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The long-pulse acceleration of the high-power positive ion beam has been demonstrated with the JT-60 positive ion source in the joint experiment among Japan Atomic Energy Agency (JAEA), Korea Atomic Energy Research Institute (KAERI) and National Fusion Research Institute (NFRI) under the collaboration program for the development of plasma heating and current drive systems. In this joint experiment, the increase of the heat load and the breakdowns induced by the degradation of the beam optics due to the gas accumulation was one of the critical issues for the long-pulse acceleration. As a result of development of the long-pulse operation techniques of the ion source and facilities of the neutral beam test stand in KAERI, 2 MW 100 s beam has been achieved for the first time. The achieved beam performance satisfies the JT-60SA requirement which is designed to be a 1.94 MW ion beam power from an ion source corresponding to total neutral beam power of 20 MW with 24 ion sources. Therefore, it was found that the JT-60 positive ion sources were applicable in the JT-60SA neutral beam injectors. Moreover, because this ion source is planned to be a backup ion source for KSTAR, the operational region and characteristic has been clarified to apply to the KSTAR neutral beam injector.

The neutral beam injection (NBI) system was designed to provide plasma heating and current drive for high performance and long pulse operation of the Korean Superconducting Tokamak Advanced Research (KSTAR) device using two co-current beam injection systems. Each neutral beam injection system was designed to inject three beams using three ion sources and each ion source has been designed to deliver more than 2.0 MW of deuterium neutral beam power for the 100-keV beam energy. Consequently, the final goal of the KSTAR NBI system aims to inject more than 12 MW of deuterium beam power with the two NBI for the long pulse operation of the KSTAR. As an initial step toward the long pulse (∼300 s) KSTAR NBI system development, the first neutral beam injection system equipped with one ion source was constructed for the KSTAR 2010 campaign and successfully commissioned. During the KSTAR 2010 campaign, a MW-deuterium neutral beam was successfully injected to the KSTAR plasma with maximum beam energy of 90 keV and the L-H transition was observed with neutral beam heating. In recent 2011 campaign, the beam power of 1.5 MW is injected with the beam energy of 95 keV. With the beam injection, the ion and electron temperatures increased significantly, and increase of the toroidal rotation speed of the plasma was observed as well. This paper describes the design, construction, commissioning results of the first NBI system leading the successful heating experiments carried in the KSTAR 2010 and 2011 campaign and the trial of 300-s long pulse beam extraction.

Neutral beam (NB) injectors for JT-60 Super Advanced (JT-60SA) have been designed and developed. Twelve positive-ion-based and one negative-ion-based NB injectors are allocated to inject 30 MW D 0 beams in total for 100 s. Each of the positive-ion-based NB injector is designed to inject 1.7 MW for 100 s at 85 keV. A part of the power supplies and magnetic shield utilized on JT-60U are upgraded and reused on JT-60SA. To realize the negative-ion-based NB injector for JT-60SA where the injection of 500 keV, 10 MW D 0 beams for 100 s is required, R&Ds of the negative ion source have been carried out. High-energy negative ion beams of 490–500 keV have been successfully produced at a beam current of 1–2.8 A through 20% of the total ion extraction area, by improving voltage holding capability of the ion source. This is the first demonstration of a high-current negative ion acceleration of >1 A to 500 keV. The design of the power supplies and the beamline is also in progress. The procurement of the acceleration power supply starts in 2010.

