ION BEAM EXTRACTION APPARATUS AND METHOD FOR CREATING AN ION BEAM

Abstract

An ion beam extraction apparatus (100), being configured for creating an ion beam (1), in particular adapted for a neutral beam injection apparatus of a fusion plasma plant, comprises an ion source device (10) being arranged for creating ions, and a grid device (20) comprising at least two grids (21, 22) being arranged adjacent to the ion source device (10) and having a mutual grid distance d along a beam axis z, wherein the grids (21, 22) are electrically insulated relative to each other, the grids (21, 22) are arranged for applying different electrical potentials for creating an ion extraction and acceleration field (3) along the beam axis z, and he ion source device (10) and the grid device (20) are arranged in an evacuable ion beam space (30) extending along the beam axis z, wherein at least one of the grids is a movable grid (21), which can be shifted along the beam axis z, and the grid device (20) is coupled with a grid drive device (40) having a drive motor (41), which is arranged for moving the movable grid (21) along the beam axis z and setting the grid distance d between the movable grid (21) and another one of the grids (21, 22). Furthermore, applications of the ion beam extraction apparatus and a method of creating an ion beam along a beam axis z are disclosed.

Claims

1. An ion beam extraction apparatus, being configured for creating an ion beam, comprising an ion source device being arranged for creating ions, and a grid device comprising at least two grids being arranged adjacent to the ion source device and having a mutual grid distance along a beam axis, wherein the grids are electrically insulated relative to each other, the grids are arranged for applying different electrical potentials for creating an ion extraction and acceleration field along the beam axis, and the ion source device and the grid device are arranged in an evacuable ion beam space extending along the beam axis, wherein at least one of the grids is a movable grid, which can be shifted along the beam axis, and the grid device is coupled with a grid drive device having a drive motor, which is arranged for moving the movable grid along the beam axis and setting the mutual grid distance between the movable grid and another one of the grids.

2. The ion beam extraction apparatus according to claim 1, wherein the movable grid has a grid support frame, which is shiftable along linear guide carriers extending parallel to the beam axis, and the drive motor is coupled with the shiftable grid support frame.

3. The ion beam extraction apparatus according to claim 2, wherein the grid drive device comprises at least one pair of a rotating spindle nut and a drive spindle, which is coupled with the shiftable grid support frame, and the spindle nut is rotatable by the drive motor.

4. The ion beam extraction apparatus according to claim 3, wherein the grid drive device comprises multiple pairs of spindle nuts and drive spindles, which are coupled with the shiftable grid support frame at different edge sections thereof.

5. The ion beam extraction apparatus according to claim 4, wherein the drive spindles comprise one primary drive spindle which is directly coupled with the drive motor and at least one secondary drive spindle which is coupled with the primary drive spindle via a chain or belt drive.

6. The ion beam extraction apparatus according to claim 3, wherein the drive motor is a pressurized air motor.

7. The ion beam extraction apparatus according to claim 1, wherein the drive motor is arranged in a surrounding outside of the evacuable ion beam space, and the drive device is coupled with the movable grid using membrane bellows for vacuum sealing.

8. The ion beam extraction apparatus according to claim 1, further comprising a position measurement device being arranged for sensing a position of the movable grid.

9. The ion beam extraction apparatus according to claim 8, wherein the position measurement device comprises at least one of a drive monitor coupled with the grid drive device and a position sensor coupled with the movable grid.

10. The ion beam extraction apparatus according to claim 8, further comprising a grid position control unit being coupled with the grid drive device and the position measurement device and being configured for a loop control of setting the grid distance.

11. The ion beam extraction apparatus according to claim 1, further comprising a mechanical stop arrangement being arranged for limiting a range of setting the grid distance.

12. The ion beam extraction apparatus according to claim 1, further comprising a cooling device with cooling medium supply lines being arranged for cooling the grid device, wherein the cooling medium supply lines coupled with the movable grid are routed out of the evacuable ion beam space by sliding pipes, which are vacuum sealed by bellows.

13. The ion beam extraction apparatus according to claim 1, wherein the movable grid is arranged directly adjacent to the ion source device.

14. The ion beam extraction apparatus according to claim 1, wherein the ion beam extraction apparatus is configured as a neutral beam injection apparatus of a fusion plasma plant, wherein the ion source device is a plasma source with an ion exit window, and the at least two grids comprise a plasma grid being arranged at the ion exit window of the plasma source and an extraction grid being coupled with a high voltage power supply.

15. The ion beam extraction apparatus according to claim 1, which is configured for use as a neutral beam injection apparatus of a fusion plasma plant, an ion generator of an ion implantation plant, an ion generator of a coating plant, an ion generator of a medical application, or an ion thruster.

16. A method of creating an ion beam along a beam axis, comprising creating ions with an ion source device, and passing the ions through an ion extraction and acceleration field along the beam axis, wherein the ion extraction and acceleration field is created with a grid device comprising at least two grids being arranged adjacent to the ion source device and having a mutual grid distance along the beam axis, wherein the ion source device and the grid device are arranged in an evacuated ion beam space extending along the beam axis, wherein the grid device is adjusted by moving a movable grid of the at least two grids along the beam axis, wherein the grid distance is set by a grid drive device which is coupled with the grid device.

