Patent Application
20230230793
The invention relates to an ion beam extraction apparatus and to a method for creating an ion beam, in particular using an ion source device and at least two grids for accelerating ions by electrical potentials along a beam axis, like e. g. an ion beam extraction apparatus employed as a neutral beam injection apparatus of a fusion plasma plant. Further applications of the invention are available in the fields of e. g. ion implantation, coating techniques and medical particle irradiation.
In the present specification, reference is made to the following prior art illustrating the technical background of the invention, in particular relating to ion beam extraction in a neutral beam injection apparatus:
[1] B. Streibl et al. “MACHINE DESIGN, FUELING, AND HEATING IN ASDEX UPGRADE” in “Fus. Sci. Technol.” 44 (2003) 578.
Neutral Beam Injection (NBI) is a well-established additional heating method for fusion plasmas, typically providing heating powers up to tens of megawatts to the plasma. In an NBI system ions, usually either positive or negative hydrogen ions, are generated in an ion source and extracted through a large grid with multiple apertures and a gross extraction area of several hundred to several thousand cm2 by means of a voltage applied between this first grid, referred to as plasma grid, and a successive grid, the extraction grid. The extracted ions are either accelerated to the final beam energy in this single grid gap, or post-accelerated in successive acceleration stages by several more grids. The fully accelerated beam then passes through a neutraliser that converts a fraction of the ions into neutral particles. The neutral beam is transmitted to the fusion plasma in a toroidal confinement device through a duct.
NBI beamlines are usually quite long in order to host all aforementioned components; the beam-line for the fusion reactor ITER will measure about 26 m from source to plasma edge and that on the much smaller fusion experiment ASDEX Upgrade is about 7 m long. At the same time the beam duct is narrow, constrained by the toroidal field coils of the toroidal magnetic confinement device. Therefore the NBI beam has to have a low divergence in order to keep the transmission losses small.
The divergence of an ion beam, or more precisely, a beamlet extracted from a single aperture depends on the geometry of plasma and extraction grid, the voltage difference applied to these grids, called extraction voltage Vex, and the extracted current, Iex. The ratio Π=Iex/Vex3/2 is called perveance. For a given grid geometry there exists an optimum perveance, Πopt=Iopt/Vex3/2, for which the beamlet divergence is minimal.
The optimum perveance is approximately proportional to (a/d)2, where a is the diameter of the plasma grid aperture and d is the extraction gap, i.e. the grid distance between the plasma grid and the extraction grid. For providing the optimum perveance, conventional NBI systems are configured with fixed design parameters, in particular with a fixed grid distance.
However, due to the requirement of operating NBI at optimum perveance, the power of the extracted ion beam has a strong dependence on the extraction voltage, Pex=Iex Vex=Πopt Vex5/2. Neglecting the energy dependence of the neutralisation yield, the neutral beam injected into the plasma inherits the same dependence of power on the extraction voltage.
Curve A of
NBI systems are designed to deliver the desired power at a beam energy that is optimised for the parameters of the device that they are installed on when it operates at typical plasma parameters. However, parameter studies, advanced plasma control, or avoidance of beam shine through at low plasma density among other reasons require changing—i.e. in most cases reducing—the beam energy from its design value. As explained above, this leads to a strong reduction of the injected neutral beam power.
Thus, conventional ion beam extraction techniques have a substantial disadvantage in terms of limited degrees of freedom for adapting the ion beam extraction to particular application conditions. This disadvantage does not occur with NBI systems only, but also with other ion beam extraction systems, e. g. for implanting or coating applications.
The objective of the invention is to provide an improved ion beam extraction apparatus and an improved method for creating an ion beam, avoiding disadvantages of conventional techniques. In particular, ion beam extraction is to be provided with increased variability of setting parameters of ion beam extraction, reduced ion beam power dependency on extraction voltage and/or improved capability of creating the ion beam with minimum divergence.
The above objectives are solved by an ion beam extraction apparatus and a method for creating an ion beam comprising the features of the independent claims. Advantageous embodiments of the invention are defined in the dependent claims.
According to a first general aspect of the invention, the above objective is solved by an ion beam extraction apparatus, being configured for creating an ion beam. The ion beam extraction apparatus comprises 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 (axial direction of the ion beam extraction apparatus). The grid distance is a length of a gap or spacing between the grids in a direction parallel to the beam axis. The grids are electrically insulated relative to each other. Furthermore, the grids are arranged for applying different electrical potentials for creating an ion extraction and acceleration field along the beam axis. The ion source device and the grid device are arranged in an evacuable ion beam space extending along the beam axis.
