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 negative ions. The source operational parameters are investigated to investigate the source plasma characteristics and to optimize the extraction properties before cesium injection, which means, in this study, that negative ions were mostly produced by reactions in the plasma volume. The magnetic filter field generated by a plasma grid (PG) current flowing through the PG can cool down the electron temperature below 1 eV, but simultaneously increased the plasma density in the extraction region. The 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 PG 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 m−2 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.
华中科技大学正在研发一种射频驱动的负氢离子源，旨在研究负离子的产生与提取物理过程。本文通过改变离子源运行参数，研究离子源等离子体特性，并在注入铯之前优化提取性能，即本研究中负离子主要产生于等离子体体相反应。由等离子体栅极（PG）电流产生的磁滤场可将电子温度降至1 eV以下，但同时会提高提取区的等离子体密度。文中讨论了垂直于PG滤场方向上的等离子体不对称性，并与其他离子源进行了比较。随着PG电流增大，不对称因子呈先增大后减小的趋势。相对于离子源本体对PG施加正偏压，可有效抑制共提取电子，从而降低电子与H⁻离子的电流比。在射频功率为20 kW、充气压力为0.3 Pa的条件下，实现了能量为12 keV、电流密度约为19 A m⁻²的负离子束提取，电子与离子电流比约为54。