Plasma electron source for cold-cathode discharge device or the like

• US 4,458,180 A
• Filed: 02/18/1982
• Issued: 07/03/1984
• Est. Priority Date: 02/18/1982
• Status: Expired due to Term

• Current Assignee: Picker Medical Systems, Ltd. (Koninklijke Philips N.V.)
• Original Assignee: Elscint Limited (Elbit Imaging Limited)
• Inventors: Sohval, A. Robert, Cooperstein, Gerald, Fleischer, David, Hearn, David R., Goldstein, Shyke A.
• Primary Examiner(s): LAROCHE, EUGENE
• Application Number:
  US06/349,766
• Time in Patent Office: 866 Days
• Field of Search: 315/111.21, 315/111.31, 315/111.81, 313/230, 313/231.41, 313/359, 313/363, 378/122, 378/134, 250/426
• 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 35/00 X-ray tubes
• CPC Class Codes: H01J 37/32009 Arrangements for generation of plasma specially adapted for examination or treatment of objects, e.g. plasma sources plasma generation in general H05H1/24

Abstract

A plasma electron source has an apertured cathode, and a housing for defining a cavity behind the aperture. A trigger electrode, in communication with the cavity, is responsive to a short-duration trigger pulse for establishing plasma in the cavity. The plasma is sustained in the cavity subsequent to the termination of the trigger pulse by a bias circuit, which biases the cavity at a relatively low voltage with respect to the cathode for a period of time much longer than the duration of the trigger pulse.

Claims

1. A plasma electron source for a vacuum device comprising:

(a) a cathode having aperture means;

(b) a housing for defining a cavity behind said aperture means;

(c) a trigger electrode in communication with the cavity, and responsive to a short-duration trigger pulse for establishing plasma in the cavity; and

(d) means for confining the plasma to the cavity.

2. A plasma electron source according to claim 1 including means for sustaining the plasma in the cavity subsequent to the termination of the trigger pulse.

3. A plasma electron source according to claim 2 wherein the means for sustaining the plasma includes means for driving current through the plasma for a period of time relatively long as compared to the duration of the trigger pulse.

4. A plasma electron source according to claim 3 including an intermediate electrode disposed between and electrically insulated from both the cathode and the trigger electrode, and wherein the means for sustaining the plasma includes a bias circuit for biasing the intermediate electrode relative to the cathode.

5. A plasma electron source according to claim 4 wherein the bias circuit includes a pulse-forming network that biases the intermediate electrode at a negative voltage with respect to the cathode.

6. A plasma electron source according to claim 5 wherein the bias relative to the cathode is in excess of 100 volts.

7. A plasma electron source according to claim 6 wherein the duration of the trigger pulse is of the order of 1 microsecond.

8. A plasma electron source according to claim 7 wherein the duration of the bias voltage is of the order of 50 microseconds.

9. A plasma electron source according to claim 3 wherein the means for driving current through the plasma includes means for rapidly and repeatedly applying trigger pulses to the trigger electrode.

10. A plasma electron source according to claim 4 wherein the trigger electrode is associated with an insulator that has a surface separating the trigger electrode from the intermediate electrode, and across which surface flashover takes place when a trigger pulse is applied to the trigger electrode for generating a plasma in the cavity.

11. A plasma electron source according to claim 10 wherein the cavity is defined, at least in part, by a cylindrical conductive member that constitutes the intermediate electrode, the trigger electrode being a rod that is coaxial with the cylindrical conductive member.

12. A plasma electron source according to claim 11 wherein the cavity is defined, at least in part, by a cylindrical non-conductive member interposed between the intermediate electrode and the cathode.

13. A plasma electron source according to claim 11 wherein the cavity is defined, at least in part, by a conductive cup-shaped member electrically isolated from the cathode and interposed between the non-conductive member and the cathode.

14. A plasma electron source according to claim 10 wherein the trigger electrode has an enlarged head that tapers toward the insulator.

15. A plasma electron source according to claim 10 wherein the head of the trigger electrode sits in a countersunk recess in the intermediate electrode.

16. A plasma electron source according to claim 15 wherein the countersunk recess has a conical surface that tapers towards the insulator.

17. A plasma electron source according to claim 10 wherein each of the trigger and intermediate electrodes have free ends that engage said surface on said insulator.

18. A plasma electron source according to claim 17 wherein the cathode defines a surface, and said surface on said insulator is non-parallel to the surface defined by the cathode.

19. A plasma electron source according to claim 18 wherein said surface on said insulator is substantially perpendicular to the surface defined by the cathode.

20. A plasma electron source according to claim 19 wherein the free ends of the trigger and intermediate electrodes define a pair of opposite faces, the spacing between which increases in a direction away from said surface on said insulator.

21. A plasma electron source according to claim 20 including an auxiliary electrode in the cavity, and wherein the intermediate electrode has a curved portion adjacent the auxiliary electrode, one of the last-mentioned electrodes being electrically connected to said bias circuit whereby the arc produced in the cavity subsequent to flashover stabilizes in the space between said curved portion and said auxiliary electrode.

22. A plasma electron source according to claim 21 wherein the bias circuit is connected to the intermediate electrode.

23. A plasma electron source according to claim 21 wherein the bias circuit is connected to the auxiliary electrode.

24. A plasma electron source according to claim 21 wherein the auxiliary electrode has a plurality of components connected to respective ones of a plurality of bias circuits that are selectively actuatable.

25. A plasma electron source according to claim 21 wherein the auxiliary electrode and the curved portion of the intermediate electrode are constructed and arranged so that the stabilized arc is oriented, generally, in a direction parallel to said surface on the cathode.

26. A plasma electron source according to claim 25 wherein the aperture means in the cathode overlies said space between said curved portion and said auxiliary electrode.

27. A plasma electron source according to claim 4 wherein the secondary of a pulse transformer is coupled to the trigger electrode.

28. A plasma electron source according to claim 4 wherein the bias circuit includes an active element having a control terminal for controlling the state of the elements, the active element being connected between the intermediate electrode and a negative voltage for applying the latter to the intermediate electrode when a control signal is applied to the control terminal.

29. A plasma electron source according to claim 4 wherein a capacitor is coupled to the trigger electrode.

30. A plasma electron source according to claim 5 wherein the pulse-forming network produces a pulse whose amplitude varies with time.

31. A plasma electron source according to claim 5 wherein the pulse-forming network produces a pulse of uniform amplitude.

32. A high-voltage switch comprising the plasma electron source of either claim 1 or claim 2 in combination with an anode spaced from the front surface of the cathode of said source, and means for applying a high positive voltage to the anode relative to the cathode, whereby electrons in the plasma in the cavity are drawn to the anode through the aperture therein.

33. A plasma electron source according to claim 1 wherein the means for confining the plasma to the cavity is a baffle located in the cavity in the vicinity of the aperture means.

34. A plasma electron source according to claim 33 wherein the projected area of the baffle on the cathode comprehends an area larger than the area of the aperture means.

35. A plasma electron source according to claim 1 wherein the aperture means includes a plurality of apertures inclined relative to the front surface of the cathode.