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For the ITER neutral beam system, reasearch and development of a vacuum-insulated MeV accelerator has been carried out at Japan Atomic Energy Research Institute. After a successful 1 MV insulation in vacuum for more than 2 h, H--ion-beam acceleration test is in progress to fulfill the ITER requirement of the current density of 200 A/m2 at 1 MeV, with good beam optics quality of <7 mrad divergence. For such a high-current-density H--ion-beam acceleration, the Kamaboko negative-ion source was operated at high input power, with increased filament number and magnetic filter strength. The optimum perveance was investigated by scanning the extraction and acceleration voltages, together with the beam divergence measurement. As a consequence, H--ion beams of 146 A/m2 (total ion-beam current: 206 mA) were obtained at 836 keV, of which power density is in the “ITER relevant” level. Such high-power-density beams with a divergence as low as 5 mrad were obtained at the optimum perveance corresponding to that of the ITER (200 A/m2 at 1 MeV). [ABSTRACT FROM AUTHOR]

A plasma neutralization efficiency of high-energy negative ion beams was measured in a practical neutralizer with substantial dimensions and compared with a gas neutralization efficiency for fusion application. Hydrogen negative ion beams of 200 keV, 4 mA were neutralized with hydrogen and argon arc plasmas in the plasma neutralizer of 2 m in length and 0.6 m in diameter. To clarify the difference between the plasma and the gas neutralizations, the high-density plasmas were produced at low operating pressure. By means of ∼30 G transverse magnetic fields that were applied locally at openings for beam path, leakage of the primary electrons to the outside of the neutralizer was suppressed sufficiently. The plasma densities were 1011–1012 cm-3 at low operating pressures of 0.002–0.05 Pa for both plasmas while an arc power was ∼40 kW. Neutralization efficiency was increased by using plasmas whose ionization degree was 10%–15%. The maximum neutralization efficiency for hydrogen plasma increased from 55% of the gas neutralization to 65%. The maximum neutralization efficiency for argon plasma increased from 51% of the gas neutralization to 55%. The line densities maximizing the plasma neutralization efficiencies decreased to 65% of those for the gas neutralizations. Neutralization efficiencies in the plasmas and the gases were in good agreement with those calculated from the cross sections. It is concluded that a practical plasma neutralizer with substantial dimensions can increase neutralization efficiency at a low line density as predicted from the numerical analysis based on the cross section data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Development of an electrostatic accelerator for 1 MeV, 1 ampere class H- ion beams has been carried out as a Proof-of-Principle (PoP) of high current accelerators required in neutral beam system of fusion reactors such as ITER. A unique feature of such fusion oriented accelerators is to utilize vacuum insulation not only inside but also outside of the accelerator for high voltage insulation, since conventional gas insulation is not applicable due to excess radiation induced conductivity in the gas. The PoP accelerator is surrounded by an insulator stack as a vacuum boundary with a vacuum gap of 50 mm all around the accelerator. One of the key technologies to achieve insulation of MV voltage was reduction of electric field strength at triple junction of the insulator (made of FRP, fiber reinforced plastic) stack. By lowering the stress to 1.7 kV/mm, the accelerator sustained 1 MV stably for more than 2 hours. By tuning the ion source with/without cesium under stable accelerator operation, the PoP accelerator generated H- ion beams of 70 mA at 1 MeV, and 110 mA (80 A/m2) at 900 keV stably for several hundreds shots (pulse length, 0.5–1 s limited by the target). The dark current flowed inside/on the surface of the FRP insulator due to photoelectron effect was estimated to be less than μA in case of 0.1 A electron beam acceleration (maximum current observed in the experiment). This is in the same order to that due to conductivity of the insulator material. So far, high voltage breakdown due to photoelectron has never been observed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines hydrogen negative ion beam acceleration using a five-stage, multiaperture electrostatic accelerator. Development of a high power negative ion source/accelerator for a neutral beam injector; High current density beam acceleration; Heat load in the accelerator.

Abstract: This paper reports R&D progress of the MeV class accelerator, beam focussing technique and a high voltage (HV) bushing toward the ITER Neutral Beam system at JAEA. A H− ion beam of 320mA (current density: 140A/m2) was successfully accelerated up to 796keV by fixing air leak in the ion source chamber caused by heat loading due to backstream positive ions. From analyses of a three-dimensional beam trajectory, a countermeasure against beamlet deflection due to space charge repulsion among beamlets was identified to compensate the deflection by proper aperture offset in the electron suppression grid. For development of the HV bushing, a half-size (Ø800mm) mockup single stage bushing was manufactured and tested, that demonstrated the dc voltage holding capability of up to 220kV in a condition with vacuum inside. Electrostatic analyses and design of the HV bushing are in progress, in which some design improvements are suggested to reduce electric field strength on the surface of ceramic insulator. [Copyright &y& Elsevier]