A VACUUM TUBE FOR AN ELECTROSTATIC GENERATOR

APPROVED:

iJ Major Professor

Minor Prof e r/sbr

j jct-1-.cL.f Director, of the Department- of Physic:

7 DeanLof 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. INTRODUCTION 1

II. CONSTRUCTION OF THE VACUUM TUBE 15

III. EXPERIMENTAL PROCEDURE AND RESULTS 31

IV. CONCLUSIONS . . . . . 46 Generating Voltmeter Charging System Gas Handling System Corona Columns Vacuum System Vacuum Tubes •

APPENDIX 58

BIBLIOGRAPHY 77

i n

LIST OF ILLUSTRATIONS

Figure Page

1. A Vacuum Tube with "Inverted Cone" Electrode Configuration 59

2. (A) Cross Section of Corona Column with Vacuum Tube Installed (B) Large Electrode (C) Small Electrode. 60

3. Longitudinal Section of Corona Column with Vacuum

Tube Installed 61

4. Cement Applicators 62

5. Foil Cutting Tool 6 3

6. Installing the Positioning Jig 64

7. C-clamp Holding a Positioning Jig, Insulator, Gasket, Electrode, and Jigging Block 65

8. Assembling Electrode-Insulator Subassemblies in

the Small Jig 66

9. Large Jig Holding the Vacuum Tube 67

10. Terminal Potential Versus BB Corona Current Prior to Installation of Vacuum Tubes 68

11. Terminal Potential Versus Generating Voltmeter Reading Prior to Alteration of the Signal Lead . 69

12. 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 75

18. 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 accel-

erating 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 acceler-

ator; 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 acceler-

ating 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 prob-

lems 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, insu-

lator breakdown, pulsed internal breakdown, and possibly the

most serious—electron loading.

Varying design parameters has reduced most of the prob-

lems, 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. It is caused by a

combination of factors. 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 brems-

strahlung 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 impor-

tant role in the above reaction. It has been found that the

threshold voltage for breakdown does not depend on the elec-

trons 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 separa-

tion. 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 con-

ducting electrode. This positive charge on the surface

lowers the potential barrier at the surface allowing elec-

trons 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 elec-

trodes 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 elec-

trodes, 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. There was also occasional elec-

trical breakdown. 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. Some electron loading was measured in

the later tests. 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 elec-

tron 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 accel-

erating 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 and 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 Acceler-ators , 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 proto-

type 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 construc-

ting 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 thermo-

plastic. 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 com-

plete 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 sub-

jected 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 elec-

trodes 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 horizon-

tally on a rack inside an oven for baking.

The same procedure as above was followed in applying

cement to the ceramic insulators, although the applicator-

insulator assembly was not inverted after insertion of the

alignment probe through the insulator. It was then verified

2 2

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 ther-

mometer 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 construc-

ting 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 incorpo-

rated, 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) con-

sisted 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 photo-

graphic 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 elec-

trodes 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 cen-

tral 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 subas-

semblies. 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° counter-

clockwise 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 counterclockwise to the hole

in the preceding electrode. 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 end-

piece. 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). The nuts on the threaded rods were

then tightened. 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 temper-

ature 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 gas-

kets 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 accom-

panying gaps for a given corona current. This condition

could produce excessive potentials across corresponding por-

tions 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 volt-

meter 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 reli-

ability 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 elec-

trodes 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 termi-

nated 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 ver-

tical 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 evac-

uated 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 elec-

trical 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

having been open to the atmosphere, its initial operation was accompanied by X-rays. To eliminate these X-rays,

nal potential was maintained at some low value and after some

the termi-

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 oper-

ating 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 pre-

sent (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 poten-

tials 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 dielec-

tric 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 opera-

tion 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 cen-

tral 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 poten-

tials characteristic peaks appeared. The majority of these

peaks were in excess of 1 MeV. 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 num-

ber two withstood 1.46 million volts. Under loaded conditions

each tube at one time withstood 1.74 million volts as deter-

mined 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 condi-

tioning. 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 oper-

ating 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 prob-

lem of electron loading. Although electron loading appeared

during the runs with both tubes, it was much harder to estab-

lish its presence. Another reason for a decrease in the

loading might have been the presence of the open beam aper-

tures 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 condi-

tioning 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 poten-

tial 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 cali-

bration 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 deter-

mined 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 volt-

meter. 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 volt-

meter 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 deter-

mine when this sound is present during operation.

