.

Directing a stream of hot air

from a 1/4 inch by 1-1/2 inch rectangular nozzle onto the cement joint containing the shattered gasket softened the cement and allowed separation of the tube at the specified cement joint.

The adhering cement and gasket were cleaned

from the surface using a razor blade.

The electrode surface

was cleaned with ethanol and fresh cement was applied to both surfaces, then allowed to dry in open air.

The entire

tube was again placed in the large jig, replacing all gaskets and reheated in the kiln.

Bubbling of the cement

appeared in each of these repaired joints. After much repeated heating and repairing, the tube was leak tight.

A second tube was then constructed, using the

same procedure as before.

Repeated heatings, however,

resulted in each of the tubes being slightly shorter than desired but within the tolerances allowed.

The tubes were

then subjected to tests under simulated operating conditions, but in the absence of the ion source.

CHAPTER BIBLIOGRAPHY 1.

Daniel, R. E., "Construction and Testing of a Charging System and a Corona Column for an Electrostatic Accelerator," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1962.

2.

Gray, Thomas Jack, "Design and Testing of a Corona Column and a Closed Gas Distribution System for a Tandem Van de Graaff Voltage Generator," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1962.

3.

Wiley, Ralph, "A Vacuum Tube for an Electrostatic Accelerator," unpublished master's thesis, Department of Physics, North Texas State University, 1963.

30

CHAPTER III EXPERIMENTAL PROCEDURE AND RESULTS The purpose in testing the accelerating tubes without a beam is to determine the maximum potential the tubes will sustain while under actual operating conditions and to become familiar with the characteristics of the machine under breakdown conditions in order that these conditions will be readily recognizable in future operation of the accelerator. The tubes were tested both individually and simultaneously while being internally evacuated and externally pressurized with sulfur hexafluoride at pressures of 20, 40, 60, and 80 psig. To insure accurate data, a calibration was performed on the BB end of the corona column at pressures of 20, 40, 60, and 80 psig.

This calibration was performed utilizing

the calibration procedure and equipment used by Daniel (1). Calibration is performed by placing a continuously variable 0-100 kilovolt potential across each of the 26 corona gaps on the BB end of the corona column by means of a high-voltage feed-through bushing which makes sliding contact between adjacent corona rings.

After the corona threshold potential

was obtained, the corona current was varied from one to twenty microamperes in steps of one microampere, and the 31

32 corresponding potential for each current was recorded. This was performed for each of the 26 corona gaps at four different pressures. While the machine is in operation, one can read the corona current on each end of the machine.

Since the corona

gaps are in series, the terminal potential is taken as the sum total of the potentials across each gap which has been determined for a given current and pressure.

It must be

assumed that the cumulative effect is not altered under actual operation.

A sum of the potentials across the corona

gaps for each corona current at each pressure was made, and the results of these summations were plotted against the corresponding corona currents (Figure 10).

It was noted

during this calibration that for a given corona current the terminal potential was generally higher than the potential obtained by Daniel (1) under the same conditions.

This was

expected since during continued operation the corona dulls the corona points; thus the potential required to extract a given current will increase with continued use.

A comparison

of needle sharpness on the corona characteristics was made by Gray (2).

His data suggested that the needles become dull

with use. It was found dxiring the calibration that the corona characteristics of several gaps did not remain the same from one pressure setting to another.

A corona gap requiring

33 consistently higher potentials than the column average during one run might require a lower potential than the column average during another run.

This may be attributed to

changes in the corona spacing caused by movement of the corona needles when contacts to the individual gaps were made. Since the original construction, mechanical vibrations and continual handling of the corona columns have caused many of the components to become loose and easily moved.

It was also

noted that the potential difference across some corona gaps consistently differed as much as 20 kilovolts from accompanying gaps for a given corona current.

This condition

could produce excessive potentials across corresponding portions of the vacuum tube and cause failure of the insulator. If possible one should determine if any correlation exists between the position of ruptured insulators and the position of these high potential gaps. Once the BB corona calibration was completed, the generating voltmeter and AA corona column were calibrated by comparison with the BB calibration.

Graphs were made of the

generating voltmeter reading versus the terminal potential {Figure 11), and the AA corona current versus the terminal potential (Figure 12) from the BB corona calibration curve at the various pressures.

Even though the generating volt-

meter calibration was a secondary calibration, it was used as the standard since no new parameters were to be introduced

34

which.would alter the relationship between the generating voltmeter and the terminal potential.

It should be noted

that the low signal-to-noise ratio in the generating voltmeter encountered by Daniel (1) has been minimized by using a shielded signal lead.

