Importance of pre-ionisation for the non-chain discharge-pumped HF laser

The importance of pre-ionisation for the non-chain discharge-pumped HF lase r is studied through experiments on an X-ray photo-triggered laser using mi xtures of Ne, SF6, and ethane. The discharge dynamic in Ne/SF6 mixtures or pure SF6, as well as the stabilisation effect induced by C2H6 and consequen ces for the laser performance, are investigated for pre-ionisation electron density values, n(eo), ranging from 10(6) cm(-3) up to 10(9) cm(-3), as we ll as for the so-called discharge self-breakdown mode. Without ethane, the minimum n(eo) value which is needed to complete 100% homogeneous charge dep osition in the plasma is a very sharply increasing function of the SF6 pres sure. This hinders performance optimisation when the molecule used to react with F-atoms, for instance H-2, has no effect on the discharge dynamic. Th e minimum ethane partial pressure that:is needed to stabilise the discharge depends on n(eo), the pumping pulse duration, the deposited electric charg e, and the SF6 pressure. Discharges in Ne/SF6 can be much more efficiently stabilised by addition of a small amount of ethane than by an increase of n (eo). A pre-ionisation density as low as 10(6) cm(-3) is sufficient to achi eve the maximum laser energy value, but total suppression of the pre-ionisa tion has a detrimental effect on the active medium homogeneity.