Velocity Distribution of $\hbox{H}^{-}$ Ions in Low Temperature Hydrogen Plasma
A Monte Carlo solution of the Boltzmann equation is applied to the calculation of the velocity distribution of negative atomic ions H- in uniform H/H2 plasmas in a wide parameter range. Results are shown for the energy distribution n(epsiv), the 2-D velocity distribution, and the energy-dependent coefficients of the spherical harmonics expansion of its angular part, for different values of the reduced electric field E/N, the atomic fraction, and the H+ mole fraction. By increasing E/N or the atomic fraction at constant ion energy, the velocity distribution becomes highly anisotropic, and at same time, n(epsiv) features a suprathermal tail consistent with analytical results.
Monte Carlo modeling of gas phase ion transport under thermal gradients and external fields
A null collision Monte Carlo method is proposed to calculate the transport kinetics of charged particles in a non equilibrium background of atoms or molecules. The method is an implementation of the null collision Monte Carlo method of Skullerud, extended in order to include the translational distribution of neutrals, also in non uniform cases. As such, it delivers an exact solution of the linear Boltzmann equation. Two equivalent formulations of the method are proposed: an inductive one based on physical considerations, and a more formal one in which the method is deduced from the appropriate transport equation. As a test case, the method is applied to the thermal diffusion of H 3 + ions diluted in H 2 under an isobaric thermal gradient and with a transversal electric field.
Numerical studies on the optimisation of volume-produced H- ions in the single-chamber system
Optimisation of volume-produced H- ions is studied by using a set of particle balance equations in a steady-state hydrogen plasma with a single-chamber system. The dependence of production of both H- ions and vibrationally excited hydrogen molecules H2* (vibrational level V) on plasma parameters (i.e. electron temperature Te, electron density ne, hydrogen gas pressure p, density ratio nfe/ne of fast primary electrons ef to slow plasma electrons e, energy of fast electron Efe, etc.) is explored because it is expected that H- ions are produced by the following two-step process, i.e. H2+ef to H2*(V)+Ef; H2*(V)+e to H-+H. Particular attention is also paid to wall effects, i.e. neutral particles-wall interaction, on H- production. So, a wall recombination coefficient gamma 1 for H and a wall de-excitation collision parameter gamma 2 for H2*(V) are treated as numerical parameters. It is confirmed that most H- ions are produced by the above-mentioned two-step process, and that the presence of ef with energies in excess of 40 eV is reasonable for H2*(V) production. With increasing Te (above 1 eV), H- yield decreases monotonically. Besides, ne, nfe/ne and p have some optimum values for H- production. However, the optimum condition for H- formation is not compatible with that for H2*(V) production. Another significant point is that the ion species ratios depend strongly on the wall parameters, i.e. gamma 1 and gamma 2. For H- production, the optimum condition is that gamma 1 approximately=1 and gamma 2<<1.