Negative ion-based beam injector

Abstract

A negative ion-based beam injector comprising a negative ion source and an accelerator. The ions produced by the ion source are pre-accelerated before injection into a high energy accelerator by an electrostatic multi-aperture grid pre-accelerator, which is used to extract ion beams from the plasma and accelerate to some fraction of the required beam energy. The beam from the ion source passes through a pair of deflecting magnets, which enable the beam to shift off axis before entering the high energy accelerator. The negative ion-based beam injector can be combined with a neutralizer to produce about a 5 MW neutral beam with energy of about 0.50 to 1.0 MeV. After acceleration to full energy, the beam enters the neutralizer where it is partially converted into a neutral beam. The remaining ion species are separated by a magnet and directed into electrostatic energy converters. The neutral beam passes through a gate valve and enters a plasma chamber.

Claims

1. A negative ion-based beam injector comprises an ion source configured to produce a negative ion beam, an accelerator spaced apart from the ion source, and a transition zone interposing the ion source and the accelerator, the transition zone comprises a low energy beam transport line, wherein the low energy beam transport line includes bending magnets that deflect the beam orthogonally to the beam's direction of motion and focus the beam onto the axis of the accelerator.

2. The injector of claim 1, wherein the ion source includes a plasma container and plasma drivers.

3. The injector of claim 2, wherein internal walls of the plasma container are configured to maintain elevated temperatures of 150-200 C.

4. The injector of claim 1, wherein the ion source includes a pre-accelerator.

5. The injector of claim 4, wherein the pre-accelerator comprises an electrostatic grid having a plurality of electrodes, wherein each of the plurality of electrodes having a plurality of apertures.

6. The injector of claim 5, further comprising a distributing manifold for directly supplying cesium on the electrostatic grid of the pre-accelerator.

7. The injector of claim 5, further comprising a pumping system to pump gas out from a pre-acceleration gap.

8. The injector of claim 5, wherein at least one of the plurality of electrodes is positively biased to pre-accelerate negative ions in the negative ion beam.

9. The injector of claim 5, wherein the plurality of apertures are configured for focusing and passing negative ions to form the negative ion beam.

10. The injector of claim 4, wherein the pre-accelerator includes external magnets to deflect co-extracted electrons in an ion extraction and pre-acceleration region.

11. The injector of claim 4, further comprising a pumping system to pump gas out from a pre-acceleration gap.

12. The injector of claim 1, further comprising a neutralizer interconnected to the accelerator.

13. A negative ion-based beam injector comprises an ion source configured to produce a negative ion beam, the ion source including a pre-accelerator having an electrostatic grid, an accelerator spaced apart from the pre-accelerator, and a distributing manifold for directly supplying cesium on the electrostatic grid of the pre-accelerator.

14. The injector of claim 13, further comprising a transition zone interposing the ion source and the accelerator.

15. The injector of claim 14, wherein the transition zone comprises a low energy beam transport line.

16. The injector of claim 15, wherein the low energy beam transport line includes cesium traps.

17. The injector of claim 15, wherein the low energy beam transport line includes bending magnets that deflect the beam orthogonally to the beam's direction of motion and focus the beam onto the axis of the accelerator.

18. The injector of claim 14, wherein the ion source includes a plasma container and plasma drivers.

19. The injector of claim 18, wherein internal walls of the plasma container are configured to maintain elevated temperatures of 150-200 C.

20. The injector of claim 13, wherein the pre-accelerator includes external magnets to deflect co-extracted electrons in an ion extraction and pre-acceleration region.

21. The injector of claim 13, further comprising a neutralizer interconnected to the accelerator.

22. A negative ion-based beam injector comprises an ion source configured to produce a negative ion beam, the ion source including a pre-accelerator having external magnets to deflect co-extracted electrons in an ion extraction and pre-acceleration region, and an accelerator spaced apart from the pre-accelerator.

23. The injector of claim 22, further comprising a neutralizer interconnected to the accelerator.

Description

BRIEF DESCRIPTION OF FIGURES

(1) The details of the example embodiments, including structure and operation, may be gleaned in part by study of the accompanying figures, in which like reference numerals refer to like parts. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, all illustrations are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than literally or precisely.

(2) FIG. 1 is a plan view of a negative ion-based neutral beam injector layout.

(3) FIG. 2 is a sectional isometric view of the negative ion-based neutral beam injector shown in FIG. 1.

