Search

Voltage holding capability of a 500 kV, 22 A three-stage electrostatic accelerator, where large- area grids of 0.28 [m.sup.2] and large fiberglass reinforced plastic (FRP) insulators of 1.8 m in diameter are used, was examined. High voltage was independently applied to each acceleration stage, where the voltage holding capabilities of 130 kV were obtained. To identify whether the breakdowns occur in the gaps between the grids or the FRP insulators, high voltages were applied to the accelerator with and without the grids. Breakdown voltages without grids, i.e., the FRP insulator itself reached 170 kV of design value for each stage. These results show that the breakdown voltage of the accelerator was mainly determined by the gaps between the large-area grids. In this paper, the influence of non-uniform electric field and multi-stage grids on the voltage holding capabilities was also discussed. Index Terms--Electrodes, electrostatic accelerator, ion source, vacuum breakdown.

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. Index Terms--Beam optics, injection power, JT-60 Super Advanced (JT-60SA), negative ion beam, negative-ion-based NBI (N-NBI), neutral beam, voltage holding capability.

The power loading of electrons ejected from the negative ion accelerator to the beamline was first measured in the negative-ion-based neutral beam injector on JT-60U. At 0.3 Pa of the operating pressure in the arc chamber, the heat flux and the total power load for the single segment were ~8 W/[cm.sup.2] and 27 kW for the D- ion beam of 300 keV and 3.4 A, respectively. The normalized total power loading on the electron dump was no more than 2.6% of the electric power in the acceleration power supply. About 70% of the total power is originated by the electrons stripped from D- ions due to collisions with residual gas molecules in the accelerator. The calculation of the stripped electron trajectories shows that the electrons stripped in the second acceleration gap are the main origin of the power loading in the beamline. Index Terms--Electrons, JT-60U, negative ion sources, stripping losses.

This paper reviews recent progress in developing high current density ion sources for positive and negative halogen ion beams. These sources have produced [Cl.sup.-] current densities almost equal to their positive chlorine current densities, and also close to the current densities of [Ar.sup.+] beams extracted under similar conditions. The emittance of the CI and positive chlorine beams was at least as low as that of an [Ar.sup.+] beam extracted from the same source. The coextracted electron ratios (e/[Cl.sup.-]) as low as six-seven were much lower than would be expected from the mass ratio dependence of mobility, and appear to be due to the existence of a novel plasma near the extraction plane composed primarily of positive ions and negative ions with few electrons. The results of research across the past eight decades into discharges with electronegative gases and vapors lend insight into the processes occurring in recent experiments, which employed innovations developed in the magnetic fusion energy program to produce high current D beams. Index Terms--Electronegative plasmas, fusion reactors, ion accelerators, ion beam applications, ion-ion plasmas, ion sources, negative ions, neutral beams.

Voltage holding capability in a 500 keV electrostatic accelerator with large FRP insulators was examined without the beam acceleration. When high voltage was applied, the light with a broad peak at a wavelength of 420 nm was mostly emitted inside the accelerator even though breakdown did not occur. The voltage holding capability has a strong correlation with the light intensity. Stable voltage holding was realized under the conditions in which light emission was well suppressed. Index Terms--Insulators, ion sources, lightning, large-scale systems.

This paper reviews the operational phase of the negative-ion-based neutral beam systems on the JT-60U tokamak in Naka, Japan and the large helical device (LHD) stellarator in Toki, Japan. These systems were the first high power negative ion beam systems to be deployed for any application, and thus represented large advances in the state of the art for negative ion sources and accelerators, especially since the ions used were hydrogen and deuterium, which have only a modest electron affinity. This paper reviews the systems, the principal problems encountered, and the improvements they engendered, as well as the progress of these systems to the present time. The role of neutral beams in fusion is also discussed, and some of the contributions of the negative ion systems to the physics programs of JT-60U and LHD are briefly reviewed. These systems have been central to the success of JT-60U and LHD, and the knowledge gained about their characteristics should provide a strong basis for the development of the next generation of negative-ion-based neutral beams for ITER and other large fusion devices. Index Terms--Fusion reactors, ion accelerators, ion beam applications, ion sources, magnetic confinement, negative ions, neutral beams.