The JT-60 Super Advanced (JT-60SA) tokamak aims to perform the ITER support and to demonstrate steady-state high-beta plasma project with the collaboration between Japan and EU. To attain these objectives, the negative-ion-based NBI (N-NBI) system is required to inject 10 MW for 100 s at the beam energy of 500 keV. On JT-60U, the present N-NBI ion source has injected 3.2 MW for 21 s at 320 keV; however, three key issues should be solved for the JT-60SA N-NBI ion source. One is to improve the voltage holding capability of the large negative ion source, where the available acceleration voltage has been limited to less than ~400 kV due to breakdowns. The accelerator of the JT-60U ion source is composed of large three-stage grids and three fiberglass reinforced plastic (FRP) insulators. Recent R&D tests suggested that the FRP insulators were not the main factor to trigger the breakdowns at the early conditioning stage. The accelerator with a large area of grids and their supporting structure may need a high margin in the design of electric field and a long time for conditioning. The second issue is to reduce the power loading of the acceleration grids. It was found that some beamlets were strongly deflected due to beamlet-beamlet interaction and strike on the grounded grid in the accelerator. Moreover, the electrons generated in the accelerator caused the grid loading and the overheating of the beamline components. The acceleration grids for JT-60SA are to be designed by taking account of the beamlet-beamlet interaction and the applied magnetic field in 3-D simulation. Third is to maintain the D production for 100 s. Although a constant D- beam power was confirmed on JT-60U for 21 s, an active cooling system is required to keep the temperature of the plasma grid (PG) under optimum condition during 100-s operation. A simple cooling structure is proposed for the active cooled PG, where a key is the temperature gradient on the PG for uniform D- production. In the present schedule, design work, reflecting the latest R&D progress, will continue until ~2011. The modified N-NBI ion source will start on JT-60SA in 2015.

Modification of JT-60U to a superconducting device (so called JT-60SA) has been planned to contribute to ITER and DEMO. The NBI system is required to inject D 0 beams of 34 MW for 100 s. The upgraded NBI system for JT-60SA consists of 12 positive-ion-based NBI (P-NBI) units and one negative-ion-based NBI (N-NBI) unit. The injection powers of each P-NBI and N-NBI units are 2 MW at 85 keV and 10 MW at 500 keV, respectively. On JT-60U, the long pulse operations of 30 s at 2 MW and 20 s at 3.2 MW have been achieved on the P-NBI and N-NBI units, respectively. Both units have demonstrated no injection power degradation for long pulse operation. It has been also found that the thermal time constants of the main key components with active cooling, such as the ion source of the present NBI system, are less than ∼20 s. Therefore, the pulse extension up to 100 s is expected to need some modifications mainly the power supply system. In addition, the voltage-holding capability of the negative ion source is required to be improved. The detailed technical design of the NBI system for JT-60SA is presented.

The 500 keV negative-ion based neutral beam injector for JT-60U started operations in 1996. The availability of the negative ion based neutral beam injection system has been improved gradually by modifying the ion source and optimizing its operation parameters. Recently, the extension of the pulse duration up to 30 s has been intended to study quasi-steady state plasma on JT-60U. The most serious issue is to reduce the heat load on the grids for long pulse operation. Two modifications have been proposed to reduce the heat load. One is to suppress the spread of beamlet-bundle which may be caused by beamlet–beamlet interaction in the multi-aperture grid due to the space charge force. Indeed, the investigation of the beam deflection, which was measured by the infrared camera on the target plate set 3.5 m away from the grid, indicates that the spread of beamlet-bundle is in proportion to the current density. Field-shaping plates were attached on the extraction grid to modify the local electric field. The plate thickness was optimized to steer the beamlet deflection. The other is to reduce the stripping loss, where the electron of the negative ion beam is stripped and accelerated in the accelerator and then collides with the grids. The ion source was modified to reduce the pressure in the accelerator column to suppress the beam-ion stripping loss. To date, long pulse injection of 19 s of 1.5–1.6 MW at a high energy beam of 360 keV, 9–10 A for D− has been obtained by one ion source with these modifications.

In the negative ion based neutral beam injector (N-NBI) for JT-60U, an extension of the pulse duration up to 30 s has been intended to study quasi-steady state plasmas, where the NBI pulse duration is significantly longer than the plasma current diffusion time. Achieving long pulse injection requires a reduction of the heat load on the grids of the ion source. The ion source was modified to reduce the pressure within the grid structure by increasing the vacuum conductance of the acceleration grids. The acceleration efficiency was improved from 0.74 to 0.79. The heat load of the grounded grid, which was the main heat load of the acceleration grids, decreased from 9.2 to 7.0% at a constant arc chamber pressure of 0.3 Pa. These improvements seemed to be caused by the reduction of beam ion stripping loss. Up to now, a long pulse injection of 17 s at the power of 1.6 MW with beam energy of 366 keV was obtained by using one ion source. The temperature rise of the cooling water from the grounded grid saturated at less than 35 °C during operation, so the heat load of the grid is significantly reduced for steady state operation at this power level.

