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

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)×{}10¹² cm^-3, electron temperature {~}2-4 eV] is studied with the aid of a simple model for the neutral-particle balance and H_α emission. This model includes molecular and atomic reactions, wall interactions, and nonthermal electrons. A large fraction of the H_α is produced by dissociative excitation of H₂ and dissociative recombination of H₂⁺, while the remainder is produced by excitation of H atoms, most of which have energies that are close to the characteristic H₂ dissociation energies. The H_α linewidth is thus insensitive to the discharge operating conditions and equals {~}0.27 {} when only slow ({~}0.3 eV) dissociatively excited atoms are present or {~}0.35 {} when fast (> 1 eV) atoms, apparently also produced in dissociation reactions, are present as well.