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

• US 4,570,106 A
• Filed: 03/20/1984
• Issued: 02/11/1986
• 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, Hearn, David R., Goldstein, Shyke A., Fleischer, David
• Primary Examiner(s): Boudreau, Leo H.
• Assistant Examiner(s): DeLuca, Vincent
• Application Number:
  US06/591,453
• Time in Patent Office: 693 Days
• Field of Search: 315/111.81, 315/111.21, 315/111.31, 313/230, 313/231.41, 313/359.1, 313/363.1, 378/122, 378/134, 250/426
• US Class Current: 315/111.81
• CPC Class Codes: H01J 27/022 Details
• 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. An X-ray source including a plasma electron source for a vacuum device comprising:

(a) a cathode having a front surface and a rear surface;

(b) anode means spaced apart from the front surface of the cathode;

(c) aperture means in said cathode between said front surface and said rear surface;

(d) a housing defining a cavity behind said aperture means;

(e) means for establishing plasma in the cavity;

(f) means for applying a positive voltage to the anode means sufficient to generate X-rays when electrons in said plasma are drawn through the aperture means in the cathode and impact on the anode means; and

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

2. An X-ray source according to claim 1, wherein the aperture means in the cathode constitutes a single aperture.

3. An X-ray source according to claim 1 wherein the aperture means in the cathode constitutes a plurality of apertures.

4. An X-ray source according to claim 1 including means for focusing electrons that are drawn through the aperture means of the cathode onto the anode means.

5. An X-ray source according to claim 4 wherein the means for focusing operates to create a potential well in the space between the means and the cathode in the vicinity of the aperture means.

6. An X-ray source according to claim 4 wherein the means for focusing includes a conductor on the surface of the cathode in the vicinity of the aperture means.

7. An X-ray source according to claim 4 wherein the means for focusing includes a non-conductor associated with said aperture means.

8. An X-ray source according to claim 1 wherein said establishing means comprises a trigger electrode in communication with the cavity, and responsive to a short-duration trigger pulse for establishing plasma in the cavity.

9. An X-ray source according to claim 8 including means for sustaining the plasma in the cavity subsequent to the termination of the trigger pulse.

10. An X-ray source according to claim 9 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.

11. An X-ray source according to claim 10 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.

12. An X-ray source according to claim 11 wherein the bias circuit includes a pulse-forming network that biases the intermediate electrode at a negative voltage with respect to the cathode.

13. An X-ray source according to claim 12 wherein the bias relative to the cathode is in excess of 100 volts.

14. An X-ray source according to claim 13 wherein the duration of the trigger pulse is of the order of 1 microsecond.

15. An X-ray source according to claim 14 wherein the duration of the bias voltage is of the order of 50 microseconds.

16. An X-ray source according to claim 10 wherein the means for driving current through the plasma includes means for rapidly and repeatedly applying trigger pulses to said trigger electrode.

17. An X-ray source according to claim 11 wherein said 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 said trigger electrode for generating a plasma in the cavity.

18. An X-ray source according to claim 17 wherein the cavity is defined, at least in part, by a cylindrical conductive member that constitutes the intermediate electrode, said trigger electrode being a rod that is coaxial with the cylindrical conductive member.

19. An X-ray source according to claim 18 wherein the cavity is defined, at least in part, by a cylindrical non-conductive member interposed between the intermediate electrode and the cathode.

20. An X-ray source according to claim 18 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.

21. An X-ray source according to claim 17 wherein said trigger electrode has an enlarged head that tapers toward the insulator.

22. An X-ray source according to claim 17 wherein the head of the trigger electrode sits in a countersunk recess in the intermediate electrode.

23. An X-ray source according to claim 22 wherein the countersunk recess has a conical surface that tapers towards the insulator.

24. An X-ray source according to claim 8 wherein the means for confining the plasma to the cavity is a baffle located in the cavity in the vicinity of the aperture means.

25. An X-ray source according to claim 24 wherein the projected area of the baffle on the cathode comprehends an area larger than the area of the aperture means.

26. An X-ray source according to claim 8 wherein the aperture means includes a plurality of apertures inclined relative to the front surface of the cathode.

27. An X-ray source according to claim 11 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.

28. An X-ray source according to claim 12 wherein the pulse-forming network produces a pulse of uniform amplitude.

29. An X-ray source according to claim 12 wherein the pulse-forming network produces a pulse whose amplitude varies with time.

30. An X-ray source according to claim 11 wherein the secondary of a pulse transformer is coupled to said trigger electrode.

31. An X-ray source according to claim 11 wherein a capacitor is coupled to said trigger electrode.

32. An X-ray source according to claim 17 wherein each of the electrodes have free ends that engage said surface on said insulator.

33. An X-ray source according to claim 32 wherein the cathode defines a surface, and said surface on said insulator is non-parallel to the surface defined by the cathode.

34. An X-ray source according to claim 33 wherein said surface on said insulator is substantially perpendicular to the surface defined by the cathode.

35. An X-ray source according to claim 34 wherein the free ends of the trigger electrode and the intermediate electrode define a pair of opposite faces, the spacing between which increases in a direction away from said surface on said insulator.

36. An X-ray source according to claim 35 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.

37. An X-ray source according to claim 36 wherein the bias circuit is connected to the intermediate electrode.

38. An X-ray source according to claim 36 wherein the bias circuit is connected to the auxiliary electrode.

39. An X-ray source according to claim 36 wherein the auxiliary electrode has a plurality of components connected to respective ones of a plurality of bias circuits that are selectively actuatable.

40. An X-ray source according to claim 36 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.

41. An X-ray source according to claim 40 wherein the aperture means in the cathode overlies said space between said curved portion and said auxiliary electrode.

42. A CT scanner comprising a plurality of X-ray sources according to claim 1 mounted on a source ring whose axis defines the axis of the scanner, and a plurality of detectors mounted on a rotatable detector ring with a diameter smaller than the source ring for receiving X-rays produced by the sources, the detector ring having an axis colinear with the axis of the source ring.