The noise level has been reduced to

.006 volts as indicated by the generating voltmeter. Although a calibration curve of the AA corona current versus the terminal potential was obtained, very little reliability should be expected from this curve.

To gain access

to the central terminal, each corona ring on the AA end has to be removed and replaced; this process conceivably alters the individual corona gap spacing and the characteristics of the corona column.

The AA corona current is lower than the

BB for a given terminal potential due to the presence of an additional corona ring on the AA end, producing a smaller potential gradient down the column. The vacuum system constructed for evacuating the vacuum tubes was external to the pressure tank.

It was composed of

two independent pumping assemblies to allow simultaneous evacuation of the tube from both ends of the tank.

To

allow each assembly to pump on the entire system, a tube with O-rings on each end was placed in the central terminal to make a vacuum connection between the large terminating electrodes on each of the vacuum tubes.

Attached to the opposite

end of each vacuum tube by means of set screws was a vacuum

35 feed-through assembly (Figure 13).

Each assembly passed

through the large tank flange and was constructed to seal the insulating gas within the tank, to provide a vacuum connection between the internal vacuum tube and the external vacuum system, to provide pressure against the vacuum tube for making a seal against the O-ring at the central terminal, and to prevent stress from being placed on the vacuum tube once in place. Each feed-through assembly was attached externally to a 1-1/2 inch evacuation tube approximately 2 feet long terminated by a 2 inch tee (Figure 14); the opposite side of the tee was sealed by a flange holding a Phillips' vacuum gauge. The bottom of each tee was connected to a cold trap, which was never utilized, followed by a chevron baffle and a Consolidated Vacuum Corporation 2 inch diffusion pump charged with their "Convelex 10" pump oil, which possesses a low ultimate pressure, good resistance to cracking, and a high operating temperature.

To attain a temperature suitable for

operation, the pump was cooled by forcing compressed air at 30 psig through the cooling coils designed for use with water cooling and by placing 94 volts a.c. across the heating element.

Any higher voltage setting resulted in melting of

the soft solder around the cooling coils nearest to the heating element.

All seals in the vacuum assemblies were

made with O-rings lightly coated with stop-cock grease.

The

36

diffusion pumps were backed by Welch "duo-seal" fore pumps, and a Hastings vacuum gauge was placed between each diffusion pump and fore pump. Installation of the vacuum tubes requires that the corona column be removed from the tank and rotated to a vertical position.

Then the tubes are inserted from the top.

The procedure has been to insert the second tube constructed, designated tube number two, in the BB end of the corona column, installing the vacuum feed-through assembly in the large flange which remains affixed to the BB end of the column.

This assembly exerts pressure on the tube, which

maintains its position while the corona column is inverted. The first tube constructed, designated tube number one, is inserted and the bellows assembly portion of the vacuum feed-through assembly is installed.

The mechanical rigidity

possessed by the tubes is evident from the torque that must be applied to them during installation.

The corona column

is inserted in the tank, and the charging system installed, followed by the installation of the vacuum system.

When

testing only one accelerating tube, the same procedure was followed, excluding installation of the second tube.

A small

aluminum disc was used to seal the open end of the tube inside of the central terminal.

The disc was held in place

by the pressurized insulating gas.

This arrangement allows

37 utilization of only one-half the vacuum system, thus reducing the pumping speed. To determine the efficiency of the vacuum system and the pressure gradient down the vacuum tube while being evacuated from only one end, an open-air experiment was performed on one of the vacuum tubes.

This experimental arrangement

(Figure 15) consisted of a vacuum tube attached to the vacuum system normally utilized when the tube was installed in the tank.

An O-ring seal at the terminal end of the tube was

made against a flat plate which held a second Phillips' vacuum gauge.

It was observed that when the gauge directly -7 over the diffusion pump read 10 rum of Hg, the gauge at the -5 opposite end of the tube read 5.2 x .10

mm of Hg.

It may

be assumed that the pressure at the central terminal would be lower if both pumps were being utilized. A majority of the information obtained concerning electrical breakdown of the vacuum tubes and the reaction of the machine to these breakdowns came from testing the tubes individually. Each tube was installed and evacuated until the Phillips' vacuum gauge indicated a pressure on the order -7 of 10

mm of Hg; then the tank was pressurized to 20, 40,

60, or 80 psig of SFg.

Each time a tube was installed after

accompanied X-rays. eliminateits these X-rays, the termihaving been by open to the To atmosphere, initial operation was nal potential was maintained at some low value and after some

38 unpredictable length of time the X-rays would suddenly stop. The fact that this effect was observed only during initial operation of each tube suggests that the cause was due to the presence of foreign matter inside of the tube.

