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Abstract. The importance of pre-ionisation for the non-chain discharge-pumped HF laser is studied through experiments on an X-ray photo-triggered laser using mixtures of Ne, SF[sup 6], and ethane. The discharge dynamic in Ne/SF[sup 6] mixtures or pure SF[sup 6], as well as the stabilisation effect induced by C[sup 2]H[sup 6] and consequences for the laser performance, are investigated for pre-ionisation electron density values, n[sup eo], ranging from 10[sup 6] cm[sup -3] up to 10[sup 9] cm[sup -3], as well as for the so-called discharge self-breakdown mode. Without ethane, the minimum n[sup eo] value which is needed to complete 100% homogeneous charge deposition in the plasma is a very sharply increasing function of the SF[sup 6] pressure. This hinders performance optimisation when the molecule used to react with F-atoms, for instance H[sup 2], has no effect on the discharge dynamic. The minimum ethane partial pressure that is needed to stabilise the discharge depends on n[sup eo], the pumping pulse duration, the deposited electric charge, and the SF[sup 6] pressure. Discharges in Ne/SF[sup 6] can be much more efficiently stabilised by addition of a small amount of ethane than by an increase of n[sup eo]. A preionisation density as low as 10[sup 6] cm[sup -3] is sufficient to achieve the maximum laser energy value, but total suppression of the pre-ionisation has a detrimental effect on the active medium homogeneity. PACS: 42.55.K; 52.80 [ABSTRACT FROM AUTHOR]

Abstract. A detailed experimental study of an X-ray photo-triggered HF laser, with an active volume of 312 cm[sup 3], has been performed in Ne/SF[sub 6]/H[sub 2] and Ne/SF[sub 6]/C[sub 2]H[sub 6] mixtures. Parameters involved have been the storage line capacitance and the circuit inductance, the capacitors charging voltage, the RH-molecule type and partial pressure, and the X-ray dose for the preionization. High laser performance has been achieved with C[sub 2]H[sub 6]: an output energy up to 3 J corresponding to a specific energy of 9.6 J/1 at an efficiency of 4.7%, which strengthens the advantage of the photo-triggering technique to energize high-power HF lasers. However the optimum performance achieved with H[sub 2], 5.75 J/l and 3.5%, are lower. It is shown, through a time-resolved study of the electrical discharge and spatial dynamics correlated to laser power and energy measurements, that discharge instabilities are responsine for the poor laser performance of the mixture with H[sub 2]. These instabilities, which lead to are development, are characteristics of the discharge in Ne/SF[sub 6]. It is demonstrated for the first time that addition of a heavy hydrocarbon, such as C[sub 2]H[sub 6], to that mixture induces the discharge stabilization so that the laser emission arises in a homogeneous active medium. This effect allows us to achieve better laser performance than with H[sub 2]. [ABSTRACT FROM AUTHOR]