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 fo r 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 fi eld, electron and positive-ion densities in the cathode region, as wel l as the time variations of the charged-particle densities and excited species concentrations in the plasma obtained with a simplified kinet ic model. Results show that under normal conditions of operation the c athode electric field can reach values as high as several 10(6) V/cm. The influence of photoemission and secondary emission due to ion impac t on the cathode is discussed. A transition from capacitive to resisti ve behavior of the sheath is seen to occur rapidly due to the multipli cation of cathode-emitted electrons in the large sheath electric field , leading to a fast increase in the ion conduction current in this reg ion. The possible mechanisms of streamer formation in the sheath are a lso discussed. Results are presented for different values of the secon dary electron emission coefficient due to ion impact from the cathode and it is shown that under conditions of low secondary emission (on th e order of 10(-2) or less), the electrical and chemical behavior of th e plasma can be strongly affected by the presence of the sheath. Final ly, the possibility of discharge instabilities related to the sheath e volution is briefly discussed on the basis of the numerical results.