US3585445A

US3585445A - Method for preserving electron gun assemblies

Info

Publication number: US3585445A
Application number: US803774A
Authority: US
Inventors: George V Miram; Samuel W Woolsey
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1969-03-03
Filing date: 1969-03-03
Publication date: 1971-06-15
Anticipated expiration: 1988-06-15

Abstract

A method for preserving electron gun assemblies is disclosed. In the method, an electron gun assembly, including a thermionic cathode emitter, is assembled in a vacuum envelope structure. The envelope is evacuated, baked out, and the cathode processed in the conventional manner to activate the cathode. Beam voltages are applied to draw beam current from the gun to test proper operation thereof. These tests may include a test of beam diameter and permeance in the presence of a beam focusing magnetic or electric field. The processed and tested electron gun is then valved off from the rest of the envelope by means of a valve connected to the envelope portion including the electron gun. The valved-off gun is then detached from the vacuum envelope for preserving the electron gun in an operable condition for subsequent use in a second vacuum envelope structure.

Description

United States Patent (72] Inventors George V. Miram Daly City; Samuel W. Woolsey, Los Altos, both of, Calif. [2]] Appl. No. 803,774 [22] Filed Mar. 3, 1969 [45] Patented June 1S, 1971 [73] Assignee Varian Associates Palo Alto, Calif.

[54] METHOD FOR PRESERVING ELECTRON GUN ASSEMBLIES 4 Claims, 2 Drawing Figs.

[52] U.S.Cl 316/1, 316/24, 316/26 [51] lnt.Cl H0lj 9/00 [50] Field of Search 316/1, l7, l8, i9, 24, 26

[56] References Cited UNITED STATES PATENTS 3,366,434 1/1968 Mengel 316/1 Primary Examiner-John F. Campbell Assistant Examiner-Richard Bernard Lazarus Attorneys-Stanley Z. Cole and Leon F. Herbert ABSTRACT: A method for preserving electron gun assemblies is disclosed. In the method, an electron gun assembly, including a thermionic cathode emitter, is assembled in a vacuum envelope structure. The envelope is evacuated, baked out, and the cathode processed in the conventional manner to activate the cathode. Beam voltages are applied to draw beam current from the gun to test proper operation thereof. These tests may include a test of beam diameter and permeance in the presence of a beam focusing magnetic or electric field. The processed and tested electron gun is then valved off from the rest of the envelope by means of a valve connected to the envelope portion including the electron gun. The valved-off gun is then detached from the vacuum envelope for preserving the electron gun in an operable condition for subsequent use in a second vacuum envelope structure.

PATENTEUJUNISIHYI 3.585445 LYSTRON KLYSTRON KLYSTRON FIG.2

UNCHER LERATOR ACCELERATOR ACCE ERAIOR INVENTORS GEORGE V.M|RAM 33 SAMUEL WWOOLSEY BY 3%, MM;

ATTORNEY KLYSTRON METHOD FOR PRESERVING ELECTRON GUN ASSEMBLIES The invention described herein was made in the course of, or under Subcontract 5603 under Contract W-7405 Eng -26 with the Atomic Energy Commission.

DESCRIPTION OF THE PRIOR ART Heretofore, a valve has been provided between the electron gun portion of a beam tester and the beam-collecting portion of the beam tester for valving off the electron gun to prevent contamination of the gun during periods wherein the collector portion of the beam tester may be exposed to atmospheric pressure. Such an arrangement is described in Research Report EE-495 of Cornell University School of Electrical Engineering, dated 1 May l96l, further identified as RADC-TN-l-l 88, Linear Beam Microwave Tube Technical Report No. 9, under Contract No. AF30(602)-l696. It was found in this prior work that the provision of a ball valve between the electron gun and, the remaining portion of the beam tester permitted the operator to valve the electron gun off from the remaining portion of the vacuum envelope of the beam tester during times when the beam test'er was not in use to prevent contamination-of the electron gun by backstreaming of oil from the diffusion pumps connected to the main beam tester envelope section. A separate vacuum pump was connected to the electron gun assembly fol" maintaining the vacuum in that portion of vacuum envelope containing the gun when the gun was valved-off from the remaining portion of the envelope.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of a method for preserving electron gun assemblies.

One feature of the present invention is the provision of a valved-off vacuum envelope containing a processed electron gun assembly for subsequent use in a second vacuum envelope structure, whereby the operating parameters of the electron gun can be guaranteed and whereby substantial shelf life can be obtained.

Another feature of the present invention is the same as the preceding feature including a glow discharge getter ion vacuum pump connected to the valved-off electron gun assembly for maintaining a low pressure and for periodically monitoring the gas pressure within the valved-off electron gun.

