Growth of thermal constrictions in a weakly ionized gas discharge in helium

Neutral and charged particle densities, radial velocities, and temperatures are calculated as functions of time for cylindrical discharges in helium at pressures from 1 to 1000 Torr and a radius of 1 cm. Ionization, recombination, ambipolar diffusion, specific heat of the neutrals, and variation of gas pressure in space and time are included. Ionization of excited states, energy loss by radiation, and space charge sheaths are neglected. Under most conditions, increased ionization in regions of decreased gas density causes a constriction of the discharge which is accentuated at high pressures and high recombination rates. Greater circuit resistance stabilizes the discharge. At low pressures, the electron density profile expands. Initial steep gradients in electron density cause constrictions off axis.