Once this

material was removed, the X-rays ceased. The characteristics of the machine under optimum operating conditions were determined from several runs during which breakdown was not observed.

It was observed that the

sum of the corona currents exceeded the sum of the charging currents by a small amount.

It was also noted that the

corona current on the end of the machine in which the vacuum tube was installed was actually greater for a given terminal potential than it would have been had the tube not been present (Figure 16).

This increase in current was possibly due

to the corona points in the vacuum tube, which were designed and installed as a protective device to prevent excess potentials from occurring across the insulators.

These corona

points, which were in parallel with the corona needles in the corona column, underwent some discharge increasing the current flowing down the corona column.

This effect was not observed

at 40 psig but became pronounced as the tank pressure was increased.

Visual examination of the vacuum tubes verified

that these corona points did experience some discharge. Three additional types of electrical breakdown occurred . during the testing.

These included a continuous breakdown

39 accompanied by electron loading characteristics, violent internal tube sparks, and violent tank sparks, all of which are common problems with Van de Graaff accelerators. Tank sparks occurred in the machine whenever the dielectric strength of the insulating gas was exceeded and was responsible for establishing the upper limit on the terminal potential when operating the generator without vacuum tubes. Tank sparks occurred frequently with the tubes installed while operating at low SFg pressures and occasionally while operating at higher pressures.

These sparks produced a loud

resonating noise and a bright flash of light visible through the tank ports.

They completely discharged the central term-

inal but in no way affected the operating characteristics of the vacuum tube.

Following a tank spark, the charging system

quickly reestablished the terminal potential. Tube sparks possessed all of the characteristics of tank sparks but were followed by a large flux of X-rays which increased in intensity as the charging system attempted to reestablish the terminal potential.

Tube sparks also

produced a gas load within the vacuum tubes, resulting in a lower vacuum.

If a large tube spark occurred while operating -7 at an indicated vacuum of 10 mm of Hg, xt would usually -4 lower the vacuum momentarily to about 10 mm of Hg; however, a fairly rapid pumpout occurred and in less than a minute a —7 vacuum of 2 or 3 x 10 could be reestablished. The increased

40 tube pressure was a highly favorable condition for electron loading which usually followed each tube spark; although on a few occasions a sudden appearance of heavy electron loading preceded a tube spark.

If the charging potential was not

immediately decreased following a spark, additional sparking and/or heavy electron loading would always occur producing a gas load requiring several hours to evacuate. Electron loading occurred under all operating conditions and did not necessarily require a tube spark to initiate. Electron loading appeared to be highly pressure dependent and rarely self-initiated in a well evacuated vacuum tube. Electron loading was recognized by one or a combination of characteristics which did not always occur simultaneously. During electron loading, it was noted that the sum of the charging currents began to exceed the normally higher sum of the corona currents.

Also for a given terminal potential,

the corona current in the end of the machine in which the tube was installed became equal to or lower than the current it would draw had the tubes not been present (Figure 16). This condition was opposite to that existing during operation without loading.

One could usually observe the

transition of the corona current if a tube went into an electron loading configuration (Figure 17).

On occasion,

the tubes would suddenly go into heavy electron loading which was characterized by a drop in all current meter

41 readings except the up-charge reading.

Apparently the

decrease in the corona current was caused by the electrons in the loading process moving up the tube in parallel with the corona current.

At all times for an established termi-

nal potential, the charging currents reaching the central terminal must equal the discharging currents.

The electrons

in the loading process apparently constituted an unmeasured portion of the discharging current, decreasing the measured corona current required to establish the balance.

When

testing the vacuum tubes individually during electron loading, the electrons undoubtedly struck the upper end of the vacuum tube and the aluminum disk making the vacuum seal at the central terminal creating X-rays.

By monitoring these X-rays

one could determine when loading was occurring; however, there were occasions when evidences of electron loading existed although no X-rays were detected. During operation, a Model 404 Technical Manufacturing Corporation 400 channel multichannel analyzer was utilized in hopes of obtaining an additional estimate of the terminal potential.

It was postulated that the spectrum of the X-rays

produced by electron loading or a spark would have a sudden drop in intensity at energies at or slightly below the energy corresponding to the terminal potential.

The spark spectrum

indicated X-rays of energies exceeding 4 MeV; these were probably produced by 'stack up' or more than one X-ray

42 entering the scintillation counter simultaneously, producing a signal corresponding to the combined energies of the X-rays. The spectrum taken with electron loading occurring at low terminal potentials resembled an exponential decay possessing no definite terminating point, while at higher terminal potentials characteristic peaks appeared. peaks were in excess of 1 MeV.

