Balmer-alpha emission and hydrogen-atom energy in ion-source discharges

Abstract

The structure of the hydrogen Balmer-alpha line emission profiles from three types of neutral-beam-injector ion-source plasmas [filling pressure ∼10 mTorr, electron density ∼(1-2)×${10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$, electron temperature ∼2-4 eV] is studied with the aid of a simple model for the neutral-particle balance and ${\mathrm{H}}_{\alpha }$ emission. This model includes molecular and atomic reactions, wall interactions, and nonthermal electrons. A large fraction of the ${\mathrm{H}}_{\alpha }$ is produced by dissociative excitation of ${\mathrm{H}}_2$ and dissociative recombination of ${\mathrm{H}}_2^{+}$, while the remainder is produced by excitation of H atoms, most of which have energies that are close to the characteristic ${\mathrm{H}}_2$ dissociation energies. The ${\mathrm{H}}_{\alpha }$ linewidth is thus insensitive to the discharge operating conditions and equals ∼0.27 Å when only slow (∼0.3 eV) dissociatively excited atoms are present or ∼0.35 Å when fast (> 1 eV) atoms, apparently also produced in dissociation reactions, are present as well.