Two-dimensional simulations of the transition from Townsend to glow discharge and subnormal oscillations

The transition from Townsend to glow discharge is studied via two-dimensional simulations of discharges with moderate pd values corresponding to the right branch of the Paschen curve (p is the gas pressure and d the electrode spacing). The discharge model is coupled to the external circuit model enabling simulations of subnormal oscillations during the discharge transition from the Townsend to normal regime. This transition is accompanied by radial constriction of the discharge and formation of a normal cathode spot. The electron thermal flux backscattered to the cathode is observed to be important in the Townsend regime, which contributes significantly to discharge instability by virtue of the secondary emission coefficient gamma becoming an increasing function of E/N. The simulations are carried out for different gas pressures and different parameters of the external circuit. The predicted results are in agreement with experimental observations and previously published semi-analytical models.