Key issues for the filament-arc ion source of EAST neutral beam injector toward high-power and long-pulse operation

Abstract

Neutral beam injection is an effective plasma heating and current drive method for magnetic confinement devices. For supporting the high-confinement plasma operation of the fully superconducting tokamak EAST, a high-power and long-pulse neutral beam is foreseen to be injected into EAST. To achieve such a mission, the most important task is to develop a qualified ion source which can launch a positive ion beam for pulse length up to 100 s with the beam power larger than 2 MW. During the research and development on the EAST positive ion source, several key issues have been studied, including the heat load on electrode grids, the backstreaming electrons deposition, and the gas discharge efficiency. The capability of heat removal of the electrode grids has been demonstrated via a series of simulations and experiments. However, the backstreaming electrons deposition was extremely dense on the electron dump of the ion source, leading to its serious melting damage. According to the simulation of the trajectories of backstreaming electrons, the central magnets on the electron dump were removed as a preliminary solution. The melting damage was successfully avoided without the central magnets and the modified ion source has been operated without malfunction until now. But the total deposition of backstreaming electrons on the electron dump was increased and the magnetic confinement of primary electrons was reduced. Consequently, the arc efficiency was lower and the fraction of the neutral atoms with half beam energy was higher for the modified ion source.