The majority of these

Since the

energy level

for the possibly target materials found in the vacuum system is much lower than this energy, these peaks were possibly due to gamma radiation from some induced nuclear reaction. The maximum potential obtainable across the vacuum tubes was limited by tube sparking.

It was found that if the tubes

were under electron loading a substantially higher potential was obtainable than when loading did not occur.

While

testing individually under optimum conditions, tube number one on an average withstood 1.45 million volts and tube number two withstood 1.46 million volts.

Under loaded conditions

each tube at one time withstood 1.74 million volts as determined from the generating voltmeter. \

Conditioning of a tube

was possible, but the gas load created by tube sparks coupled

\

with the "slow" pumping speed usually prevented much conditioning.

The largest electron locking observed immediately

followed tube sparks. Under normal operating conditions no visible damage to the tubes occurred; however, on two separate occasions an

43 insulator on tube number one cracked when a tube spark occurred.

The method of repair has been to place a coating

of the vinyl cement over the cracked insulator, to replace the entire column in the large jig, and to heat in the kiln to 150° Celsius.

This procedure seemed to have no effect on

the operating characteristics of the tube.

On both occasions

during which the rupture occurred, the tube was being tested individually under a heavy gas load accompanied by electron loading.

In both cases there appeared a sudden increase in

the corona currents followed immediately by a violent tube spark, causing the tube to rupture.

The sudden increase in

corona current was an indication that the electron loading suddenly ceased.

The rupture was created either by the surge

in corona which produced very high potentials across many of the insulators, exceeding the dielectric strength of one, or by mechanical stresses placed on the tube by the Shockwave created by the tube spark. The characteristics of the Van de Graaff while operating with either one or two tubes remain very nearly the same.

The most significant differences are the pumping

speed and decrease in electron loading.

With both pumping

assemblies operating simultaneously on the two tubes, the pumping speed was rapid enough to allow continuation of a run following a tube spark without a decrease in terminal potential.

This allowed conditioning of the vacuum tubes.

44 The more rapid pumping speed also aided in reducing the problem of electron loading.

Although electron loading appeared

during the runs with both tubes, it was much harder to establish its presence.

Another reason for a decrease in the

loading might have been the presence of the open beam apertures on each end of the central terminal.

Many of the

electrons associated with the loading process might possibly have left one tube, traversed the central terminal, and entered the second tube.

This condition would produce an

oscillation of the electrons through the central terminal and would reduce the number of collisions. The maximum terminal potential obtained after conditioning with the presence of both tubes under evacuation and in the absence of any detectable electron loading was 1.64 million volts as determined from the generating voltmeter. On a typical run without electron loading an average potential of 1.49 million volts was obtained before a tube spark occurred.

With electron loading, a potential of 1.64 million

volts was attained on two separate occasions.

CHAPTER BIBLIOGRAPHY Daniel, R. E. , "Construction and Testing of a Charging System and a Corona Column for an Electrostatic Accelerator," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1962. Gray, Thomas Jack, "Design and Testing of a Corona Column and a Closed Gas Distribution System for a Tandem Van de Graaff Voltage Generator," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1962.

45

CHAPTER IV CONCLUSIONS Generating Voltmeter The generating voltmeter has proven to be the only reliable indicator of the terminal potential.

The gener-

ating voltmeter reading is not dependent on conditions existing in the vacuum tubes, as are the corona currents which have previously been used in determining terminal potentials.

It would be desirable to have a digital a.c.

voltmeter or an a.c. voltmeter with a linear scale easily readable to three-place accuracy instead.of the Ballentine a.c. voltmeter currently used, which possesses a logarithmic scale that is difficult to read at the higher values.

A

calibrated oscilloscope could be used; however, there is a d.c. component associated with the signal from the generating voltmeter due to the capacitance between the stator of the voltmeter and the central terminal.

The significant 24 0

cycle a.c. component of the signal is superimposed on top of the d.c. signal, which increases with the terminal potential, creating an inherent drift in the signal trace which must be continually corrected, making the oscilloscope troublesome to use.

46

47 Due to the production of X-rays by the Van de Graaff after installation of the vacuum tubes, it became necessary to move the machine controls from beside the generator to a shielded area several feet away.

Upon resuming testing, it

was noted that the generating voltmeter reading for a given terminal potential was significantly lower than previous readings.