In the neutral beam injection (NBI) system, an extension of the pulse duration up to 30 sec has been intended to study quasi-steady state plasma on JT-60U. The four positive-ion based (P-NBI) units, which tangentially inject neutral beam to plasma, were mainly modified on the electric power supplies and the beam limiters to extend the pulse duration up to 30 sec with 2 MW at 80 keV per each. The seven P-NBI units, each of which perpendicularly injects for 10 sec, were conducted to operate in series for the total pulse duration of 30 sec. The ion source of the negative-ion based (N-NBI) unit, whose target beam energy is 500 keV for 10 sec, was also modified to reduce the heat load of the grid for long pulse operation. The reduction of the re-ionization of the neutral beam in the beam drift duct was a key to achieve a long pulse injection. It was found that the pressure rise in the beam drift duct, which gives the re-ionization rate, depended on the temperature of the re-ionization plates during NBI injection. Up to now, it was attained successfully that the pulse duration of the tangential P-NBI unit was extended up tomore » 30 sec. 310 MJ of the total integrated injection energy into JT-60U plasma was achieved, including the negative-ion based NBI operation for 17 sec at 366 keV.« less

The 500-keV negative-ion based neutral beam injector for JT-60U started operation in 1996. The beam power has been increased gradually through optimizing operation parameters of the ion sources and conquering many troubles in the ion source and power supplies caused by a high voltage break-down in the accelerator. However, some issues remain to be solved concerning the ion source for increasing further the beam power and the beam energy. The most serious issue of them is non-uniformity of source plasma in the arc chamber. Various countermeasures have been implemented to improve the non-uniformity. Some of those countermeasures have been found to be partially effective in reducing the non-uniformity of the source plasma, and as the result the ion source, so far, has accelerated negative-ion beams of 17.4A at 403keV with deuterium and 20A at 360keV with hydrogen against the goal of 22A at 500keV. The neutral beam injection power into the plasma has reached 5.8MW at 400keV with deuterium. Further efforts to ...

The negative ion based neutral beam injection system for JT-60 has operated since 1996 injecting neutral beam into JT-60 plasmas. A power flow measurement in the beam line and ion source with a water calorimeter had shown that 40%–50% of accelerated beam particles were intercepted on the two accelerator grids and the grounded grid at an ion source gas pressure of 0.2–0.3 Pa. Much of the beam loss was not caused by stripping loss of the negative ions, but rather by direct impingement of the negative ions onto the grids. After reducing the acceleration area by masking the edge area (about 13% of the extraction area) of the accelerator grid so as to minimize the edge effect of magnetic field in the arc chamber, the loss in the accelerator decreased by roughly 25%. In comparing a deuterium beam with a hydrogen beam, the neutral beam power with deuterium is lower by 30% than that of hydrogen at the same arc power, although the heat load onto the grounded grid does not change so much. The power deposition ratio...

A beam injection experiment with the negative-ion based NBI system (N-NBI) started in March 1996 on JT-60U. After achieving the first neutral beam injection of 180 keV, ∼0.1 MW for 0.4 s into the JT-60U plasmas, the operation parameters of the ion source and power supply had been optimized for increasing the beam energy and beam current. In September 1996, a deuterium neutral beam of 2.5 MW at 350 keV was injected into JT-60U using two ion sources. In the operation with hydrogen at the beginning of 1997, a negative ion beam current of 18.4 A at 350 keV has been obtained, and a neutral beam of 3.2 MW at 350 keV for 1 s has been injected into the plasma with one ion source. A neutralization efficiency of negative ion beam has been confirmed to be about 60% at the beam energies of 250–385 keV as predicted theoretically.

