CATHODE SHEATH FORMATION IN A DISCHARGE-SUSTAINED XECL LASER

A one-dimensional longitudinal model of a XeCl discharge including the cathode region, the plasma, and the external circuit has been used for conditions close to experiments for 50-100 ns laser pulse durations and electron power deposition in the MW/cm3 range in a 300 cm3 chamber. This model provides the space and time variations of the electric field, electron and positive-ion densities in the cathode region, as well as the time variations of the charged-particle densities and excited species concentrations in the plasma obtained with a simplified kinetic model. Results show that under normal conditions of operation the cathode electric field can reach values as high as several 10(6) V/cm. The influence of photoemission and secondary emission due to ion impact on the cathode is discussed. A transition from capacitive to resistive behavior of the sheath is seen to occur rapidly due to the multiplication of cathode-emitted electrons in the large sheath electric field, leading to a fast increase in the ion conduction current in this region. The possible mechanisms of streamer formation in the sheath are also discussed. Results are presented for different values of the secondary electron emission coefficient due to ion impact from the cathode and it is shown that under conditions of low secondary emission (on the order of 10(-2) or less), the electrical and chemical behavior of the plasma can be strongly affected by the presence of the sheath. Finally, the possibility of discharge instabilities related to the sheath evolution is briefly discussed on the basis of the numerical results.