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The temperature control system of the large-size plasma grid has been developed to realize the long pulse production of high-current negative ions for JT-60SA. By using this prototype system for the JT-60SA ion source, 15 A negative ions has been sustained for 100 s for the first time, which is three times longer than that obtained in JT-60U. In this system, a high-temperature fluorinated fluid with a high boiling point of 270 degree Celsius is circulated in the cooling channels of the plasma grids (PG) where a cesium (Cs) coverage is formed to enhance the negative ion production. Because the PG temperature control had been applied to only 10% of the extraction area previously, the prototype PG with the full extraction area (110 cm x 45 cm) was developed to increase the negative ion current in this time. In the preliminary results of long pulse productions of high-current negative ions at a Cs conditioning phase, the negative ion production was gradually degraded in the last half of 100 s pulse where the temperature of an arc chamber wall was not saturated. From the spectroscopic measurements, it was found that the Cs flux released from the wall might affect to the negative ion production, which implied the wall temperature should be kept low to control the Cs flux to the PG for the long-pulse high-current production. The obtained results of long-pulse production and the PG temperature control method contributes the design of the ITER ion source. [ABSTRACT FROM AUTHOR]

R&D of high power density negative ion beam acceleration has been carried out at MeV test facility in JAEA to realize ITER neutral beam accelerator. The main target is H- ion beam acceleration up to 1 MeV with 200 A/m² for 60 s whose pulse length is the present facility limit. For long pulse acceleration at high power density, new extraction grid (EXG) has been developed with high cooling capability, which electron suppression magnet is placed under cooling channel similar to ITER. In addition, aperture size of electron suppression grid (ESG) is enlarged from 14 mm to 16 mm to reduce direct interception on the ESG and emission of secondary electron which leads to high heat load on the upstream acceleration grid. By enlarging ESG aperture, beam current increased 10% at high current beam and total acceleration grid heat load reduced from 13% to 10% of input power at long pulse beam. In addition, heat load by back stream positive ion into the EXG is measured for the first time and is estimated as 0.3% of beam power, while heat load by back stream ion into the source chamber is estimated as 3.5 ~ 4.0% of beam power. Beam acceleration up to 60 s which is the facility limit, has achieved at 683 keV, 100 A/m² of negative ion beam, whose energy density increases two orders of magnitude since 2011. [ABSTRACT FROM AUTHOR]

We have developed the integrated 2D PIC code for the analysis of the negative ion beam optics, in which an overall region from the source plasma to the accelerator is modeled. Thus, the negative ion trajectory can be solved self-consistently without any assumption of the plasma meniscus form initially. This code can reproduce the negative ion beam halo observed in an actual negative ion beam. It is confirmed that the surface produced negative ions which are extracted near the edge of the meniscus can be one of the reasons for the beam halo: these negative ions are over-focused due to the curvature of the meniscus. The negative ions are not focused by the electrostatic lens, and consequently become the beam halo. [ABSTRACT FROM AUTHOR]

A meniscus of plasma-beam boundary in H- ion sources largely affects the extracted H- ion beam optics. Recently it is shown that the beam halo is mainly caused by the meniscus, i.e. ion emissive surface, close to the plasma grid (PG) where its curvature is large. The purpose of this study is to clarify the effect of H- surface production rate on plasma meniscus and beam halo formation with PIC (particle-in-cell) modeling. It is shown that the plasma meniscus and beam halo formation is strongly dependent on the amount of surface produced H- ions. [ABSTRACT FROM AUTHOR]

Acceleration of a 500 keV beam up to 2.8 A has been achieved on a JT-60U negative ion source with a three-stage accelerator by overcoming low voltage holding which is one of the critical issues for realization of the JT-60SA ion source. In order to improve the voltage holding, preliminary voltage holding tests with small-size grids with uniform and locally intense electric fields were carried out, and suggested that the voltage holding was degraded by both the size and local electric field effects. Therefore, the local electric field was reduced by tuning gap lengths between the large size grids and grid support structures of the accelerator. Moreover, a beam radiation shield which limited extension of the minimum gap length was also optimized so as to reduce the local electric field while maintaining the shielding effect. These modifications were based on the experiment results, and significantly increased the voltage holding from <150 kV/stage for the original configuration to 200 kV/stage. These techniques for improvement of voltage holding should also be applicable to other large ion sources accelerators such as those for ITER. [ABSTRACT FROM AUTHOR]

This paper describes the development of the neutral beam (NB) systems on JT-60SA, where 30-34 MW D0 beams are required to be injected for 100 s. A 30 s operation of the NB injectors suggests that existing beamline components and positive ion sources on JT-60U can be reused without the modifications on JT-60 SA. The JT-60 negative ion source was modified to improve the voltage holding capability, which leads to a successful acceleration of 2.8 A H- ion beam up to 500 keV of the rated acceleration energy for JT-60SA. [ABSTRACT FROM AUTHOR]

Voltage holding of -1 MV is an essential issue in development of a multi-aperture multi-grid (MAMuG) negative ion accelerator, of which target is to accelerate 200 A/m2 H- ion beam up to the energy of 1 MeV for several tens seconds. Review of voltage holding results ever obtained with various geometries of the accelerators showed that the voltage holding capability was about a half of designed value based on the experiment obtained from ideal small electrode. This is considered due to local electric field concentration in the accelerators, such as edge and steps between multi-aperture grids and its support structures. Based on the detailed investigation with electric field analysis, accelerator was modified to reduce the electric field concentration by reshaping the support structures and expanding the gap length between the grid supports. After the modifications, the accelerator succeeded in sustaining -1 MV for more than one hour in vacuum. Improvement of the voltage holding characteristics progressed the energy and current accelerated by the MeV accelerator. Up to 2010, beam parameters achieved by the MAMuG accelerator were increased to 879 keV, 0.36 A (157 A/m2) at perveance matched condition and 937 keV, 0.33 A (144 A/m2) slightly under perveance. [ABSTRACT FROM AUTHOR]

In the ITER NB, conventional gas insulated beam source (GIBS) cannot be utilized because of the radiation-induced conductivity of the insulation gas. Thus a vacuum insulated beam source (VIBS), where the whole beam source is immersed in vacuum, has been developed at JAERI. Recently, voltage holding capability of the VIBS was drastically improved by the large stress ring, which reduces the electric field concentration at the triple junction. Up to now, a high current density H- beam of 102 A/m2 (140 mA) at 800 keV has been accelerated. The beam acceleration was quite stable and accomplished for several hundreds shots. The degradation of the voltage holding due to the beam acceleration and/or Cs seeding was not observed. Thus the development of vacuum insulated beam source has solved technical issues of high voltage insulation of 1 MV level under the presence of the H- ion beams. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]