A measurement of the resistance of the signal lead

excluded the possibility of a resistive drop in the signal. It was found that the capacitive reactance of the shielded signal lead was responsible for the loss in signal intensity. It would be advisable to establish immediately a new and very accurate calibration of the generating voltmeter. This would be desirable at this time since BB corona calibration data have just been acquired and since the previously used generating voltmeter calibration curves are invalid because of the new signal lead.

For analyzing data obtained

on tube number one, at the new location, a correlation between the new generating voltmeter readings and the old readings was made, by assuming that the BB corona currents and the corresponding terminal potentials remained the same while operating in each, position during runs in the absence of electron loading.

A generating voltmeter calibration curve

was obtained (Figure 18).

It was noted that the voltmeter

readings at the tyo positions differed by a constant determined by the tank pressure.

At 40 psig, the new voltmeter

48 reading was determined by multiplying the old reading by .806, and at 60 psig the reading was multiplied by .847. Future operators of the machine should be aware of the most common difficulty encountered with the generating voltmeter.

The electric motor used to drive the rotor blades

on the generating voltmeter is characterized by a very rapid slowing once turned off.

The rotational inertia of the rotor

blades often unscrews the threaded shaft supporting the rotor from the motor shaft, allowing the rotor to drop slightly and extend inside the tank.

As the potential is

increased, sparking occurs to the protruding shaft.

Under

the above circumstances the signal from the generating voltmeter will be d.c. with spikes due to the sparking, which can be observed with an oscilloscope.

A prism should be placed

inside one of the observing ports to allow visual examination of the rotor blades in case trouble occurs.

The operator

should also become familiar with the sound associated with the whirling blades of the rotor and should be able to determine when this sound is present during operation. From the reliability of the data obtained from the generating voltmeter, it is believed that this component of the Van de Graaff is suitable for future operation, although a more accurate a.c. voltmeter should be installed and a calibration should be obtained.

49 Charging System Because a sag has developed in the Lucite corona column support, the brushes through which the pellet string passes have become misaligned.

Aluminum shims have been

placed under several of the brushes in an attempt to reestablish alignment, but the problem still exists.

Due to

the flexibility of the pellet string, this misalignment has not presented any problem. A high-frequency vibration is created by the pellet string passing through the brushes along the corona column. This vibration is transferred throughout the machine and tends to loosen any screws or nuts not firmly secured.

This

vibration has caused unscrewing of the corona set screws along the vacuum tubes, the brass balls and plungers along the inside of the corona column, and the set screws on the charging brushes.

The corona set screws on the vacuum

tubes have been coated with clear fingernail polish, alleviating this problem.

The brass balls and plungers pose a

problem which is difficult to solve unless the corona columns are completely reassembled, coating the screws with a bonding agent.

The brushes should be checked and cleaned

frequently to minimize the risk of shorting the brushes or of the pellet strings pulling the brushes from their supports, The pellet string to date has operated for 55.8 hours. The original split pellet used in connecting the ends of the

50 string was broken and replaced with a modified design.

The

nylon cable has become frayed at the base of the male portion of the split pellet, and at one point between two pellets on the string one of the nylon lines comprising the nylon cable has become severed. of wear.

These are the only signs

It is difficult to estimate when repairs should

be made on the male half of the split pellet.

If the string

breaks at this point, the results could be catastrophic. Since the pellet string continually stretches, repairs can be made on the present pellet.string if necessary without reducing the length below the operational minimum.

Each time

the pellet string has been removed from the tank it has been wiped with a clean lint-free rag to remove any accumulated oil and dirt.

Each time the pellet string has been installed,

it has been lightly coated with 30 weight motor oil.

This

cleaning and oiling makes a definite difference in the amount of force required to move a pellet string through the brushes and helps to minimize the possibility of breakage. During operation, the up-charge current still exceeds the down-charge current even though the resistors in the central terminal have been changed in accordance with the suggestion made by Daniel (2).

If a matched up-charge and

down-charge is desired, additional resistance can be added. The up-charge current is very stable, is determined by the charging potential, and is independent of conditions existing

51 along the corona columns or within the vacuum tubes; however, the down-charge current is susceptible to these conditions. The charging system is, therefore, not always stable; this can be observed from occasional oscillations occurring in the down-charge current. Gas Handling System The gas handling system has performed satisfactorily during the present series of experiments; however, some modifications should be made to facilitate more rapid pumping speeds at low gas pressures.

A 3/4 inch by-pass with valve

should be installed around the small regulator at the entrance to the surge tank.

