Development of High Performance Negative Ion Sources and Accelerators for MeV Class Neutral Beam Injectors

摘要

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 H~- beams 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 H~- ions 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 H~- ion beam of 310 A/m~2 was extracted at very low pressure of 0.1 Pa. This satisfies the ITER requirement of current density at 1/3 of the ITER design pressure (0.3 Pa).