A negative-ion-based neutral beam injector (N-NBI) has been constructed for JT-60U. The N-NBI is designed to inject 500 keV, 10 MW neutral beams using two ion sources, each producing a 500 keV, 22 A D- ion beam. In the preliminary experiment using one ion source, a D- ion beam of 13.5 A has been successfully accelerated with an energy of 400 keV (5.4 MW) for 0.12 s at an operating pressure of 0.22 Pa. This is the highest D- beam current and power in the world. Co-extracted electron current was effectively suppressed to the ratio of Ie/I D - < 1. The highest energy beam of 460 keV, 2.4 A, 0.44 s has also been obtained. To realize 1 MeV class NBI system for ITER (International Thermonuclear Experimental Reactor), demonstration of ampere class negative ion beam acceleration up to I MeV is an important mile stone. To achieve the mile stone, a prototype accelerator and a I MV, I A test facility called MeV Test Facility (MTF) were constructed. Up to now, an H- ion beam was accelerated up to the energy of 805 keV with an acceleration drain current of 150 mA for 1 s in a five stage electrostatic multi-aperture accelerator.

The negative ion based neutral beam injector heating system for JT-60U has contributed to core plasma heating and noninductive current drive experiments on JT-60U. For increasing further the beam power and beam pulse duration, the serious issue is improvement of source plasma nonuniformity in the ion source. Various countermeasures have been devised to solve the nonuniformity. The first is to adjust the spatial distribution of the arc discharge through regulating the arc current limiting resistors to be connected in series to each of the filament groups. The second is to change the arc discharge mode through controlling the filament temperature. These measures have been found to be very effective.

Significant progress in the extension of pulse durations of powerful negative ion beams has been made to realize the neutral beam injectors for JT-60SA and ITER. In order to overcome common issues of the long-pulse production/acceleration of negative ion beams in JT-60SA and ITER, new technologies have been developed in the JT-60SA ion source and the MeV accelerator in Japan Atomic Energy Agency.As for the long-pulse production of high-current negative ions for the JT-60SA ion source, the pulse durations have been successfully increased from 30 s at 13 A on JT-60U to 100 s at 15 A by modifying the JT-60SA ion source, which satisfies the required pulse duration of 100 s and 70% of the rated beam current for JT-60SA. This progress was based on the R&D efforts for the temperature control of the plasma grid and uniform negative ion productions with the modified tent-shaped filter field configuration. Moreover, each parameter of the required beam energy, current and pulse has been achieved individually by these R&D efforts. The developed techniques are useful to design the ITER ion source because the sustainment of the caesium coverage in the large extraction area is one of the common issues between JT-60SA and ITER.As for the long-pulse acceleration of high power density beams in the MeV accelerator for ITER, the pulse duration of MeV-class negative ion beams has been extended by more than 2 orders of magnitude by modifying the extraction grid with a high cooling capability and a high transmission of negative ions. A long-pulse acceleration of 60 s has been achieved at 70 MW m−2 (683 keV, 100 A m−2) which has reached the power density of JT-60SA level of 65 MW m−2. No degradations of the voltage holding capability of the acceleration voltage and the beam optics due to the distortion of the acceleration grids have been observed in this power density level.These results are the longest pulse durations of high-current and high-power-density negative ion beams in the world.

The pulse duration of the negative ion based NBI system has been extended from 10 s to 25 s to study long pulse plasmas on JT-60U. A feedback control technique has been demonstrated to keep the arc power constant by controlling the filament voltage for long pulse operation. Thus it was clearly observed that the negative ion beam current increased with the temperature of the plasma grid at constant arc power. A tapered filament is employed to improve its durability for the next operational campaign. Moreover, a high voltage holding test indicates that the reduction in the outgassing from the FRP (fiberglass-reinforced plastic) insulator may be a key to suppress the breakdowns in the ion source

The first results of the performance test of the large negative ion source for a JT‐60U negative‐ion‐based neutral beam injector (N‐NBI) are presented. The ion source consists of a cesium seeded multicusp plasma generator, where negative ions are produced via volume and surface processes, a 110 cm×45 cm multiaperture extractor, and a three‐stage electrostatic accelerator. After negative ion production and voltage holding tests in test stands, the ion source was installed in the N‐NBI system and the full power test began. Up to now, the ion source has produced 400 keV, 5.9 A (2.4 MW) D− ion beams, the world highest D− current and beam power, with a pulse duration of 0.1 s.