According to the invention, at least one of the grids is a movable grid, which can be shifted along the beam axis. 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 grid distance between the movable grid and another one, preferably the next neighbouring, of the grids.
According to a second general aspect of the invention, the above objective is solved by a method of creating an ion beam along a beam axis, comprising the step of 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. The ion source device and the grid device form an evacuated ion beam space extending along the beam axis.
According to the invention, the method of creating the ion beam includes a further step of adjusting the grid device by moving a movable grid of the at least two grids along the beam axis, wherein the grid distance is set by a drive motor of a grid drive device which is coupled with the grid device. Advantageously, the grid distance can be set such that the particle energy can be chosen independently from the particle current in a wide range at simultaneously minimum divergence. Preferably, the inventive method of creating the ion beam or one of the embodiments thereof is conducted with the ion beam extraction apparatus according to the first general aspect of the invention or one of the embodiments thereof.
The ion source device generally comprises an ion source wherein ions are created by collisions of neutral particles (e. g. atoms or molecules) with energetic electrons which are created either by an arc discharge or by coupling high frequency waves into it. In particular, the ion source device is an apparatus including a neutral particle supply and an energetic electron supply arranged for ionizing the neutral particles. The ion source device is configured for an operation at a high electrical potential, e. g. a potential of at least 1 kV up to 1 MeV. The grid device comprises two or more grids, each comprising a plane or curved electrode with apertures allowing a passage of ions. With the plane design, the electrodes are arranged parallel to each other and perpendicular to the beam axis, i. e. in radial directions relative to the beam axis. In case of the curved design, the grids are arranged with a constant mutual distance with the center of the curvature on the beam axis. Furthermore, the grids are arranged for applying electrical potentials (grid potentials) which create the electric ion extraction and acceleration field along the beam axis.
With preferred applications of the invention, e. g. in a neutral beam injection system, the grids comprise a plasma grid (also called first grid) arranged next to the ion source device and an extraction grid (also called second grid) arranged adjacent to the plasma grid. Optionally, at least one further post-accelerating grid can be provided downstream of the extraction grid.
The movable grid is one of the grids, which can be shifted relative to the remaining structure of the ion beam extraction apparatus along the beam axis. The movable grid can be shifted parallel to the beam axis while keeping the perpendicular orientation and the radial position relative to the beam axis. Advantageously, this can be achieved with high precision. In particular, the movable grid can be shifted with the grid drive device in the completely assembled and evacuated ion beam extraction apparatus, e. g. during the operation thereof. The grid drive device including the drive motor is configured for adjusting the movable grid at a predetermined position on the beam axis. According to the positions of the movable grid and the neighbouring grid, the grid distance therebetween is set. Preferably, one single grid of the group of grids is movable. Alternatively, two or more grids can be movable.
The inventors have found that the above disadvantages of conventional NBI systems can be avoided with the grid device in which the grid distance, in particular the extraction grid gap, can be varied in situ. Changing the grid distance allows optimizing perveance, making it possible to match the actual perveance (Πopt=Π) for a wide range of combinations of extraction voltage Vex and extracted current Iex. This means for example that the injected power can be kept roughly constant while changing the beam energy or vice versa the power can be changed at constant beam energy. Advantageously, the invention provides the variable grid gap so that the ion beam extraction can be provided with an extended operational space. Shifting the movable grid provides an additional degree of freedom in setting operation parameters of the ion beam extraction, in particular for shaping the ion beam and in particular minimizing the divergence of the ion beam.
Furthermore, the inventors have found that the movable grid can be implemented despite of the fact that the technical realization of an ion beam extraction apparatus, e. g. in an NBI system, is challenging. The whole process of ion formation, extraction and acceleration takes place in a high vacuum environment with a low impurity content. The ion source device and the grids, like the plasma grid and the extraction grid (and possible acceleration grids), are set on different, but high electric potentials of e. g. several 10 kV up to 1000 kV. Mechanical support and supplies (electric, cooling) are properly insulated against each other and against ground potential, and suitable vacuum feedthroughs are provided for all supplies. The grids are high precision components which preferably are provided with internal water cooling (to cope with the plasma heat load) and typically have a shape and position tolerance of a few hundredths of a millimeter for each of the single beamlet apertures (several hundred to several thousand apertures per grid, e.g. 774 for ASDEX Upgrade).