(4) FIG. 3 is a plan view of a high-power injector of neutrals for the ITER tokomak.

(5) FIG. 4 is an isometric cut-away view of the discharge chamber with a peripheral multipole magnetic field for the JET neutral beam injector.

(6) FIG. 5 is a chart showing the integral yield of negative ions formed by bombarding a Mo+Cs surface with neutral H atoms and positive molecular H as a function of incident energy. Yields are boosted by utilizing DC cesiation as compared to only precesiating the surface.

(7) FIG. 6 is a plan view of a negative ion source for the LHD.

(8) FIG. 7 is a schematic of a multi-aperture ion-optical system for the LHD source.

(9) FIGS. 8A and 8B are top and side views of the LHD neutral beam injector.

(10) FIG. 9 is a sectional view of an ion source.

(11) FIG. 10 is a sectional view of a low energy hydrogen atoms source.

(12) FIG. 11 is a graph showing the trajectories of H.sup. ions in the low energy tract.

(13) FIG. 12 is an isometric view of an accelerator.

(14) FIG. 13 is a graph showing the ion trajectories in the accelerating tube.

(15) FIG. 14 is an isometric view of the triplet of quadrupole lenses.

(16) FIG. 15 is a graph showing a top view (a) and a side view (b) of the ion trajectories in an accelerator of a high energy beam transport line.

(17) FIG. 16 is an isometric view of a plasma target arrangement.

(18) FIG. 17 is a graph showing results of two-dimensional calculations of ion beam deceleration in the recuperator.

(19) It should be noted that elements of similar structures or functions are generally represented by like reference numerals for illustrative purpose throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the preferred embodiments.

DETAILED DESCRIPTION

(20) Each of the additional features and teachings disclosed below can be utilized separately or in conjunction with other features and teachings to provide a new negative ion-based neutral beam injector. Representative examples of the embodiments described herein, which examples utilize many of these additional features and teachings both separately and in combination, will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Therefore, combinations of features and steps disclosed in the following detail description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the present teachings.

(21) Moreover, the various features of the representative examples and the dependent claims may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. In addition, it is expressly noted that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure, as well as for the purpose of restricting the claimed subject matter independent of the compositions of the features in the embodiments and/or the claims. It is also expressly noted that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure, as well as for the purpose of restricting the claimed subject matter.

(22) Embodiments provided herein are directed to a new negative ion-based neutral beam injector with energy of preferably about 500-1000 keV and high overall energetic efficiency. The preferred arrangement of an embodiment of a negative ion-based neutral beam injector 100 is illustrated in FIGS. 1 and 2. As depicted, the injector 100 includes an ion source 110, a gate valve 120, deflecting magnets 130 for deflecting a low energy beam line, an insulator-support 140, a high energy accelerator 150, a gate valve 160, a neutralizer tube (shown schematically) 170, a separating magnet (shown schematically) 180, a gate valve 190, pumping panels 200 and 202, a vacuum tank 210 (which is part of a vacuum vessel 250 discussed below), cryosorption pumps 220, and a triplet of quadrupole lenses 230. The injector 100, as noted, comprises an ion source 110, an accelerator 150 and a neutralizer 170 to produce about a 5 MW neutral beam with energy of about 0.50 to 1.0 MeV. The ion source 110 is located inside the vacuum tank 210 and produces a 9 A negative ion beam. The vacuum tank 210 is biased to 880 kV which is relative to ground and installed on insulating supports 140 inside a larger diameter tank 240 filled with SF.sub.6 gas. The ions produced by the ion source are pre-accelerated to 120 keV before injection into the high energy accelerator 150 by an electrostatic multi aperture grid pre-accelerator 111 (see FIG. 9) in the ion source 110, which is used to extract ion beams from the plasma and accelerate to some fraction of the required beam energy. The 120 keV beam from the ion source 110 passes through a pair of deflecting magnets 130, which enable the beam to shift off axis before entering the high energy accelerator 150. The pumping panels 202 shown between the deflecting magnets 130 include a partition and cesium trap.

