Numerical Model of Laser Induced Fluorescence in a Hydrogen Plasma.

摘要

Discrepancies between observed and predicted level populations before and during laser induced fluorescence in a hydrogen plasma are partially resolved using improved numerical models. The plasma parameters investigated (.45 Torr < or = 1 eV) correspond to the z-pinch plasma produced in the LIF experiments. Using a rate equation model, it is found that the H- ion and the molecular species H2, H2(+), and H3(+) play important roles in such plasmas. The dissociative recombination of H2(+) and H3(+) preferentially populate the n = 3 and n = 2 atomic hydrogen levels, respectively. Vibrationally excited H2 produces H- via dissociative attachment, while the mutual neutralization of H- and H+ populates the n = 3 level. In addition, the initial z-pinch appears to cause electrons and ions to stream directly to the walls, where they remain attached for several hundred microseconds. This reduces the plasma density by over 50% and causes ambipolar diffusion to overshadow recombination in the electron density decay. With these additional species and processes, many of the experimental observables, both before and during laser illumination, are reproduced. The agreement with experiment is further improved by a kinetics equation model which confirms the importance of radiation trapping and its role in producing a hot, non-Maxwellian bump in the electron distribution in the vicinity of 10 eV.