Numerical analysis of primary electrons in a tandem-type negative ion source

Kato, Kyohei*; Takado, Naoyuki*; Hatayama, Akiyoshi*; Hanada, Masaya; Seki, Takayoshi; Inoue, Takashi

To clarify physics mechanism of plasma spatial nonuniformity observed in tandem-type negative-ion sources, primary electron-transport process has been analyzed by a three-dimensional Monte Carlo simulation code. In the model, equations of motion for electrons are numerically solved. Geometry and magnetic-field configuration of the JAEA 10 Ampere negative ion source are taken into account. Various collision processes with neutral particles are also included in the model. The simulation results show that (1) the primary electrons have been lost from the source region to the extraction region due to magnetic drift in the magnetic filter, and then (2) there is another magnetic drift near the sidewalls, where a sum of magnetic field of the filter and the cusp field for plasma confinement allows electron drift towards the extraction region. A sequence of these magnetic drifts would increase the electron temperature in local area of extraction region, which resulted in loss of negative ions.