In the negative ion based NBI for JT-60, the maximum injection power reached so far is 5.2 MW at 350 keV. However the beam power and pulse duration are limited mainly by a mismatched perveance of the accelerated beam due to the source plasma non-uniformity. The source plasma uniformity was investigated by measuring the arc current distribution and ion saturation current in the plasma generator. A large non-uniformity has been found at the bottom side along a longitudinal direction in the ion source. As countermeasures against the source plasma non-uniformity, the adjustment of resistors in series with are current circuit connected to each of eight filament groups and the masking both fringes of the plasma grid are effective.

A 500 keV negative-ion based NBI system is under construction for NB current drive and plasma core heating in high density plasmas in JT-60U. Part of the beamline and the high voltage power supply required for a verification test of an ion source have completed in March 1995. After having done a high potential test of the power supply, the negative-ion generation and acceleration tests have started in June 1995 aiming at deuterium beams of 500 keV, 22 A. In initial experiment, deuterium negative-ion beams of 410 keV, 6.1 A (2.5 MW) for 0.2 sec, so far, have been achieved. This is the world highest D/sup -/ current and negative ion beam power. The construction of the total system will be completed by the beginning of 1996, and the beam injection will start in March 1996.

A 500 keV negative ion source for JT-60U and a 1 MeV ion source for ITER are being developed at JAERI. A beam acceleration test of the JT-60U negative ion source that is designed to produce a 500 keV, 22 A D beam for 10 s, has started. The ion source consists of a cesium seeded volume negative ion generator and a three-stage multi-aperture accelerator. Up to now, a D ion beam of 410 keV, 6.1 A, 0.2 s, 2.5 MW was accelerated. This is the world record of deuterium negative ion beam current and negative ion beam power. On the other hand, to demonstrate negative ion acceleration up to an energy of 1 MeV for ITER, we constructed a five-stage electrostatic accelerator and a 1 MV/1A test facility called "MeV Test Facility (MTF)". The accelerator was conditioned up to a high voltage of 760 kV without beam. The H ion beam was successfully accelerated up to the energy of 700 keV with a drain current of 230 mA for 1 s.

In order to investigate the origin of the non-uniformity of the source plasmas in the JT-60 negative ion source, the distribution of H + ions and H 0 atoms, which were converted to H − ions on the cesium covered surface, were measured by Langmuir probes and emission spectroscopy. Both of H + ions and H 0 atoms at the top and bottom of the ion source were about twice higher and lower than those in the center region, respectively. In particular, they were highly localized near the side wall of the top of the ion source. The analyses of the primary electron trajectories showed that this non-uniformity was due to the B x grad B drift of primary electrons emitted from filaments. These results suggest that the non-uniformity of the negative ions results in a locally poor beam optics and significant direct interception of the H − ions on the acceleration grid. This is the reason why the negative ion beams are non-uniform in the JT-60 negative ion source. c

Beam extraction and acceleration test in the Negative Ion Based Neutral Beam Injector for JT‐60U has been started using one ion source that is designed to produce a 500 keV, 22 A D− ion beam. Deuterium negative ions are produced in a cesium‐seeded semi‐cylindrical plasma generator and accelerated by a multi‐aperture three‐stage electrostatic accelerator. In the preliminary experiment of beam acceleration, the D− ion beam of 13.5 A was successfully accelerated to 400 keV for a pulse duration of 0.12 s. The negative ion beam power was 5.4 MW. The operating gas pressure in the plasma generator was as low as 0.22 Pa. The highest energy beam of 460 keV, 2.4 A, 0.44 s was also obtained. The ratio of extracted electron current to extracted negative ion current is estimated Ie/ID−

In the JT-60U negative-ion-based neutral beam injection system, the effect of negative ion and electron deflection by the plasma grid (PG) magnetic filter was studied. After a long-pulse operation of up to 19s, a local melting was observed on the beamline near the ion source, facing the electron drift side of the PG magnetic field. It is confirmed that the experimental deflection of the negative-ion beam agrees well with the three-dimensional beam simulation result by taking account of the measured magnetic field. By using the code, it is found that some stripped electrons produced in the first acceleration gap pass through the down pitch of the multiple apertures in the next stage of acceleration grids, and then collide on the beamline around the melted location.