Broad-beam electron source

• US 4,684,848 A
• Filed: 10/15/1985
• Issued: 08/04/1987
• Est. Priority Date: 09/26/1983
• Status: Expired due to Term

• Current Assignee: Kaufman & Robinson, Inc.
• Original Assignee: Kaufman & Robinson, Inc.
• Inventors: Kaufman, Harold R., Robinson, Raymond S.
• Primary Examiner(s): Chatmon, Saxfield
• Application Number:
  US06/787,665
• Time in Patent Office: 658 Days
• Field of Search: 315/111.21-111.91, 313/231.31, 313/231.41, 313/362.1
• US Class Current: 315/111.81
• CPC Class Codes: H01J 3/025 Electron guns using a discharge in a gas or a vapour as electron source gas-filled discharge tubes with gaseous cathodes H01J15/00
• CPC Class Codes: H01J 37/077 Electron guns using discharge in gases or vapours as electron sources

Abstract

A broad-beam electron source has a chamber into which is introduced an ionizing gas. Electrons are emitted between a cathode and an anode assembly to ionize that gas. The electrons within the plasma are drawn outwardly from the chamber through an apertured wall, which constitutes a screen, and thereafter are accelerated toward a target in a well-directed beam. A comparatively copious supply of electrons is developed, while yet requiring only low voltages in connection with its generation and resulting in correspondingly low electron energies. Ions produced external to the electron source itself are utilized to assist in neutralizing the charge density of the electron beam in order to help maintain its definition. For insulative targets, secondarily emitted electrons permit conservation of surface charge.

Claims

1. An electron source comprising:

a chamber;

means for introducing an ionizable gas into said chamber;

means for developing a plasma throughout a region within said chamber and the size of which region is large compared to the Debye length in said plasma;

and a selective means constituting a bounding wall of said chamber effectively defining a screen with an array of apertures transmissive of electrons produced from said plasma together with means, including the impressing of a potential on said screen, to enable broad extraction from said chamber of a defined broad beam of said produced electrons, while screening flow of ions from within said chamber, said selective means accelerating an effective plurality of electron beamlets which exhibit balanced electron-emlssive characteristics that coalesce into a defined beam.

2. An electron source as defined in claim 1 in which said developing means includes within said chamber an anode assembly and a cathode together with means for impressing a potential difference between said anode assembly and said cathode to effect emission of ionizing electrons from said cathode at a velocity sufficient to ionize said gas and thereby generate said plasma.

3. An electron source as defined in claim 1 in which said plasma is homogeneous throughout said chamber.

4. An electron source as defined in claim 1 in which said selective means includes an accelerating grid spaced beyond said screen from said chamber together with means for biasing said accelerating grid to cooperate with said screen and complete definition of said beam of produced electrons.

5. An electron source as defined in claim 1 in which said screen permits transmission through said apertures of sufficient positive ions from said plasma to inhibit mutual repulsion between electrons in said defined beam.

6. An electron source as defined in claim 1 in which the apertures in said screen effectively are sufficiently small to discourage the flow therethrough of positive ions from said plasma, while yet being sufficiently large to permit copious transmission therethrough of said produced electrons.

7. An electron source as defined in claim 1 in which the open area fraction in said screen is sufficiently small to maintain stability of emission from said cathode, while insuring uniformity of emission through said apertures.

8. An electron source as defined in claim 4 in which the acceleration of the produced electrons is subjected to a magnetic field, but the magnetic field integral crossed by the trajectories of said electrons is sufficiently small to inhabit any substantial deflection of produced electrons transmitted from said apertures.

9. An electron source as defined in claim 4 in which a decelerating grid is spaced further beyond said accelerating grid from said chamber, in which said decelerating grid is biased to repel background electrons otherwise penetrating the space between said source and a target.

10. An electron source as defined in claim 9 in which said accelerating grid is biased and formed to repel background positive ions which tend to escape from space between said source and a target through apertures in said grids.

11. An electron source as defined in claim 1 in which a mesh is disposed over each of said apertures in said screen, said mesh having openings which at least approximate the Debye shielding distance in order to unify the potential across each aperture to that of said screen.

12. An electron source as defined in claim 1 wherein said source is combined into a system in which said produced electrons are directed to a target, and in which said target is conductive and connected to a neutral potential level within said source.

13. An electron source as defined in claim 1 wherein said source is combined into a system in which said produced electrons are directed to a target, and in which said target is non-conductive but secondarily electron emissive to a degree sufficient to prevent excess electron accumulation on said target, so as to conserve surface charge.

14. An electron source as defined in claim 1 wherein said source is combined in a system in which said produced electrons are directed to a target, and in which said target is non-conductive but the surface charge on the target is conserved with low energy ions generated by said electrons directed to said target.

15. An electron source as defined in claim 1 in which the diameter of the apertures in said screen have a size which is less than approximately that of the Debye distance, thereby tending to exclude low-energy electrons from said defined beam and reducing energy spread in said beam.

16. An electron source as defined in claim 2 in which the potential on said screen, relative to said cathode, is adjustable to enhance collimation of said electrons or enable control of the level of electron current produced.

17. A method of developing a broad beam of electrons which comprises:

introducing an ionizable gas within a confined space;

creating ionization of said gas to produce a plasma that yields a copious supply of electrons along with positive ions, said plasma being created throughout a region the size of which is large compared to the Debye length in said plasma;

and extracting said electrons from said plasma as an effective plurality of beamlets of uniform electron-emission characteristics and which together define a broad-area beam thereof and directing said beam toward a target.

18. The method as defined in claim 17 which includes the inhibiting of the extraction of said ions from said plasma.

19. The method as defined in claim 17 in which ions also are extracted from said plasma.

20. The method as defined in claim 17 in which the extraction of said ions from said plasma is inhibited beyond significant utility in controlling the definition of said beam.

21. The method as defined in claim 17 which includes accelerating said electrons extracted from said plasma and thereby completing definition of said beam.

22. The method as defined in claim 21 which further includes decelerating said electrons following said accelerating thereof.

23. The method of producing a broad beam of low-energy electrons that comprises the use of:

a chamber,

means for introducing an ionizable gas into said chamber;

an anode assembly disposed in said chamber;

a cathode disposed in said chamber;

means for impressing a potential difference between said anode assembly and said cathode to effect emission of ionizing electrons from said cathode at a velocity sufficient to ionize said gas and thereby generate a plasma throughout a region within said chamber and the size of which is large compared to the Debye length in said plasma;

and a selective means constituting a bounding wall of said chamber effectively defining a screen with an array of apertures;

the new use comprising;

sizing said apertures sufficiently small to avoid significant disturbance to said plasma and maintain the existence of said plasma, while extracting said electrons into a defined beam with said defined beam being in the form of an effective plurality of electron beamlets which exhibit balanced electron-emission characteristics that coalesce into said defined beam;

and permitting the transmission of electrons copiously through said screen, while reducing the flow of ions therethrough.