Development of high performance negative ion sources and accelerators for MeV class neutral beam injectors

Abstract

Operation of accelerator at low pressure is an essential requirement to reduce stripping loss of the negative ions, which in turn results in high efficiency of the NB systems. For this purpose, a vacuum insulated beam source (VIBS) has been developed at JAERI, which reduces the gas pressure in the accelerator by enhanced gas conductance through the accelerator. The VIBS achieves the high voltage insulation of 1 MV by immersing the whole structure of accelerator in vacuum with long (~ 1.8 m) insulation distance. Results of the voltage holding test using a long vacuum gap of 1.8 m indicate that a transition from vacuum discharge to gas discharge occurs at around 0.2 Pa m in the long vacuum gap. So far, the VIBS succeeded in acceleration of 20 mA (H-) beam up to 970 keV for 1 s. The high voltage holding capability of the VIBS was drastically improved by installing a new large stress ring, which reduces electric field concentration at the triple junction of the accelerator column. At present the VIBS sustains 1 MV stably for more than 1200 s. Acceleration of ampere class Hbeams at high current density is to be started soon to demonstrate ITER relevant beam optics. Operation of negative ion source at low pressure is also essential to reduce the stripping loss. However, it was not so easy to attain high current density Hions at low pressure, since destruction cross section of the negative ions becomes large if the electron temperature is > 1 eV, in low pressure discharge. Using strong magnetic filter to lower the electron temperature, and putting higher arc discharge power to compensate reduction of plasma density through the filter, an Hion beam of 310 A/m 2 was extracted at very low pressure of 0.1Pa. This satisfies the ITER requirement of current density at 1/3 of the ITER design pressure (0.3 Pa).

A novel diagnostic technique for H−(D−) densities in negative hydrogen ion sources

“…Large negative ion sources are in operation at JT60-U [5,6] and LHD [7,8], representing roughly a 2/3 and 1/3 ITER size, respectively. These sources are filament driven arc sources, based on the so-called KAMABOKO type [9].…”

Acceleration of 100A/m2 Negative Hydrogen Ion Beams in a 1 MeV Vacuum Insulated Beam Source

“…The H 2 gas is considered to prevent the creeping discharge along the FRP insulator surface. It was found that the VIBS could sustain 1 MV under the presence of H 2 in the range of 0.07 -0.2 Pa [9,10]. This pressure range corresponds to that of the accelerator during the operation, and it was confirmed that the VIBS can sustain 1 MV stably for more than 2 hours under the H 2 gas feed of this range.…”

Design activities on helical DEMO reactor FFHR-d1

“…The beam heat loads on the accelerator grids and injection port limit the pulse length of the injection. The stripped electrons [60] are the main cause of the former heat load. Reduction of gas load in the accelerator and distribution of the electron load on the grids will be investigated.…”