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1 MeV, Ampere Class Accelerator R&D For ITER
Abstract: The present objective of the 1 MeV electrostatic accelerator R&D for the ITER NB system is acceleration of ampere class negative ion beams up to the beam energy of 1 MeV. However, there was no attempt of such high current and high energy acceleration in the past. The JAERI MeV accelerator has been designed extrapolating available vacuum insulation design guidelines (the clump theory and Paschen law) to Mega Volt and long vacuum gap. It was effective to reduce electric field concentration at triple junction by a large stress ring to prevent flashover along insulator surface. By the vacuum insulation technology above, the accelerator sustained 1 MV for 8,500 s continuously. Operating the KAMABOKO source with high power arc discharge (≤40 kW), H- ion beams of 146 A/m2 (total ion current: 0.206 A) have been obtained stably at the beam energy of 836 keV (pulse length: ≥ 0.2 s). Strong enhancement of negative ion surface production has been attained by stopping vacuum leaks due to SF6 permeation through Viton and O ring damages by backstream ions, followed by increase of the H- ion current density toward 200 A/m2 without saturation. The paper reports recent progress of the accelerator development at JAERI. Bremsstrahlung generation in the accelerator is also estimated from EGS4 analysis, and then discussion on the breakdown possibility follows.
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