Thermal Instability of a Model Hydrogen Plasma

Abstract

The thermal stability of a hydrogen plasma is analyzed with the aid of a simple model atom which possesses a continuum of unbound states but only one bound level (the ground state). If ionization is collisional and recombination is radiative, the model plasma is thermally unstable if and only if its kinetic temperature exceeds 17300 K. Although this plasma emits only free-bound radiation (the free- free emission is neglected), the instability is caused by the same ionization effects which lead to instability in plasmas emitting line radiation. The instability criterion and the growth rate (if it is not too large) are not affected by the presence of a uniform magnetic field unless the motions are constrained to be perpendicular to the field. The applicability of equations of steady-state ionization to calculations of thermal instability is examined.