The orifice on the pressure

regulator is too small, and once the gas pressure falls below approximately 25 pounds, the rate of gas flow through the regulator becomes lower than the evacuation rate of the compressor, and the compressor has to be stopped frequently to allow gas to flow into the surge tank.

The by-pass valve

could be throttled at the lower gas pressures, maintaining pressure in the surge tank and preventing its evacuation. This would greatly decrease pumping time. The gas used in testing has not been dehumidified since testing began.

There have been no unusual operating charac-

teristics to indicate a need for dehumidification. Leaks have been found at the pipe joints in the gas handling system and gas losses have been observed from both

52 the storage tank and the Van de Graaff tank when under pressure.

Losses could be reduced by placing valves at both

inlets to the Van de Graaff tank, preventing leakage through the gas handling system. Corona Columns The calibration of the BB corona column brought out the fact that the present corona columns fail to establish uniform potential gradients between the central terminal and ground.

This fact can be illustrated by observing the dif-

ferences in the potential across the 26 corona gaps at a typical gas pressure and corona current.

It was also noted

during the calibration that the coronas are highly unstable and produce continual voltage fluctuations across succeeding corona rings. The weight of the corona columns and central terminal has produced a sag in the corona column.

When the vacuum

tubes are installed in the corona columns, they both possess a downward slope toward the central terminal.

When installed,

it is impossible to look through the beam apertures of the vacuum tubes and see out the opposite end; however, some light can be seen entering from the opposite aperture.

It

may be necessary to eliminate this sag and align the accelerating tubes if a beam is to be injected into the accelerator. Until present experimentation, it has been common procedure to estimate the terminal potential from the calibration

53

of the BB corona current and the terminal potential.

This

procedure is acceptable as long as the vacuum tubes are not installed.

Installation of these tubes alters the char-

acteristics of the corona columns, and there no longer exists a correlation between the corona currents and terminal potential.

While operating the machine without electron loading,

it has been noted that the corona currents are higher than if the tubes were not installed, and with electron loading the corona currents drop below this value.

This is seen as

an indication of the presence of electron loading and becomes more evident when operating at higher tank pressures.

It

is evident that the generating voltmeter provides the most reliable means of determining the terminal potential. Vacuum System The vacuum system constructed for evacuating the vacuum tubes has operated satisfactorily.

The small beam aperture

is responsible for the slow pumping speed observed and not the vacuum system.

To improve the performance of the vacuum

system and provide localized diffusion pump cooling facilities, several modifications should be made. It has been suggested that oil vapors from the diffusion pumps diffuse into the vacuum tubes and are responsible for much of the electron loading (1).

Pump oil might be respon-

sible for the breakdown accompanying the initial operation

54 of the Van de Graaff with the vacuum tubes installed.

To

alleviate the possibility of pump oil playing a role in electron loading, the cold traps should be utilized.

The

cold traps would also help attain lower pressures. Modification of the cooling system on the diffusion pumps should be made in accordance with a local design successfully tested.

To alleviate the necessity of a con-

tinual supply of compressed air for cooling, the modification incorporates a small blower to force air through special cooling coils installed in place of the present cooling coils. A voltage regulator is used to obtain a suitable operating temperature. It has been noted since the conclusion of experimentation that the Phillips1 vacuum gauges are not accurate for _7 measuring pressures in the range of 10

mm of Hg.

This was

determined from observing the pressure reading on a vacuum system containing a Phillips' gauge and a recently purchased ionization gauge.

This fact does not have any direct bearing

on the experiments performed since the pressure gradient down the tubes was so large; the gauges were used only for a relative vacuum measurement. The Vacuum Tubes It is evident from experimentation why the problems of electrical breakdown are so vaguely understood.

There are

55 too many uncontrollable parameters associated with the operation of a machine. The vacuum tubes constructed will consistently sustain a potential of 1.45 million volts, which corresponds to a potential gradient down the BB corona column of 892 kilovolts per foot, a value comparable to the better accelerators in existence.

After conditioning, these tubes have sustained

potentials as high as 1.64 million volts, corresponding to a gradient of 1.01 million volts per foot.

Failure of the

machine to attain higher potentials cannot be traced to either of the tubes; each has very similar operational characteristics . When operating with either one or both tubes installed, electron loading is frequently present.

It is not always

easy to establish when loading is occurring, and loading is less prevalent when the tubes are installed simultaneously. Electron loading is highly dependent on tube pressure and increases with tube pressure.

Electron loading is usually

initiated by a tube spark which produces a gas load, increasing the tube pressure.

The beam aperture in the vacuum tubes is

too small to provide a rapid pumping speed under operating conditions incurred. The corona points installed in the vacuum tubes draw current during operation.

