Beam extraction under Cs-free conditions in HUST negative ion source

Abstract

A radio-frequency driven negative hydrogen ion source is under development at Huazhong University of Science and Technology, which aims to investigate the physics of production and extraction of the negative ions. The source operational parameters are investigated to research the source plasma characteristics and optimize the extraction properties before cesium injection, which means negative ions were mostly produced by reactions in the plasma volume in this study. The magnetic filter field generated by a PG (plasma grid) current flowing through the plasma grid can cool down the electron temperature below 1 eV, but simultaneously increased the plasma density in the extraction region. This plasma asymmetry in the direction perpendicular to the PG filter field, is discussed and compared with that in other ion sources. As the PG current increased, the asymmetry factors tended to increase first and then decrease. Biasing the plasma grid positively with respect to the source body can reduce the co-extracted electrons effectively, resulting in reducing the electron to H− ion current ratio. An extracted negative beam with a current density of about 19 A/m2 and an energy of 12 keV was achieved under 20 kW RF power and 0.3 Pa filling pressure, with a ratio of electron current to ion current of about 54.