(23) The gas efficiency of the ion source 110 is assumed to be about 30%. A projected negative ion beam current of 9-10 A corresponds to 6-7 l.Math.Torr/s gas puff in the ion source 110. The neutral gas flowing from the ion source 110 builds up to an average pressure in the pre-accelerator 111 of about 210.sup.4 Torr. At this pressure, the neutral gas causes 10% striping loss of the ion beam inside the pre-accelerator 111. Between the deflecting magnets 130 there are dumps (not shown) for neutral particles, which arise from the primary negative ion beam. There are also dumps (not shown) for positive ions back streaming from the high energy accelerator 150. A low energy beam transport line region 205 with differential pumping from pumping panels 200 is used immediately after pre-acceleration to decrease the gas pressure down to 10.sup.6 Torr before it reaches the high energy accelerator 150. This introduces an additional 5% beam loss, but since it happens at a low pre-acceleration energy the power loss is relatively small. The charge exchange losses in the high energy accelerator 150 are below 1% at the 10.sup.6 Torr background pressure.

(24) After acceleration to full energy of 1 MeV the beam enters a neutralizer 170 where it is partially converted into a neutral beam. The remaining ion species are separated by a magnet 180 and directed into electrostatic energy converters (not shown). The neutral beam passes through the gate valve 190 and enters a plasma chamber 270.

(25) The vacuum vessel 250 is broken down into two sections. One section contains the pre-accelerator 111 and low energy beam line 205 in the first vacuum tank 210. Another section houses a high energy beam line 265, the neutralizer 170 and charged particles energy converters/recuperators in a second vacuum tank 255. The sections of the vacuum vessel 250 are connected through a chamber 260 with the high energy accelerator tube 150 inside.

(26) The first vacuum tank 210 is the vacuum boundary of the pre-accelerator 111 and low energy beam line 205 and the larger diameter tank or outer vessel 240 is pressurized with SF.sub.6 gas for high voltage insulation. The vacuum tanks 210 and 255 act as the support structure for the interior equipment, such as the magnets 130, cryosorption pumps 220, etc. Heat removal from the internal heat-bearing components will be accomplished with cooling tubes, which have to have insulation breaks in the case of the first vacuum tank 210, which is biased to 880 kV.

(27) Ion Source:

(28) A schematic diagram of the ion source 110 is shown in FIG. 9. The ion source includes: electrostatic multi-aperture pre-accelerator grids 111, ceramic insulators 112, RF-type plasma drivers 113, permanent magnets 114, a plasma box 115, water coolant channels and manifolds 116, and gas valves 117. In the ion source 110, a cesiated molybdenum surface of the plasma pre-accelerator grids 111 is used to convert the positive ions and neutral atoms formed by the plasma drivers 113 into negative ions in a plasma expansion volume (the volume between the drivers 113 and the grids 111, indicated by the bracket labeled PE in FIG. 9) with magnetic-multipole-bucket containment as provided by the permanent magnets 114.

(29) A positive bias voltage for collection of the electrons to the plasma pre-accelerator grids 111 is applied to optimized conditions for negative ion production. Geometric shaping of apertures 111B in plasma pre-accelerator grids 111 is used to focus H.sup. ions into the apertures 111B of the extraction grid. A small transverse magnetic filter produced by external permanent magnets 114 is used to decrease the temperature of electrons diffused from the driver region or plasma emitter region PE of plasma box 115 to the extraction region ER of the plasma box 115. Electrons in the plasma are reflected back from the extraction region ER by the small transverse magnetic filter field produced by external permanent magnets 114. The ions are accelerated to 120 keV before injection into the high energy accelerator 150 by the electrostatic multi-aperture pre-accelerator plasma grids 111 in the ion source 110. Before acceleration to high energy, the ion beam is about 35 cm in diameter. The ion source 110 therefore has to produce 26 mA/cm2 in the apertures 111B assuming 33% transparency in the pre-accelerator plasma grids 111.

(30) Plasma, which feeds the plasma box 115, is produced by an array of plasma drivers 113 installed on a rear flange 115A of the plasma box, which is preferably a cylindrical water-cooled copper chamber (700 mm diameter by 170 mm long). The open end of the plasma box 115 is enclosed by the pre-accelerator plasma grids 111 of the extraction and acceleration system.

(31) It is assumed that the negative ions are to be produced on the surface of the plasma grids 111, which are covered with a thin layer of cesium. Cesium is introduced into the plasma box 115 by use of a cesium supply system (not shown in FIG. 9).

