The efficacy of heating low-pressure H2 in a microwave discharge

The relative ease with which a low-pressure hydrogen stream may be heated in an electrical discharge suggests that such a system be considered in current efforts to develop thrusters for spacecraft orbit raising purposes. In this work a detailed model of a microwave discharge in flowing, low-pressure hydrogen is used to interpret and clarify experimental measurements of atom concentration, electron energy, and electron density. The radially averaged, constant-pressure model accurately reproduces the experimental data and also calculated the rates of a number of gas-heating and wall-heating processes as well as rates of energy deposition into coolant and working fluid streams. The calculated gas-heating rates indicate that the gas heating is due primarily to the thermalization of the energetic atoms produced by dissociation of H₂ via excitation of theb ³∑ _u ⁺ state. The calculations also indicate that the energy flux to the quartz tube is significantly influenced by Lyman and Werner band radiation and by heterogeneous atomic recombination processes and, to a much lesser degree, by electron-ion recombination processes. The fraction of power input which is ultimately transferred to the gas stream is a decreasing function of the power input and varies from 0.24 to 0.12.