From the reliability of the data obtained from the gener-

ating 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 cali-

bration 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 rees-

tablish 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, alle-

viating this problem. The brass balls and plungers pose a

problem which is difficult to solve unless the corona col-

umns 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 por-

tion 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. These are the only signs

of wear. 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 cen-

tral 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 modi-

fications 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 com-

pressor, 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 pres-

sure. 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 accel-

erating tubes if a beam is to be injected into the accelerator.

Until present experimentation, it has been common pro-

cedure 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 poten-

tial. 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 facili-

ties, 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 experimenta-

tion 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 rela-

tive 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 oper-

ation 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 char-

acteristics .

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 fol-

lowing 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 inci-

dent 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 gasket, electrode, and jigging

a positioning block.

jig, insulator,

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

t r >

• H

Q) t n

U r t i

h I

i

i

C h

t r >

• r H

P 4

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

1.9-

1.6

1.7-

1.6

1.5-

j 1.4-

a 1.3 -f < I- | 2-a 1

U) S I.I CL

-I L 0

< I .9 8 01 « h i • ©" H

.7

.6

.6

.4 -

.3-

.2-

.1 -

8 0 PSIG

60 PSIG

4 0 PSIG 20 P8I©

- H - h - _J J.—j.—j—^—|—j—J—,—j„

£.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

6 0 PSI6

1 | I | I [l-[~| | 1 | 1 | 1 | I [-l-fl-f+l 1 1 1 1 1 1 ! 1 ! 1 ! | ! | 1 1 l-f 0 1 2 3 4 6 6 7 8 9 10 IE 12 IS 14 15 !G I?

AA CORONA CURRENT (MA)

19 2 0 Si 22.

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 r t i >

- P " H £

o •p f d 5-1 d ) G CD t r >

O • H - P rtf

- P U) 0

- P u <D

i—I W

1 I

i—I

t n • H P n

73

Q)

£ 1

- P

e

o fd >

a) - P fd 3 o fd > a)

fi fd

CO CO O 5-1 U fd

fd • H - P C a) n CD m 4-1 • H

CD U

3 CO CO a) U

PA

a)

4->

t7> G - H

• H £

U

0 )

•P a) Q i i

L O

(T> • H En

74

OPERATI M© CONDITIONS

X - HQ TUBE

o - WITHOUT LOADING

• - WITH LOADING

r i .s

60 FS1Q

I. O I I I 1 | ! 1-4 4 -| M H-j-1 i ! { j —I—4—I—f—f- H-i-j-l- I I I j I I I I | 1( I I | 1 M } -f~

© I 2 3 4 8 6 7 6 8 10 II

BB CORONA CURRENT (HA)

F i g . 16—A_ renreripntpshi vp r-nmnGrJi cnn -p

75

2«0

1.9

L7

L©

b*

ty HLS O a j

<

i«.4 K 111

1.3

1.2

i.l

OPERATING CONDITIONS

O - WITH TUBE

X - WITHOUT TUBE

60 P8I0

I.O H i f-f-+-| 1 | I I • I

0 1 2 3 4 S G 7 6 9 10

B0 CORONA CURRENT (|4 A>

I I

Fig. 17—A representative curve of terminal potential versus BB corona current as spontaneous electron loading occurs

76

2 . 3

2.2-

2.1-

2.0-

6 0 P 8 I Q 9 I. I

4 0 P 8 I 0

§.0 3 . 5 4.0 4.5 6.0 5 . 5

G E N E R A T I N G V O L T M E T E R ( A . O . V O L T S )

6.0

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.