(32) The ion source 110 is surrounded by permanent magnets 114 to form a line cusp configuration for primary electron and plasma confinements. The magnet columns 114A on the cylindrical wall of the plasma box 115 are connected at the rear flange 115A by rows of magnets 114B that are also in a line-cusp configuration. A magnetic filter near the plane of the plasma grids 111 divides the plasma box 115 into the plasma emitter PE and the extraction region ER. The filter magnets 114C are installed at a flange 111A next to the plasma grids 111 to provide a transverse magnetic field (B=107 G at the center) which serves to prevent energetic primary electrons coming from the ion drivers 113 from reaching the extraction region ER. However, positive ions and low energy electrons can diffuse across the filter into the extraction region ER.

(33) An electrode extraction and pre-acceleration system 111 comprises five electrodes 111C, 111D, 111E, 111F and 111G, each having 142 holes or apertures 111B formed orthogonal there through and used to provide a negative ion beam. The extraction apertures 111B are each 18 mm in diameter, so that total ion extraction area of the 142 extraction apertures is about 361 cm.sup.2. The negative ion current density is 25 mA/cm.sup.2 and is required to produce a 9 A ion beam. The magnetic field of the filter magnets 114C is extended into the gaps between the electrostatic extractor and pre-accelerator grids 111 to deflect co-extracted electrons onto grooves at the inner surface of the apertures 111B in the extracting electrodes 111C, 111D, and 111E. The magnetic field of the magnetic filter magnets 114C together with the magnetic field of additional magnets 114D provides the deflection and interception of the electrons, co-extracted with negative ions. The additional magnets 114D include an array of magnets installed between the holders of the accelerator electrodes 111F and 111G of the accelerator grid located downstream from the extracting grid comprising extracting electrodes 111C, 111D, and 111E. The third grid electrode 111E, which accelerates negative ions to an energy of 120 keV, is positively biased from the grounded grid electrode 111D to reflect back streaming positive ions entering the pre-accelerator grid.

(34) The plasma drivers 113 include two alternatives, namely an RF plasma driver and an arc-discharge atomic driver. A BINP-developed arc-discharge arc plasma generator is used in the atomic driver. A feature of the arc-discharge plasma generator consists of the formation of a directed plasma jet. Ions in the expanding jet move without collisions and due to acceleration by drop of ambipolar plasma potential gain energies of 5-20 eV. The plasma jet can be directed on to an inclined molybdenum or tantalum surface of the converter (see 320 in FIG. 10), wherein as the result of neutralization and reflection of the jet a stream of hydrogen atoms is produced. The energy of hydrogen atoms can be increased beyond an initial 5-20 eV by negative biasing of the converter relative to the plasma box 115. Experiments on obtaining intensive streams of atoms with such a converter were performed in the Budker Institute in 1982-1984.

(35) In FIG. 10, the developed arrangement of a source of low energy atoms 300 is shown to include a gas valve 310, a cathode insert 312, an electrical feed through to a heater 314, cooling water manifolds 316, an LaB6 electron emitter 318, and an ion-atom converter 320. In experiments a stream of hydrogen atoms with an equivalent current of 20-25 A and energy varying in the range from 20 eV to 80 eV have been produced with an efficiency of more than 50%.

(36) Such a source can be used in the negative ion source to supply atoms with energy optimized for efficient generation of negative ions on the cesiated surface of plasma grids 111.

(37) Low Energy Beam Transport Line

(38) The H.sup. ions generated and pre-accelerated to an energy of 120 keV by the ion source 110 on their passage along the low-energy beam transport line 205 are displaced perpendicular to their direction of motion by 440 mm with deviation by peripheral magnetic field of the ion source 110 and by a magnetic field of two special wedge-shaped bending magnets 130. This displacement of the negative ion beam in the low energy beam transport line 205 (as illustrated in FIG. 11) is provided to separate the ion source 110 and high-energy-accelerator regions 150. This displacement is used to avoid penetration of fast atoms originated from stripping of the H.sup. beam on residual hydrogen in the accelerating tube 150, to reduce streams of cesium and hydrogen from the ion source 110 to the accelerating tube 150, and also for suppression of secondary ion flux from the accelerating tube 150 to the ion source 110. In FIG. 11 the calculated trajectories of the H.sup. ions in the low-energy beam transport line are shown.

(39) High Energy Beam Duct

(40) The low energy beam outgoing from the low energy beam line enters a conventional electrostatic multi aperture accelerator 150 shown in FIG. 12.