Although this was not supposed

to occur it might act to establish a more uniform potential

56

gradient down the tubes and stabilize the corona column currents. Rupture of an insulator occurred twice on tube number one while it was being tested individually.

The first punc-

ture was repaired by coating the insulator with a light coat of vinyl cement and heating.

The repair did not seem to

affect the operational characteristics of the tube.

The

second rupture occurred at the same insulator as the first. Both cracks were created by a tube spark immediately following what appeared to have been sudden termination of electron loading. The vacuum tubes are mechanically rigid and withstand moderate mechanical shock and considerable torque without breakage.

They are easily constructed and repaired, although

much time and care are required for construction. Although much data were acquired during the series of experiments, some of the conclusions about the operational characteristics of the vacuum tubes and the generator are^ drawn from a few isolated instances, but none from an incident which occurred only once unless specifically stated. Much more research needs to be performed before one can • determine if the tubes are suitable for use.

It is felt

that because of the time and expense required to build tubes with large apertures, and since an ion source will soon be available for the accelerator, experimentation on the present tubes should be continued.

CHAPTER BIBLIOGRAPHY 1.

Blewett, J. P., "Electron Loading in Ion Accelerating Tubes II," The Physical Review, LXXXI (1951), 305A.

2.

Daniel, R. E., "Construction and Testing of a Charging System and a Corona Column for an Electrostatic Accelerator," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1962.

57

APPENDIX

59

GJSp-O

Pig. 1 — A vacuum tube with "Inverted cone" electrode configuration.

60

B Fig. 2 — ( A ) Cross section of corona column with vacuum tube installed (B) Large electrode (C) Small electrode

61

V

' "siV

11

*

Pig- 3 Longitudinal section of corona column with vacuum tube installed.

62

w

U

O -P fd 0 •H •—I Qj

Cu (d

-P C a) e a) u 1 i

tr> •H Cm

63

—

Fig.

5—Foil cutting tool

64

tn -H •r~)

tT> £ •H £ o -H +J •H 0) 0 CU 0) jC +J Cn G •H i—I i—I fd 4-> CO £ H 1 I

tn •H Cm

65

m

J

Fig. 7—C-clamp holding a positioning jig, insulator, gasket, electrode, and jigging block.

66

Fig. 8--Assembling electrode-insulator subassemblies in the small jig.

67

Q) J3 2

-P g 3 3 U cd >

Q) A

-P t n £

"rH i—1 o

tr> •H

Q) t n

U rti h I i i Ch

tr> •rH P4

68

@0 PSIG

60 pate

40 PSJ©

SO Pol6

t~ I.I

!—|—!~|—I—|—{—|—j- "1 "J' • -}—S—|—[—|—I—}~i—j—1—{•—}—|—I—J—J—j—1—j—i—|—I—{-•

0

I 2 S 4 6 6 7 I 9 10 II 12 18 14 16 I© IT IS I© EO 21 22 BB CORONA CURKKNT (JUA)

Fig. 10—Terminal potential versus BB corona current prior to installation of vacuum tubes.

69

2.3

2.2

2.1 2.0 1.91.6 1.71.6 1.5j 1.4a 1.3 -f < I- 1| 2a U) S I.I CL -I L 0 < I .9 8 01 hi

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PSIG

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£.8 3.0 3.® 4.0 4.5 5.0 5,6 6.0 B.& 7.0 7.5 6.0 8.5 ©.0 GENERATING

VOLTMETER ( A X . VOLTS)

Pig. 11—Terminal potential versus generating voltmeter reading prior to alteration of the signal lead.

70

8 0 PSIG

60 PSI6

1 | I | I [l-[~| | 1 | 1 | 1 | I [-l-fl-f+l1 1 1 1 1 1 ! 1 ! 1 ! | ! | 1 1 l-f 0

1 2

3

4

6

6

7 AA

8

9

10 IE 12 IS 14 15 !G I?

19 2 0 Si 22.

CORONA CURRENT (MA)

Pig. 12 Terminal potential versus AA corona current prior to installation of vacuum tubes.

71

\ \ \ \V

SQ3

Pig. 13—-Vacuum feed-through assembly

72

g 0) +J CO

>1 CO

|

3 U rti >

-P "H

£

•op fd 5-1 d) G CD tr> O •H -P rtf -P U) 0 -P u
tn •H Pn

73

Q) £ 1

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a) -P fd

3 o fd >

a) fi fd

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•H -P C

a)

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PA

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74

OPERATI M© CONDITIONS X -

HQ TUBE

o -

WITHOUT LOADING

• -

WITH LOADING

r i.s

6 0 FS1Q

I. O

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2

3 BB

Fig.

