Modeling of Negative Ion Velocity Distribution in Hydrogen Negative Ion Source Plasmas

负离子温度是氢负源最重要的参数之一，它影响光束光学特性，决定了负离子束发散的下限。为了解影响源内h -速度分布和h -温度（平均动能）的物理机制，对h的产生、输运和破坏过程进行了数值模拟。为了计算h -生产剖面，已经开发了中性（氢分子和原子）的三维（3D）蒙特卡罗代码。该代码考虑了生产氢的各种重要反应。对于h输运和破坏，也开发了三维蒙特卡罗代码。考虑了与背景H+离子的库仑碰撞的动量和能量弛豫过程以及与背景H+和电子的破坏反应。应用这两种规范分析了典型低压条件下（气体压力~几mTorr，放电电压~ 50V）大H源“卡门贝尔ⅲ”的H速度分布。数值结果定性地再现了实验观察结果，即存在热（~ 1eV）和冷（~ 1eV）两个h离子能族。库仑碰撞对冷h离子的形成起重要作用，而等离子体势的加速对热h离子的形成起重要作用。此外，还建立了双激光光剥离技术测量h离子温度的一维PIC模型。结果表明，激光发射形成的双极势影响了测量结果，在高h离子密度下，双激光光剥离技术推导出的h温度往往大于实际温度。

The negative ion temperature is one of the most important parameters of hydrogen negative sources, since it affects the beam optics and determines the lower limit of negative ion beam divergence. To understand the physical mechanism which dominates the H-velocity distribution and H-temperature (average kinetic energy) inside the source, numerical modeling of H-production, transport and destruction processes has been done.In order to calculate H-production profile, a three dimensional (3D) Monte Carlo code for neutrals (hydrogen molecules and atoms) has been developed. The code takes into account the various important reactions for H- production. As for the H-transport and destruction, a 3-D Monte Carlo code has been also developed. Momentum and energy relaxation processes by Coulomb collisions with back ground H^+ ions are taken into account as well as the destruction reactions with back ground H+ and electrons. These two codes are applied to the analysis of the H- velocity distribution in the large H-source "Camembert III" under the typical low pressure conditions (gas pressure 〜a several mTorr, discharge voltage〜50V). The numerical results qualitatively reproduce the experimental observation, i.e., two energy groups of H-ions : hot (〜1eV) and cold (〜1eV) H-ions are present. Coulomb collisions play an important for the formation of cold H-ions, while the acceleration by the plasma potential is responsible for the formation of hot H-ions.In addition, 1D PIC modeling of the measurement of H-ion temperature by two-laser photodetachment technique has been done. It is shown that the ambipolar potential formed by the laser shot affects the measurement, and H-temperature deduced by two-laser photodetachment technique tends to be larger than the actual temperature in high H-ion density regime.