(41) The results of the calculation of the 9 A negative-ion beam acceleration taking into account the space charge contribution are shown in FIG. 13. Ions are accelerated from a 120 keV energy up to 1 MeV. The accelerating potential on the tube 150 is 880 kV, and the potential step between the electrodes is 110 kV.

(42) The calculation shows that the field strength does not exceed 50 kV/cm in the optimized accelerating tube 150 on electrodes in the zones of possible development of electron discharge.

(43) After acceleration the beam goes through a triplet 230 of industry conventional quadrupole lenses 231, 232 and 233 (FIG. 14), which are used to compensate slight beam defocusing on the exit of accelerating tube 150 and to form a beam with a preferred size on the exit port. The triplet 230 is installed inside the vacuum tank 255 of the high energy beam transport line 265. Each of the quadrupole lenses 231, 232 and 233 include a conventional set of quadrupole electromagnets that produce customary magnetic focusing fields as are found in all modern conventional particle accelerators.

(44) The calculated trajectories of a 9 A negative-ion beam with the transverse temperature of 12 eV in the accelerating tube 150, the quadrupole lenses 230 and the high energy beam transport line 265 are shown in FIG. 15. The calculation follows the beam beyond its focusing point.

(45) The calculated diameter of the neutral beam with a 6 A equivalent current after the neutralizer at the distance of 12.5 m at half-height of the radial profile is 140 mm and 95% of the beam current is in a 180 mm diameter circumference.

(46) Neutralization

(47) The photodetachment neutralizer 170 selected for the beam system can achieve more than 95% stripping of the ion beam. The neutralizer 170 comprises an array of xenon lamps and a cylindrical light trap with highly reflective walls to provide the required photon density. Cooled mirrors with a reflectivity greater than 0.99 are used to accommodate a power flux on the walls of about 70 kW/cm.sup.2. In an alternative, a plasma neutralizer using conventional technology could be used instead but with the expense of a slight decrease in efficiency. Nevertheless, 85% neutralization efficiency of a plasma cell is quite sufficient if an energy recovery system has >95% efficiency, as predicted.

(48) The plasma neutralizer plasma is confined in a cylindrical chamber 175 with multi-pole magnetic field at the walls, which is produced by an array of permanent magnets 172. General view of the confinement device is shown in FIG. 16. The neutralizer 170 includes cooling water manifolds 171, permanent magnets 172, cathode assembles 173, and LaB6 cathodes 174.

(49) The cylindrical chamber 175 is 1.5-2 m long and has openings at the ends for beam passing through. Plasma is generated by using several cathode assembles 173 installed at the center of the confinement chamber 175. Working gas is supplied near the center of the device 170. In the experiments with a prototype of such a plasma neutralizer 170, it was observed that confinement of electrons by the multi-pole magnetic fields 172 at the walls is good enough and considerably better than that of plasma ions. In order to equalize ion and electron losses, considerable negative potential develops in the plasma, so that the ions are effectively confined by the electric field.

(50) Reasonably long plasma confinement results in relatively low power of the discharge required to sustain about 10.sup.13 cm.sup.3 plasma density in the neutralizer 170.

(51) Energy Recuperation

(52) There are objective reasons for achievement of high power efficiency in our conditions. First of all, these are: a relatively small current of the ion beam and low energy spread. In the scheme described herein, with the usage of plasma or vapor-metal targets, the residual current of ions can be expected to be 3 A after the neutralizer. These streams of rejected ions with either positive or negative charge will be diverted via deflection magnet 180 to two energy recuperators, one each for positive and negative ions, respectively. Numerical simulations of the deceleration of these residual rejected ion beams with typically 1 MeV energy and 3 A in the direct converters inside the recuperators without a space-charge compensation have been carried out. The direct converter converts a substantial portion of the energy contained in the residual rejected ion beam directly to electricity and supplies the rest of the energy as high quality heat for incorporation in the thermal cycle. The direct converters follow the design of an electrostatic multi aperture decelerator, whereby consecutive sections of charged electrodes produce the longitudinal breaking fields and absorb the kinetic energy of the ions.

(53) FIG. 17 shows the results of two-dimensional calculations of ion beam deceleration in the converter. From the presented calculations, it follows that the deceleration of the ion beam with 1 MeV energy down to 30 keV energy is quite feasible, thus, the value of recuperation factor of 96-97% can be obtained.

