Electron source

Abstract

An electron source includes a plurality of electron emission cathodes and at least one control electrode. A gate current regulator is provided for regulation of current flowing through the at least one control electrode.

Claims

1. A system for regulating an electron source, the system comprising: a plurality of electron emission cathodes operable to release a current; at least one control electrode; a gate current regulator configured for regulation of a gate current flowing through the at least one control electrode via a voltage difference between the at least one control electrode and an electron emission cathode of the plurality of electron emission cathodes; and a gate current measuring unit in a control loop including the gate current regulator, the gate current measuring unit operable to measure the gate current, the gate current being a portion of the current released by the plurality of electron emission cathodes, wherein an electron current of at least one electron emission cathode of the plurality of electron emission cathodes is proportional to the gate current.

2. The system as claimed in claim 1, wherein the gate current measuring unit has a substraction circuit in the control loop.

3. The system as claimed in claim 1, wherein the gate current-measuring unit comprises paired opto-couplers.

4. The system as claimed in claim 1, wherein the gate current-measuring unit comprises a shunt, an analog-to-digital converter, and an opto-coupler.

5. The system as claimed in claim 1, wherein the plurality of electron emission cathodes is configured as field emitters or indirectly heated emitters.

6. The system as claimed in claim 5, wherein the plurality of electron emission cathodes comprises carbon nanotubes or graphene or is configured as dispenser cathodes.

7. The system as claimed in claim 2, wherein the plurality of electron emission cathodes is configured as field emitters or indirectly heated emitters.

8. The system as claimed in claim 3, wherein the plurality of electron emission cathodes is configured as field emitters or indirectly heated emitters.

9. The system as claimed in claim 4, wherein the plurality of electron emission cathodes is configured as field emitters or indirectly heated emitters.

10. A method for the operation of an electron source, the method comprising: emitting electrons with a plurality of electron emission cathodes, wherein a voltage is applied between the plurality of electron emission cathodes and a control electrode; regulating, with a gate current regulator, gate current flowing through the control electrode via the voltage applied between an electron emission cathode of the plurality of electron emission cathodes and the control electrode; and measuring the gate current with a gate current measuring unit in a control loop including the gate current regulator, the gate current comprising a portion of the electrons emitted by the plurality of electron emission cathodes, wherein an electron current of at least one electron emission cathode of the plurality of electron emission cathodes is proportional to the gate current.

11. The method as claimed in claim 10, wherein the plurality of electron emission cathodes is operated is a pulsed manner, with pulse times under 1 ms.

12. The method as claimed in claim 10, further comprising determining a transmission rate of the electron source, wherein the determining comprises subtracting the gate current flowing through the control electrode from a cathode current flowing through the plurality of electron emission cathodes.

13. The method as claimed in claim 12, wherein a change in the transmission rate is incorporated into the regulation of the gate current.

14. The method as claimed in claim 11, wherein the pulse times are under 0.1 ms.

15. The method as claimed in claim 11, further comprising determining a transmission rate of the electron source, wherein the determining comprises subtracting the gate current flowing through the control electrode from a cathode current flowing through the plurality of electron emission cathodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows one embodiment of an electron source with gate current regulation,

(2) FIGS. 2-4 show variants of a gate current measuring unit for the electron source according to FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

(3) FIG. 1 shows one embodiment of an X-ray device that includes an X-ray tube 2 and a control unit 3. The X-ray device is identified overall with the reference character 1.

(4) The X-ray tube 2 includes a tube unit 4 that includes a plurality of electron emission cathodes 5, a control grid 6 (e.g., a control electrode), and an anode 7. With respect to the basic function of the tube unit 4 (e.g., actual tubes of the X-ray device 1), attention is drawn to the prior art cited in the introduction, and to DE 10 2009 011 642 A1.

(5) In one embodiment, the plurality of electron emission cathodes 5 is configured as field emitters and emits electrons using field emission. A voltage of up to 5 kV is applied between the plurality of electron emission cathodes 5 and the control grid 6 using a grid voltage supply 8. The arrangement including the plurality of electron emission cathodes 5, the control grid 6, and the grid voltage supply 8 is configured as an electron source 9. Individual electron emission cathodes 5 or groups of electron emission cathodes 5 of the plurality of electron emission cathodes may be controlled separately, so that geometrical parameters of the electron source 9 and thus also the generated X-ray radiation may be changed without changing the arrangement of the electron source 9 (e.g., through shifting of the electron source 9).