According to a preferred embodiment of the invention, the movable grid has a grid support frame, which is shiftable along linear guide carriers extending parallel to the beam axis and the drive device is coupled with the shiftable grid support frame. Advantageously, the grid support frame provides a stable carrier of the movable grid, so that the radial position and orientation can be precisely kept when adjusting the axial position thereof. The grid support frame is a solid frame component supporting the movable grid. At least two, preferably at least three linear guide carriers are provided in engagement with the grid support frame. By the action of the drive device on the grid support frame, the movable grid is shifted for setting the grid distance.
Particularly preferred, the drive device comprises the drive motor and 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. Shifting the movable grid by the action of at least one drive spindle has particular advantages in terms of precise adjustment of the axial position of the movable grid.
If, according to a further preferred variant, the 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, keeping the perpendicular orientation of the movable grid relative to the beam axis is advantageously facilitated. Particularly preferred, 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. With this embodiment, the coupling of the drive motor with the drive spindles is facilitated.
According to a further particularly preferred embodiment of the invention, the drive motor is a pressurized air motor. The pressurized air motor is mechanically connected to the shiftable grid support frame and thus being set on the high electrical potential of the movable grid. The activation of the pressurized air motor is realized via pressurized air through plastic hoses and thus provides electrical insulation between the movable grid and all other components of the system at ground potential, thus advantageously allowing an operation while a high voltage potential is applied to the movable grid.
According to a further preferred embodiment of the invention, the drive motor is arranged in a surrounding outside of the evacuable ion beam space, and the drive motor is coupled with the movable grid support frame using membrane bellows for vacuum sealing. This allows an advantageous configuration of the drive device for an operation at atmospheric pressure.
Preferably, a position measurement device is arranged for sensing a position of the movable grid. Particularly preferred, the position measurement device comprises a drive monitor coupled with the grid drive device and/or a position sensor coupled with the movable grid. Advantageously, the position measurement device allows a precise monitoring and/or adjustment of the grid distance.
In particular, according to a further advantageous variant of the invention, the position measurement device can be included in a control loop for controlling the grid distance. Preferably, the grid distance is set using a loop control in dependency on a power parameter of the extracted ion beam. With these embodiments, a grid position control unit is preferably provided, which is coupled with the grid drive device and the position measurement device and which is configured for the loop control for setting the grid distance.
If, according to a further embodiment of the invention, a mechanical stop arrangement, including at least one mechanical stop, is arranged for limiting a range of setting the grid distance, advantages in terms of operation safety of the ion beam extraction apparatus are obtained.
According to a further preferred feature of the invention, a cooling device with cooling medium supply lines can be arranged for cooling the grid device. The cooling medium supply lines coupled with the movable grid are routed out of the evacuated space by sliding pipes, which are vacuum sealed by membrane bellows. Advantageously, the sliding pipes facilitate keeping the cooling action during the operation of the ion beam extraction apparatus, in particular during adjusting the grid distance.
While the movable grid may be any one of the grids, according to a particularly preferred embodiment, the movable grid is arranged directly adjacent to the ion source device. Accordingly, the movable grid is the first grid passed by the ions extracted from the ion source device. This embodiment has, compared with shifting e. g. the second grid, advantages for the mechanical set-up and avoiding unintended changes of relative distances of subsequent grids.
According to a preferred application of the invention, the ion beam extraction apparatus is configured as a neutral beam injection apparatus of a fusion plasma plant. Preferably, the ion source device is a plasma source with an ion exit window, 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 source, and preferably a neutraliser device is arranged downstream of the extraction grid for converting at least a portion of the accelerated ions into neutral particles. With this application in the NBI apparatus, particular advantages for creating the ion beam with minimum divergence and precise passing the ion beam through a duct port into the torus shaped reactor vessel of the fusion plasma plant are obtained.
According to further preferred applications of the invention, the ion beam extraction apparatus is 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.
Further details and advantages of the invention are described with reference to the attached drawings, which show in
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.
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).
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
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
The cooling device 70 comprises cooling medium supply lines 71 (cooling water supply lines, schematically shown in
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
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.
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 Vex and extracted ion current Iex. The grid distance to be set is obtained from numerical calculations and/or calibration data of the NBI system.
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
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
As mentioned above,
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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/067321 | 6/22/2020 | WO |