Another feature of the present invention is the same as any one or more of the preceding features wherein the valved-off electron gun assembly is connected by a suitable flange structure to a second envelope, such as a particle accelerator, which, is to use the gun when the valve is opened for placing the gun in gas communication with the second vacuum envelope.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a longitudinal view, partly in section and partly schematic, of a portion of an electron gun tester incorporating features of the present invention, and

FIG. 2 is a longitudinal view, partly in section, of a particle accelerator incorporating the detachable electron gun assembly of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, there is shown a beam tester apparatus l, for practicing the method of the present invention. The gun tester 1 includes a typical electron gun assembly 2 having a spherical concave thermionic cathode emitter 3, as of oxide coated nickel, impregnated tungsten, or other conventional emitter material, which is heated to operating temperature via heater 4 to produce a stream of electrons 5 passable through the central aperture 6 in an anode structure 7. A cylindrical high voltage insulator structure 8 forms a portion of the vacuum envelope for the gun 2, is sealed at one end to the anode 7, and includes suitable feedthrough connector assemblies at the opposite end thereof, not shown, for applying the operating potentials to the elements within the gun. A grid structure 9, having a concave surface conforming to the radius of curvature of the cathode emitter 3, is disposed in front of the emitter 3 for controlling the beam current.

The anode electrode 7 includes an annular disc portion 11 made of a magnetic permeable material such as magnetic stainless steel to form the anode pole piece of a magnetic beam focusing structure. A conductive insert 12, as of copper, is inserted into the aperture in the center of the pole piece 11 to form the proper shape of the electrostatic field i the region between the cathode and the anode while permittiiig the pole piece aperture to be shaped to produce the proper magnetic field shaping through the anode.

A magnetically confined glow discharge getter ion vacuum pump 13 is affixed to the downstream side of the anode pole piece 11 via a circular pipe-coupling flange 14. The vacuum pump 13 includes a tubular main body 15 disposed in axial alignment with the beam path for passage of the beam I therethrough. A pumping chamber 16 surrounds the tube 15 and contains therewithin a plurality of Penning anode cells disposed between a pair of cathode plates of getter material. Pumps of this general type are described in U.S. Pat. No. 3,125,283. A horseshoe-shaped magnet 17, shown in phantom lines, is disposed with its north and south pole straddling the pumping chamber 16 to produce an axially directed magnetic field through the anode glow discharge passageways therein for confining the glow discharge therewithin. Although only one magnet 17 appears in FIG. 1, a plurality of such magnets are normally spaced around the pump chamber as shown in said U.S. Pat. No. 3,125,283. The magnet 17 is not required when the beam 5 is magnetically focused by means of a magnetic beam focus solenoid 18 shown in dotted lines. Alternatively, the beam may be focused by periodic electrostatic or magnetic lenses in which case the pump 13 will require its own magnet 17.

A bakeable straight through high vacuum valve 19, such as Varian Associates Model No. 951-5052, is affixed to the vacuum pump 13 via a pair of mating bakeable

high vacuum flanges

20 and 21 affixed to the vacuum pump and straight through valve, respectively. The straight-through valve 19 includes a straight-through tubular body portion 22 coaxially aligned with the beam path 5 for passage of the beam through the valve when the valve 19 is in the open position. The other end of the valve 19 is sealed, in a vacuum-tight manner, to a disc-shaped collector pole piece 23, as of soft iron, via the intermediary of a demountable high-vacuum flange assembly 24. The flange assembly 24 includes a first flange member 25 carried at the end of the valve body 22. Flange 25 mates with a flange member 26 sealed to the collector pole piece 23. The collector pole piece 23 is centrally apertured at 27 for passage of the beam therethrough into a hollow collector structure 28 which is sealed in a vacuum-tight manner to the collector pole piece 23 via a cylindrical insulator 29. The beam collector 28 is provided with water coolant channels, not shown, for dissipating the heat generated in the collector 28, in the conventional manner.

In use, the electron gun assembly is assembled with the cathode emitter 3 disposed within a composite vacuum envelope structure defined by the envelope of the gun, pump 13, valve 19, and beam collector 28. A suitable oven is placed around the assembled components and the vacuum envelope, including the gun, is baked at 450 to outgas the various components within the vacuum system. During the baking cycle, the system is evacuated to a relatively low pressure by means of vacuum pump 13 and the thermionic cathode emitter 3 is processed in the conventional manner to activate the cathode emitter. During the bakeout cycle, the straight-through valve 19 is in the open position to allow gas communication throughout the entire vacuum envelope. After completion of the bakeout and cathode processing steps, the magnet 17 is removed from the pump 13 and the beam focus solenoid is placed in position as indicated in Fit]. 1 to produce a beamfocusing magnetic field through the beam passageway between the gun 2 and the collector 28. The beam-focusing magnetic field also supplies the magnetic field for the getter ion vacuum pump 13. Beam voltage is applied to the gun and the gun 2 is put into operation for testing the beam. The beam current is measured via conventional techniques, i.e., by measuring the current collected by the collector to assure that the gun 2 is operating properly and that it meets design specifications.

