Measurement and modeling of molecular ion concentrations in a hydrogen reflex-arc discharge

The concentrations of the ions H+, H-2(+), and H-3(+) are measured in a weakly-ionized hydrogen plasma with electron temperatures T-e=3-7 eV, electron densities N-e=10(11)-10(12) cm(-3), and background gas densities N-H2=5x10(13)-10(15) cm(-3). The relative ion concentrations are measured using an omegatron-type mass spectrometer, while the electron density and temperature are measured using a Langmuir probe together with absolutely-calibrated Halpha emission spectroscopy. The mean lifetime of ions in the plasma is estimated from fixed wall probe measurements. From the measured parameters, the expected steady-state molecular ion concentrations are modeled using the relevant rate balance equations. It is found that the observed molecular ion concentrations can be predicted within the experimental accuracy using currently available collision cross sections. (C) 2002 American Institute of Physics.