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.
Modeling of a negative ion source. II. Plasma-gas coupling in the extraction region
The production, destruction, and transport of H− in the extraction region of a negative ion source are investigated with a 1D(z)-3V particle-in-cell electrostatic code. The motion of charged particles (e, H+, H2+, and H−) in their self-consistent electric field is coupled with the neutral particles [H(n=1) and H2(X1∑g+,v=0,…,14)] dynamics and vibrational kinetics of H2. Neutral influxes into the domain are determined by the simulation of the expansion region. Surface and volumetric processes involving plasma and neutrals have been included by using different Monte Carlo collision methods. Calculations show the influence of the plasma grid bias and of the magnetic filter on the plasma parameter profiles. In particular, a transition from classical to complete reverse sheath is observed using a positively biased plasma grid. The influence of the magnetic filter is small. The importance of the hot-atom mechanism on the surface negative ion production is shown.
Effects of the energy distribution of fast electrons on H2 vibrational excitation in a tandem negative ion source
The population distribution of vibrationally excited hydrogen molecules H2(v‘), and the H− production are investigated theoretically by solving numerically a set of particle balance equations in a steady‐state pure hydrogen plasma. In particular, the enhancement of the H2(v‘) distribution is discussed for different energy distributions of fast electrons ef. Whether the energy distribution for ef is a delta function type or a plateau type, collisional excitation of H2(v‘) caused by ef is very effective to enhance the H− production and a resultant vibrational distribution becomes the plateau distribution. Besides these, if ef with energies higher than 30–40 eV are present, both H2(v‘) excitation and then H− yield hardly depend on the form of the energy distribution of ef.