  A physical and numerical model of the Townsend discharge in molecular
hydrogen and deuterium has been developed to meet the needs of designing a
plasma-based switching device for power grid application. The model allows to
predict the low-pressure branch of the Paschen curve for applied voltage in the
range of several hundred kiloVolts. In the regime of interest, electrons are in
a runaway state and ionization by ions and fast neutrals sustains the
discharge. It was essential to correctly account for both gas-phase and surface
interactions (electron emission and electron back-scattering), especially in
terms of their dependence on particle energy. The model yields results
consistent with prior data obtained for lower voltage. The three-species
(electrons, ions and fast neutrals) model successfully captures the essential
physics of the process.