(54) Previous development attempts of high power neutral beam injectors based on negative ions have been analyzed to reveal critical issues so far preventing achievement of injectors with stable steady state operation of 1 MeV and several MWs of power. Among those most important are: Control of cesium layer, and loss and re-deposition (temperature control, etc) Optimization of surface production of negative ions for extraction Separation of co-streaming electrons Non-homogeneity of ion current profile at plasma grid due to internal magnetic fields Low ion current density Accelerators are complicated and a lot of new technologies are still being developed (low voltage holding capacity, large insulators, etc) Back-streaming positive ions Advanced neutralizer technologies (plasma, photons) are not demonstrated at relevant conditions Energy conversion is not developed enough Beam blocking in the duct

(55) The innovative solutions to the problems provided herein can be grouped according to the system they are connected with, namely negative ion source, extraction/acceleration, neutralizer, energy converters, etc.

(56) 1.0. Negative ion source 110:

(57) 1.1. Internal walls of a plasma box 115 and plasma drivers 113 stay at elevated temperature (150-200 C.) to prevent cesium accumulation on their surfaces. The elevated temperature:

(58) prevents uncontrolled cesium release due to desorption/sputtering and decreasing its penetration into the ion optical system (grids 111),

(59) reduces absorption and recombination of hydrogen atoms in cesium layer at the walls,

(60) reduces consumption and poisoning of cesium.

(61) To achieve this, a high temperature fluid is circulated through all components. The temperature of the surfaces is further stabilized via active feed back control, i.e.: heat is either removed or added during CW operation and transient regimes. In contrast to this approach, all other existing and planned beam injectors use passive systems with water cooling and thermal breaks between the coolant tubes and the hot electrode bodies.

(62) 1.2. Cesium is supplied through a distributing manifold directly onto surface of the plasma grids 111, not to the plasma. Supplying cesium through a distributing manifold:

(63) provides controlled and distributed cesium supply during all beam-on time,

(64) prevents cesium shortage typically due to blocking by plasma,

(65) reduces cesium release from plasma after its accumulation and unblocking during long pulses.

(66) In contrast, existing ion sources supply cesium directly into the discharge chamber.

(67) 2.0. Pre-accelerator (100-keV) 111:

(68) 2.1. A magnetic field used to deflect co-extracted electrons in the ion extraction and pre-acceleration regions is produced by external magnets, not by magnets embedded into the grid body, as adopted in previous designs:

(69) magnetic field lines in the high-voltage gaps between the grids are everywhere concaved towards the negatively biased grids, i.e. towards the plasma grid in the extraction gap and towards the extraction grid in the pre-accelerating gap. The concavity of magnetic field lines towards the negatively biased grids prevents the appearance of local Penning traps in the high-voltage gaps and the trapping/multiplying of co-extracted electrons, as it may happen in configurations with embedded magnets.

(70) the electrodes of the ion optical system (IOS) (grids 111) without embedded low-temperature NIB magnets could be heated up to an elevated temperature (150-200 C.) and permits heat removal during long pulses by use of hot (100-150 C.) liquids.

(71) the absence of embedded magnets saves the space between the emission apertures of the grids and permits the introduction of more efficient electrode heating/cooling channels.

(72) In contrast, previous designs utilize magnets embedded into the grid body. This leads to the creation of static magneto-electric traps in the high voltage gaps that trap and multiply co-extracted electrons. This can cause a significant reduction in extracted beam current. It also prevents elevated temperature operation as well as appropriate heating/cooling performance, which is critical for long-pulse operation.

(73) 2.2. All of the electrodes of ion-optical system (grids 111) are always sustained at elevated temperature (150-200 C.) to prevent cesium accumulation at their surfaces and to increase the high-voltage strength of extracting and pre-accelerating gaps. In contrast, in conventional designs, the electrodes are cooled by water. The electrodes have elevated temperatures because there are thermal breaks between the coolant tubes and the electrode bodies, and there is no active feed back.

(74) 2.3. Initial warming up of the grids 111 at start up and heat removal during the beam-on phase is performed by running a hot liquid with a controllable temperature through the internal channels inside the grids 111.

(75) 2.4. Gas is additionally pumped out from the pre-accelerating gap through the side space and large openings in the grid holders in order to decrease gas pressure along beam line and to suppress negative ions stripping and production/multiplying of secondary particles in the gaps.