17. The method according to claim 16, wherein the grid distance is set such that particle energy can be chosen independently from particle current at simultaneously minimum divergence.

18. The method according to claim 1, wherein the grid distance is set using a loop control in dependency on a power parameter of the extracted ion beam.

19. The method according to claim 16, wherein the ion beam extraction apparatus according to claim 1 is used.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Further details and advantages of the invention are described with reference to the attached drawings, which show in

[0036] FIG. 1: a schematic illustration of features of preferred embodiments of an ion beam extraction apparatus and method according to the invention;

[0037] FIG. 2: a schematic cross-sectional partial illustration of an embodiment of the ion beam extraction apparatus adapted for an NBI system;

[0038] FIG. 3: a top view on a high voltage flange and a grid; and

[0039] FIG. 4: a diagram illustrating an operational space of the ASDEX Upgrade NBI injector in the beam-energy-vs.-neutral-beam-power plane.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0040] Preferred embodiments of the invention are described in the following with exemplary reference to an ion beam extraction apparatus included in an NBI system. It is emphasized that the invention is not restricted to this application, but rather possible in a corresponding manner e. g. for providing an ion generator of an ion implantation plant, a coating plant, a medical application, or an ion thruster. The NBI system is described with particular reference to the details of adjusting the movable grid of the grid device. Further details of the NBI system components, like e. g. details of the plasma source, the grid design, the grid support, the electric insulation, the vacuum equipment, the cooling device, the neutraliser, and the operation of the NBI system are not described as far as they are known from conventional NBI systems, like the ASDEX Upgrade NBI injector [1]. The figures are schematic illustrations. In practice, the shape and size of the illustrated components can be selected and adapted in dependency on particular application requirements.

[0041] FIG. 1 schematically shows an embodiment of the ion beam extraction apparatus 100 configured as the NBI system of a fusion plasma plant 200. Further details of the ion beam extraction apparatus 100 are illustrated in FIG. 2. The ion beam extraction apparatus 100 creates an ion beam 1, which is converted into a neutral particle beam 2 to be directed through a port duct 110 into the torus shaped reaction chamber 210 of the fusion plasma plant 200. The ion beam extraction apparatus 100 comprises the ion source device 10, the grid device 20 with three grids 21, 22 and 23, each with a grid support frame 24, the grid drive device 40 with a drive device 41, the grid position control unit 50, the mechanical stop arrangement 60 and the cooling device 70 with the cooling lines 71. The grids 21, 22 and 23 are arranged for extracting the ion beam 1 from the ion source device 10 along a beam axis z.

[0042] An evacuable ion beam space 30 is provided, including a vacuum recipient, which accommodates the components 10, 20, 40, 50, 60 and 70 at least partially in high vacuum. Additionally, a neutraliser device 80 is arranged in the evacuable ion beam space 30. The neutraliser device 80 is configured for converting a substantial fraction (about 30 to 70%, depending on beam energy) of the ion beam 1 of fast ions into a neutral particle beam 2 of fast neutral particles through interaction with neutral background gas. The residual ions at the end of the neutraliser device 80 are separated from the neutral particle beam 2 by a magnetic or electrostatic deflector onto an ion dump (not shown).

[0043] The ion source device 10 is a plasma source for creating ions from hydrogen atoms. The ions are extracted from the plasma source through an ion exit window 11, which is the open side of the plasma source (see also FIG. 2) by the effect of an electric field between the first grid 21 and the second grid 22 penetrating through the apertures of the first grid 21 into the plasma source.

[0044] The first grid 21 of the grid device 20, downstream from the ion exit window 11, is the plasma grid, which is movable as described below and therefore called the movable grid. Subsequently, the extraction grid 22 and a further acceleration grid 23 (grounded grid) are provided as the second and third grids. The extraction grid 22 and the further acceleration grid 23 remain unchanged and they are kept in a fixed position relative to the ion source device 10, in particular with respect to a high voltage flange 47. Each grid is mounted onto a grid support frame 24. The grid support frames 24 of the grids 21, 22 and 23 basically have the same structure. Each of the grids 21, 22 and 23 comprises two individual segments (see e.g. segments 21A, 21B in top view of the plasma grid 21 in FIG. 3) which are mounted onto the grid support frame 24 (see FIG. 2). The grid support frames 24 of the grids 21, 22 and 23 are nested inside each other and mounted on ceramic posts that provide the electrical insulation. Each of the grids 21, 22 and 23 extends in a plane perpendicular to the beam axis z. The grids have e. g. 774 apertures with a diameter of 8 mm. The apertures of the different grids 21, 22 and 23 are aligned relative to each other.