(6) X-ray radiation is generated in the tube unit 4 in that electrons emitted by the electron source 9 are accelerated using high voltage that may be in the order of 20 kV to 180 kV. The high voltage is generated by a high voltage supply unit 10 and applied between the plurality of electron emission cathodes 5 and the anode 7. The high voltage arrives at the anode 7. Electron current released from the plurality of electron emission cathodes 5 (e.g., cathode current) divides into two partial currents:

(7) A first partial current (e.g., gate current (IG)) flows out via the control grid 6; a second partial current reaches the anode 7 in order to generate X-ray radiation at the anode 7. The second partial current is designated an anode current (IA). The relationship between the anode current (IA) and the cathode current (IK) is defined as a transmission rate (TR) of the X-ray tube 2. The following relationship exists between the gate current (IG), the anode current (IA), and the transmission rate (TR):
IG=IA(1TR)/TR

(8) A gate current measuring unit 11 is provided for measuring the gate current, which, as shown in simplified form in FIG. 1, includes, for example, a shunt 12 and an operational amplifier 13. An auxiliary power supply for the gate current measuring unit 11 may be integrated into the grid voltage supply 8. The gate current measuring unit 11 is part of a control loop, which further includes a gate current regulator 14 and a number of actuators 15 that are each assigned to an electron emission cathode 5 of the plurality of electron emission cathodes 5. The gate current is regulated at the control loop via a voltage difference between gate (6) and cathode (5), according to the above formula.

(9) The gate current regulator 14 is connected to a microcontroller 16. The microcontroller 16, among other functions, processes set values for radiation parameters that are stored in a memory 17. A gate current supply 18 implemented in the microcontroller 16, which prescribes a nominal value of the gate current (e.g., may be calculated according to the formula above), interacts with an anode current readjustment unit 19 (e.g., likewise realized in the microcontroller 16) in order to prescribe a set value of the gate current (G.sub.soll) for the gate current regulator 14. An actual value of the gate current is accordingly designated G.sub.ist. The actual value of the cathode current IC.sub.ist processed by the anode current readjustment unit 19 is measured with the aid of a shunt 20 and digitized by an analog-to-digital converter 21. A further analog-to-digital converter 22, which interacts with the anode current readjustment unit 19, is provided for digitization of the actual value of the gate current G.sub.ist. The analog-to-digital converter 21 and the further analog-to-digital converter 22 may be integrated into the anode current readjustment unit 19. The anode current readjustment unit 19, for example, takes account of long-term, creeping changes in the transmission rate of the X-ray tube 2.

(10) The microcontroller 16 enables a targeted selection of, for example, up to several hundred electron emission cathodes 5 and interacts with a contactor 24 via a control line 23. The contactor 24 is connected between the gate current regulator 14 and the actuator 15. Each electron emission cathode 5 of the plurality of electron emission cathodes 5 is connected to the associated actuator 15 via a cathode line 25 and a vacuum feedthrough 26. Cathode-side parasitic capacitances are designated C.sub.par, and corresponding currents are designated with I.sub.Kap. The regulation of the gate current using the gate current measuring unit 11, the gate current regulator 14 and the actuators 15 is not influenced by the parasitic capacitances C.sub.par; the actual value of the gate current IG.sub.ist is measured without falsification.

(11) Different possible ways of configuring the gate current measuring unit 11 for a precise, rapid measurement of the gate current are represented in FIGS. 2 to 4.

(12) In the exemplary embodiment according to FIG. 2, the gate current measuring unit 11 includes a substraction circuit 27 for high voltages. The operational amplifier 13, which is also shown in simplified form in FIG. 1, is switched to high impedance using different resistances R1, R2. A measured voltage U.sub.S is proportional to the current flowing through the shunt 12 I.sub.Shunt.

(13) According to the embodiment shown in FIG. 3, paired opto-couplers 28, 29 are provided within the gate current-measuring unit 11. The opto-couplers function as measurement couplers or as reference couplers.

(14) In the variant according to FIG. 4, measurement of the gate current takes place via the shunt 12, the operational amplifier 13, and a fast analog-to-digital converter 30 connected downstream of the shunt 12 and the operational amplifier 13. The fast analog-to-digital converter 30 supplies a digital signal. The supplied digital signal is fed to the gate current regulator 14 by an opto-coupler 31 or optical fiber.

(15) In each of the FIGS. 2 to 4, a line connected to the control grid 6, conducting the directly regulated gate current, is identified with the reference character 32. In each of the FIGS. 2 to 4, the gate current and thus, taking account of short- and long-term influences, the anode current may be precisely regulated from 70 s.

(16) While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.