Discharge instabilities initiated by nonuniform laser extraction in electron-beam sustained discharge KrF lasers

Abstract

The photon energy of KrF lasers (λ=248 nm) is sufficiently large to photodetach electrons from the halogen negative ion (F−) and to photoionize excited states of krypton. In electron-beam (e-beam) sustained discharge KrF lasers, the contribution of photogenerated electrons to the total rate of ionization can be large enough to significantly perturb the conductivity of the plasma. If laser oscillation is spatially nonuniform, the photoelectron contribution to the conductivity deforms the local electric field. This condition can ultimately lead to a local discharge ionization instability and an arclike termination of the discharge. In this paper, the effect of nonuniform optical extraction on the discharge stability of an e-beam sustained discharge KrF laser will be studied with results from a two-dimensional time-dependent discharge kinetics and laser extraction model. Examples of stable and unstable configurations will be presented. The time at which the arclike ionization instability terminates the discharge correlates with the time that multistep ionization becomes an appreciable fraction of the total rate of ionization . A criterion for discharge stability is proposed; gradients in conductivity caused by photogenerated electrons must be either perpendicular or parallel to the bulk electric field.