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

负离子温度是氢负离子源的重要参数之一，它影响着束流的光学特性，决定着负离子束发散角的下限。为了理解H-速度分布和H-温度（平均动能）的物理机制，我们对H-产生、输运和破坏过程进行了数值模拟，为了计算H-产生分布，我们开发了中性粒子（氢分子和氢原子）的三维Monte Carlo程序。该代码考虑了H-产生的各种重要反应。对于氢的输运和破坏，还开发了一个三维蒙特卡罗程序。考虑了与背景H^+离子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。这两个程序被应用于分析大型氢源Camembert III在典型低压条件下（气压为几毫托，放电电压为50伏）的氢速度分布。数值结果定性地再现了实验观察，即，存在两个能量组的H离子：热（101 eV）和冷（101 eV）H离子。库仑碰撞对冷氢离子的形成起重要作用，而等离子体势的加速作用对热氢离子的形成起重要作用。此外，还对双激光光剥离技术测量氢离子温度进行了一维PIC模拟。结果表明，在高氢离子浓度区，激光作用形成的双极电位影响测量结果，双激光光剥离技术推导的氢离子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.