High proton ratio plasma production in a small negative ion source

Morishita, Takatoshi; Inoue, Takashi; Iga, Takashi*; Watanabe, Kazuhiro; Imai, Tsuyoshi

Negative ion beams of high current density are required for accelerator and fusion. The H source utilizes surface production that produces H from H or H. And hence, high proto yield ion source is required. Generally, a large volume plasma generator with strong plasma confinement is suitable to achieve high proton yield. On the contrary, production of high proton ratio plasma is not easy in small sources. However, in a small source (3.5 liter), high current H beam of 800 A/m was obtained. In this research, the proton ratio was investigated experimentally and analytically in a small source (1.4 liter). The measured proton ratio increased form 40% to 90% by applying the magnetic filter. From the numerical analysis, the proton ratio is low as 40% in the driver region. However, with the magnetic filter, flow of primary electrons is restrained, resulting in suppression of H production at the extraction region. In addition, molecular ions are easily destroyed by thermal electrons in the filter region. Thus the proton ratio is enhanced by the magnetic field in the small sources.