Upon completion of the gun-testing step, the straightthrough vacuum valve 19 is closed, thereby valving off the electron gun 2, pump 13, and the closed-off portion of the valve 19. The gun 2, pump 13 and valve 19 are then detached from the remaining portion of the vacuum envelope structure by disassembling the demountable flange 24. The electron gun 2 with the dependent pump and valve 19 are then removed from the gun tester 1 for storage and subsequent use in a second evacuated system, such as a particle accelerator. Periodically, the vacuum pressure within the detached gun can be monitored by placing magnet 17 around the pump 13 and energizing the vacuum pump. The current drawn by the pump is a measure of the gas pressure within the gun and can be utilized to maintain the pressure within the gun at a very low pressure, as of IO torr.

Referring now to FIG. 2, there is shown a microwave linear particle accelerator structure 31. The accelerator 31 includes a first buncher section 32 comprising a disc-loaded waveguide, of conventional design, excited with microwave energy derived from a klystron oscillator 33 via a waveguide 34. The electron gun 2, pump 13 and valve 19 are assembled to the buncher 32 via a demountable flange 24. A series of

accelerator sections

35, 36 and 37 are serially disposed along the beam path of the accelerator and are excited via suitable sources of microwave energy such as

klystrons

38, 39 and 41 via

waveguides

42,43 and 44, respectively. The accelerator structure including the buncher and accelerator sections are evacuated to a suitable low pressure as of torr via a conventional high-vacuum pump such as a diffusion pump, not shown.

When the accelerator structure has been evacuated, the straight-through valve 19 is opened such that the electron gun 2, in operation, can project its beam axially through the pump 13, valve 19 and into the buncher section 32 for acceleration by the accelerator section to relatively high particle velocities. The high-velocity beam is projected through a suitable particle permeable window structure 45 disposed at the output end of the accelerator. A beam focus solenoid structure 46 surrounds the accelerator for focusing the beam therethrough and includes a pole piece structure 47 at the gun end of the tube which is fitted to the anode pole piece 11, in the manner as indicated in FIG. 1. The beam focus solenoid 46 also pro vides the axial magnetic field for the vacuum pump 13 which may be connected to a suitable power source for maintaining and monitoring the vacuum within the gun 2 and accelerator 31. When the accelerator 31 is to be shut down or to stand for substantial periods without operation, the high-vacuum straight-through valve 19 is preferably closed to prevent contamination of the thermionic cathode emitter 3 in the event a leak develops in the accelerator section or in case work is done on the accelerator. As an alternative to solenoid 46 for focusing the beam of the accelerator periodic magnetic or electrostatic lenses may be employed.

The advantage of the processed, evacuated and demountable electron gun assembly is that the gun may be tested to assure that it meets design specifications and then these specifcations can be preserved while the gun is in storage prior to its being affixed to a suitable utilization device such as a linear accelerator. This offers many advantages since, prior to this time, the electron guns were assembled in a preprocessed state onto the accelerator and were processed in place on the accelerator structure. Leaks often developed in the accelerator and contaminants such as oil from an oil-diffusion pump often contaminated the cathode 3 poisoning same such that it would not meet design specifications. in such a case, it was difficult to tell whether the cathode failed because it was defective when shipped or because it was contaminated in the processing stage when attached to the utilization device. Use of the preprocessed, evacuated and demountable electron gun greatly reduces the failure of the guns when applied to various utilization devices.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What I claim is:

1. In a method for manufacture and preservation of an electron gun assembly the steps of, assembling an electron gun having a thermionic cathode emitter in an envelope structure, evacuating the envelope, processing the cathode to activate same, applying beam voltage to the gun to draw beam current from the gun to determine proper operation thereof, valving off a portion of the envelope containing the electron gun by means of a valve in a portion of the envelope, and detaching the valved-off and still evacuated portion of the envelope which contains the valve and processed electron gun from the remainder of the vacuum envelope structure for preserving the electron gun in an operable condition for subsequent use in a second vacuum envelope structure.

2. The method of claim 1 including the step of, connecting a glow discharge getter ion vacuum pump into that portion of the vacuum envelope structure to be detached, and periodically activating the pump to monitor the gas pressure within the detached and valved-offelectron gun.

3. The method ofclaim 1 including the step of attaching the evacuated and processed electron gun to the vacuum envelope structure of a particle accelerator, evacuating the particle accelerator envelope structure, and opening the valve in the attached envelope portion containing the electron gun for placing the processed electron gun in gas communication with the remaining portion of the vacuum envelope of the particle accelerator.

4. The method ofclaim 1 wherein the step ofprocessing the electron gun to activate the cathode emitter includes the step of baking the electron gun and valve at a temperature in excess of 400C.

Claims

2. The method of claim 1 including the step of, connecting a glow discharge getter ion vacuum pump into that portion of the vacuum envelope structure to be detached, and periodically activating the pump to monitor the gas pressure within the detached and valved-off electron gun.

3. The method of claim 1 including the step of attaching the evacuated and processed electron gun to the vacuum envelope structure of a particle accelerator, evacuating the particle accelerator envelope structure, and opening the valve in the attached envelope portion containing the electron gun for placing the processed eleCtron gun in gas communication with the remaining portion of the vacuum envelope of the particle accelerator.

4. The method of claim 1 wherein the step of processing the electron gun to activate the cathode emitter includes the step of baking the electron gun and valve at a temperature in excess of 400*C.