US3509410A

US3509410A - Insulator shielded cathode

Info

Publication number: US3509410A
Application number: US667010A
Authority: US
Inventors: Conrad M Banas; Clyde O Brown
Original assignee: United Aircraft Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1967-09-11
Filing date: 1967-09-11
Publication date: 1970-04-28
Anticipated expiration: 1987-04-28

Description

April 28, 1970 c. M. BANAS ETAL 3,509,410

INSULATOR SHIELDED CATHODE Original Filed Nov. 3, 1965 'r Y a I l I Q CD i AHMP/ F, I T s l a l 5. t 0% I H I HIGH DvgLTAGE POWER SUPPLY INVENTORS CONRAD M. BANAS CLYDE 0. BROWN ATTORN EY United States Patent 3,509,410 INSULATOR SHIELDED CATHODE Conrad M. Banas, Manchester, and Clyde 0. Brown,

Newington, Coun., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Continuation of application Ser. No. 506,237, Nov. 3, 1965. This application Sept. 11, 1967, Ser. No. 667,010 Int. Cl. H01j 61/04, 61/10 U.S. Cl. 313207 6 Claims ABSTRACT OF THE DISCLOSURE A cathode operating in a glow discharge for producing an electron beam is provided with an insulator shield for selectively suppressing the flow of electrons from those areas covered by the shield.

This application is a continuation of application Ser. No. 506,237, filed Nov. 3, 1965.

Conventional methods of producing electron beams involve liberation of electrons from the surface of a heated cathode by thermionic emission. Recently, focussed electron beams have been produced from a hollow cathode operated in a glow discharge mode. The presence of a plasma near the external surfaces of the cathode has been found to be detrimental to efiicient operation since most of the energy supplied to maintain this plasma is dissipated at the cathode. Substantial improvement in the beam power efficiency of such cathodes can be obtained by shielding the external cathode surfaces in such a manner as to eliminate the glow discharge from these regions.

Shields for these cathode discharge devices have been made of conductive materials and found to improve beam power efficiency. However, it has been noted that such shielding significantly reduces the ambient pressure range for stable electron beam mode operation of the discharge. Further, this shielding technique has been found to lead to sporadic arcing between cathode and shield. These disadvantages may be obviated and the electron beam mode operation of a glow discharge cathode may be substantially improved by shielding it with an insulator material.

It is accordingly an object of this invention to provide an insulator shield for a cathode operating in the glow discharge mode.

This and other objects of this invention will become more readily apparent upon a review of the description and the accompanying figure.

In this invention a cathode operating in the glow discharge mode is surrounded by an insulator shield which is spaced from the cathode surfaces at a predetermined distance to essentially eliminate arc formation between the cathode and the shield, and to exclude the discharge plasma from the region adjacent to the external surfaces of the cathode.

In the figure, a conventional hollow cathode 1 is shown which is enclosed in a chamber. This cathode is well known and may be of the type disclosed in U.S. Patent 3,381,157. The hollow cathode assembly in the figure is made up of two cylindrical sections 2, a solid shaft which serves as the high voltage lead and support, and 1 the hollow cathode. The hollow cathode 1 may be removably mounted to the solid portion 2 by means of a banana plug arrangement or by means of a threaded stud attached to the hollow cathode section 1 and extending into the solid section 2. The solid section 2, which is also conductive, is connected to a conductive stud 3 either by a threaded arrangement similar to that used for section 1 or forms a continuous conductive body therewith. The stud 3 extends through a high voltage bushing 4 and a hermetic seal is formed between the stud 3 and the bushing 4 by brazing at 5.

The cathode 1 is exposed to an evacuated chamber 6 in which is disposed an anode 7 at a suitable axial distance from the cathode 1. The anode 7 may also be the workpiece. The cathode 1 is also provided with an aperture 8 which has the proper dimensions necessary to provide the hollow cathode discharge. The vacuum chamber 6 is evacuated to the proper pressure needed to support the glow discharge. An inert gas atmosphere is customarily used to reduce contamination in metal-working applications. A high voltage supply 12 biases the cathode negative with respect to the anode 7.

Surrounding the cathode 1 is an insulator shield 9 which is spaced from the cathode 1 by gap 10. The gap 10 is determined as a function of the pressure and voltage operation of the cathode in accordance with wellknown gas discharge scaling relationships. The insulator is also shown to terminate a short distance 11 from the face of the cathode to enhance the efiiciency of this device. The gap 10 extends all along the body of the cathode 1 and support shaft 2.

The insulator shield 9 effectively inhibits the formation of a glow discharge in the gap 10 which is sized to suppress the formation of the plasma therein. The benefit of a shield formed from an insulating material arises from a reduction of the high potential gradients in the gap 10 and at the junction between the high voltage lead 3 and insulator 9 as compared with those obtainable with conductive shields. Reduction of the potential gradients alleviates high voltage breakdown problems. Consequently, even though the plasma which is essentially at anode potential and may partially extend within the gap 10, there is a marked reduction in high voltage arcs between the cathode and the shield when an insulator material is used.

Cathodes of solid wall construction may be effectively used with an insulating shield. Such cathodes are much simpler and easier to construct than screen cathodes and may operate with beam power efiiciencies in excess of Typical refractory materials from which the shield 9 may be made are alumina, zirconia, boron nitride, or beryllia. These shield materials are specified on the basis of their high dielectric strength at elevated temperatures. Due to radiant heating by the cathode 1, portions of the shield may approach temperatures of the order of 1500 K. under high power level conditions. For low beam power applications, a nonrefractory insulator shield could be used with advantage. 7

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described but may be used in other ways without departture from its spirit as defined by the following claims.

We claim:

1. In an electron beam apparatus,

an enclosure containing a relatively low pressure ion- 1 izable gaseous medium,

a cathode positioned Within said enclosure and comprising a hollow electrically conductive structure having at least one beam exit aperture,

an anode positioned within said enclosure,

means for applying potentials to said anode and said cathode whereby a plasma may be generated within said cathode to effect a nonthermionic electron beam mode of operation of said cathode whereby an electron beam issues from the plasma through said beam exit aperture, and

an insulating shield substantially surrounding said cathode and spaced at a short distance therefrom for suppressing any glow discharge exterior to said cathode.

2. An electron beam apparatus as in claim 1 in which said cathode is cylindrical and in which said aperture is formed in a base of said cylindrical cathode,

said insulating shield extending fully aboutsaid cathode and terminating a short distance from the apertured base of said cathode.

3. An electron beam apparatus as in claim 1 and including support means for said cathode extending from a wall of said enclosure, said insulating shield surrounding said support means.

4. An electron beam apparatus as in claim 1 in which said insulating shield is spaced from said cathode at a distance which suppresses formation of a plasma therebetween.

5. An electron beam apparatus as in claim 1 in which said insulator shield comprises a material having a high dielectric strength at elevated temperatures.

UNITED STATES PATENTS 2,112,718 3/1938 Somers 313-205 X 2,238,277 4/1941 Miller 313205 2,433,813 12/1947 Hilliard 3l3-219 X 3,242,371 3/1966 Sugawara et al. 313219 X 3,320,475 5/1967 Boring 315108 ROBERT SEGAL, Primary Examiner U.S. Cl. X.R. 313205, 339