(76) 2.5. The inclusion of positively biased grids 111 is used to repel back streaming positive ions.

(77) 3.0. High voltage (1 MeV) accelerator 150:

(78) 3.1. The high voltage accelerator 150 is not coupled directly to the ion source, but is spaced apart from the ion source by a transition zone (low energy beam transport lineLEBT 205) with bending magnets 130, vacuum pumps and cesium traps. The transition zone: intercepts and removes most of the co-streaming particles including electrons, photons and neutrals from the beam, pumps out gas emanating from the ion source 110 and prevents it from reaching the high-voltage accelerator 150, prevents cesium from flowing out of the ion source 110 and penetrating to the high-voltage accelerator 150, prevents electrons and neutrals, produced by negative ions stripping, from entering the high-voltage accelerator 150.

(79) In the previous designs, the ion source is directly connected to the high-voltage accelerator. This causes the high-voltage accelerator to be subject to all gas, charged particle, and cesium flows from the ion source and vice versa. This strong interference reduces the voltage holding capacity of the high-voltage accelerator.

(80) 3.2. Bending magnets 130 in the LEBT 205 deflect and focus the beam onto the accelerator axis. The bending magnets 130:

(81) compensate any beam offset and deflection during transport through the magnetic field of the ion source 110,

(82) offset between the axes of pre and high-voltage accelerators 111 and 150 reduces the influx of co-streaming particles to the high-voltage accelerator 150 and prevents the highly accelerated particles from back-streaming (positive ions and neutrals) into the pre-accelerator 111 and ion source 110.

(83) In contrast, previous systems have no physical separation between acceleration stages and, therefore, do not allow for axial offsets as featured herein.

(84) 3.3. The magnets of the low energy beam line 205 focus the beam into the entrance of the single aperture accelerator 150: Beam focusing facilitates homogeneity of the beam entering the accelerator 150 compared to the multi-aperture grid systems.

(85) 3.4. Application of a single aperture accelerator:

(86) simplifies system alignment and beam focusing

(87) facilitate gas pumping and secondary particle removal from high energy accelerator 150

(88) reduces beam losses onto the electrodes of high energy accelerator 150.

(89) 3.5. Magnetic lenses 230 are used after acceleration to compensate for over focusing in the accelerator 150 and to form a quasi-parallel beam.

(90) In the conventional designs, there are no means for beam focusing and deflection, except in the accelerator itself.

(91) 4.0. Neutralizer 170:

(92) 4.1. Plasma neutralizer based on a multi-cusp plasma confinement system with high field permanent magnets at the walls;

(93) increases neutralization efficiency,

(94) minimizes overall neutral beam injector losses.

(95) These technologies have never been considered for application in large-scale neutral beam injectors.

(96) 4.2. Photon neutralizerphoton trap based on a cylindrical cavity with highly reflective walls and pumping with high efficiency lasers.

(97) further increases neutralization efficiency,

(98) further minimizes overall neutral beam injector losses.

(99) These technologies have never been considered for application in large-scale neutral beam injectors.

(100) 5.0. Recuperators:

(101) 5.1. Application of residual ion energy recuperator(s): increases overall efficiency of the injector.

(102) In contrast, recuperation is not foreseen in conventional designs at all.

REFERENCES

(103) [1.] L. W. Alvarez, Rev. Sci. Instrum. 22, 705 (1951) [2.] R. Hemsworth et al. Rev. Sc. Instrum., Vol. 67, p. 1120 (1996) [3.] Capitelli M. and Gorse C. IEEE Trans on Plasma Sci, 33, N. 6, p. 1832-1844 (2005) [4.] Hemsworth R. S., Inoue T., IEEE Trans on Plasma Sci, 33, N. 6, p. 1799-1813 (2005) [5.] B. Rasser, J. van Wunnik and J. Los Surf. Sci. 118 (1982), p. 697 (1982) [6.] Y. Okumura, H. Hanada, T. Inoue et al. AIP Conf. Proceedings #210, NY, p. 169-183 (1990) [7.] O. Kaneko, Y. Takeiri, K. Tsumori, Y. Oka, and M. Osakabe et al., Engineering prospects of negative-ion-based neutral beam injection system from high power operation for the large helical device, Nucl. Fus., vol. 43, pp. 692-699, 2003

(104) While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. All references are specifically incorporated herein in their entirety. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.