[0045] The cooling device 70 comprises cooling medium supply lines 71 (cooling water supply lines, schematically shown in FIG. 1), which are coupled with the grid support frames 24, with the grids 21, 22, 23 and with a cooling medium supply 72, e. g. as it is known from standard ASDEX Upgrade ion sources. In case of the movable grid 21, the cooling medium supply lines 71 include sliding pipes, which are vacuum sealed by bellows. On the air side, the motion of the movable grid 21 is compensated by flexible metal hoses, which again are connected to the cooling supply lines 71 via insulating hoses.

[0046] The grid drive device 40 is coupled with the movable plasma grid 21 for setting the grid distance d. Changing the gap between plasma grid 21 and the extraction grid 22 in situ comprises in the illustrated example moving the plasma grid 21 along the beam axis z in the order of tens of millimeters, e. g. in a range from 5 mm to 25 mm. The movement advantageously is conducted while keeping the vacuum, with flexible supply connections, electrically insulated for high potentials and with a very high precision (parallel to the beam direction of the order of 0.1 mm). Furthermore, the mutual lateral alignment of the apertures of the different grids 21, 22 and 23 is kept during the movement at even higher precision in the order of several hundreds of millimeters. As the drive motor 41 of the grid drive device 40 is arranged outside the evacuable ion beam space 30, membrane bellows 44 are provided for keeping the vacuum and compensating of movements of further parts of the grid drive device 40. Details of the grid drive device 40 using rotating spindle nuts on the drive spindles 43 are described below with reference to FIGS. 2 and 3.

[0047] Setting the grid distance d is preferably obtained with a control loop implemented with the grid position control unit 50, which is coupled with the grid drive device 40 and a position measurement device 51. The grid position control unit 50 is a computer device being coupled with a general control of the ion beam extraction apparatus 100 and/or the fusion plasma plant 200, e. g. in a remote control room. The position measurement is preferably done on the air side via redundant measurement of the drive spindle rotation and a linear measurement, both transmitted via light fibers from the high potential to the grid position control unit 50. The position measurement device 51 is e. g. a drive monitor coupled with the grid drive device 40 for sensing a current adjustment position of the movable grid 21, e. g. by counting rotations of the drive spindles. The mechanical stop arrangement 60 comprises two mechanical stops which are implemented to prevent damage should one of the position measurement device 51 and the grid position control unit 50 fail.

[0048] Changing and adjusting the grid distance to a particular predetermined value is conducted in dependency on the particular application conditions of the NBI system, in particular for optimizing perveance in relation to a certain extraction voltage V.sub.ex and extracted ion current I.sub.ex. The grid distance to be set is obtained from numerical calculations and/or calibration data of the NBI system.

[0049] FIG. 2 is a cross sectional view of a portion of the ion beam extraction apparatus 100 including the grid drive device 40 with further details, wherein two different positions of the movable grid 21 with a largest grid distance d (FIG. 2A) and a smallest grid distance d (FIG. 2B) relative to the extraction grid 22 are shown. In the upper section of FIG. 2, the ion source device 10 with the ion exit window 11 is illustrated. The beam axis z is vertically oriented in the drawing plane. The extraction grid 22 and the further acceleration grid 23 are shown downstream of the movable grid 21.

[0050] The grid support frame 24 of the movable grid 21 is supported in vacuum in the ion beam space 30 by drive spindles 43. Four fine threaded drive spindles 43A, 43B are provided as further shown in the top view of FIG. 3. Thus, the grid support frame 24 of the movable grid 21 is supported via the spindles 43 with high accuracy. Support shafts 45 of the spindles 43 to the plasma grid support frame 24 act with guide bushings as high precision linear guides. The spindles 43 and their linear guides are positioned in air for better access and lubrication. The membrane bellows 44 enclosing the support shafts 45 serve as vacuum sealing. The drive motor 41 rotates the spindle nut 42 which is supported via ball bearings on the ion source base flange. This rotation of the nut moves the spindle and therefore movable grid 21.

[0051] In order to improve synchronous rotation of all four spindles 43A, 43B, they are coupled outside the vacuum by a chain drive 46 on a high voltage flange 47 (see FIG. 3). One of the gearwheels 48 of the chain drive 46 is driven by the drive motor 41 of the grid drive device 40. The drive motor 41 is a pressurized air motor facilitating operation when the ion source device 10 is at high voltage. By this mechanism the gap d between extraction grid 22 and the plasma grid 21 can be adjusted, e. g. in the range from 5 mm to 25 mm.

[0052] As mentioned above, FIG. 4 shows the operational space of the ASDEX Upgrade NBI injector in the beam-energy-vs.-neutral-beam-power plane. With the variable grid distance d, operation is no longer restricted to the black line of curve A, but to a continuous area between the dashed lines B and C. The boundaries of the area between the dashed lines B and C are given by the optimum perveances at minimum and maximum gap d (5 and 25 mm in the example), and further the maximum current that the high voltage power supply (HV PS) can deliver, the maximum power that the residual ion dump can take, and other system specific limitations represented by the further dashed lines in FIG. 4.

[0053] The features of the invention disclosed in the above description, the drawings and the claims can be of significance individually, in combination or sub-combination for the implementation of the invention in its different embodiments.