4

8

CORONA

6

7

6

CURRENT (HA)

16—A_ renreripntpshi vp r-nmnGrJi cnn ^-p

8

10

} -f~ II

75

2«0

1.9

OPERATING O -

WITH

CONDITIONS TUBE

X - WITHOUT

TUBE

L7

L© b*

ty HLS O a j < i«.4 K 111 1.3

1.2

6 0 P8I0

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I.O 0

1

2

3 B0

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CORONA

H i f-f-+-| 1 | I I • I G 7 6 9 10

I I

CURRENT (|4 A>

Fig. 1 7 — A representative curve of terminal potential versus BB corona current as spontaneous electron loading occurs

76 2. 3

2.2-

2.12.0-

6 0 P8IQ

9 I. I 40

§.0

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GENERATING

4.5 VOLTMETER

6.0

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(A.O. VOLTS)

Fig. 18—Terminal potential versus generating voltmeter reading after alteration of the signal lead.

BIBLIOGRAPHY Books Livingston, M. S., and Blewett, J. P., Particle Accelerators, New York, McGraw-Hill Book Company, 1962. Articles Alvarez, L. W., "Energy Doubling in D. C. Accelerators," The Review of Scientific Instruments, XXII (1951), 705, Blewett, J. P., "Electron Loading in Ion Accelerating Tubes 1 1 T h e Physical Review, LXXXI (1951), 305A. Chick, D. R., Hunt, S. E., Jones, W. M., and Petrie, D. P. R., "A Van de Graaff Accelerator Tube of Very Low Retrograde Electron Current," Nuclear Instruments and Methods, V (1959), 518. Cranberg, L., "The Initiation of Electrical Breakdown in Vacuum," Journal of Applied Physics, , and Henshall, J. B., "Small-Aperture Diaphragms in Ion-Accelerator Tubes," Journal of Applied Physics, XXX (1959), 708. Firth, K., and Chick, D. R., "The 'Screening' of Neutral Particles in High Voltage Ion Accelerator Tubes," Journal of Scientific Instruments, XXX (1953), 167. Hunt, S. E., Cheetham, F. C., and Evans, W. W., "The Performance and Conditioning of 'Inverted Cone' Van de Graaff Accelerating Tubes," Nuclear Instruments and Methods, XXI (1963), 101. Jones, F. L., "Electrical Discharges and the Vacuum Physicist," Vacuum, III (1953), 116. Mansfield, W. K., and Fortescue, R. L., "Prebreakdown Conduction Between Electrodes in Continuously Pumped Vacuum Systems," British Journal of Applied Physics, VIII (1957), 73. McKibben, J. L., "Control of Current Loading and Sparks in Ion Accelerating Tubes by Back-Biased Diaphragms," Bulletin of American Physical Society, I (1956), 61. 77

78 Bernsrs, E. D., Eppling, F. J., Knecht, D. J., and Herb, R. G., "New Electrostatic Accelerator," The Review of Scientific Instruments, XXX (1959), 855. Trump, J. G. Van de Graaff, R. J., "The Insulation of High Voltages in a Vacuum" Journal of Applied Physics, XVIII (1957), 327. Turner, C. M., "Ionization Loading of Electrostatic Generators," The Physical Review, XCV (1943), 599. "Electron Loading in Ion Accelerating Tubes I," The~Physical Review, LXXXI (1951), 305A. Van Atta, L. C., Northrop, D. L., Van Atta, C. M., and Van de Graaff, R. J., "The Design, Operation, and Performance of the Round Hill Electrostatic Generator," The Physical Review, XLIX (1936), 761. Webster, E. W., Van de Graaff, R. J., and Trump, J. G., "Secondary Electron Emission from Metals under Positive Ion Bombardment in High Extractive Fields," Journal of Applied Physics, XXIII (1952) , 264. Unpublished Materials Daniel, R. E., "Construction and Testing of a Charging System and a Corona Column for an Electrostatic Accelerator," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1962. Gray, Thomas Jack, "Design and Testing of a Corona Column and a Closed Gas Distribution System for a Tandem Van de Graaff Voltage Generator," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1962. Michael, Irving, "The Development and Performance of a New Electrostatic Accelerator," unpublished doctoral dissertation, Department of Physics, University of Wisconsin, Madison, Wisconsin, 1958. Wiley, Ralph, "A Vacuum Tube for an Electrostatic Accelerator," unpublished master's thesis, Department of Physics, North Texas State University, 1963.