On the Efficiency of Nonchain Electric-Discharge HF (DF) Lasers

Abstract

An important operating parameter of any laser which determines the field of application for the laser is its efficiency. Therefore, to increase the efficiency is a practically significant problem. Nonchain electric-discharge HF (DF) lasers have been studied for a long time, and high energies and average radiation powers have been achieved with an electrical efficiency of ~3–4% [1–5]. However, no work has been performed with the goal to increase the efficiency of nonchain HF (DF) lasers and to reveal the factors that affect the efficiency. The maximum electrical efficiency of an HF laser (up to 6.3%) has been obtained for mixtures of elegas with hydrogen pumped with short (20 ns) pulses from a strip line; the pulse energy was not over 0.14 J [6]. For SF6–С2Н6 mixtures, a specific lasing energy of about 9 J/l has been obtained with an electrical efficiency of up to 4.7% [7]. Earlier we have shown that if the discharge is maintained homogeneous, the intrinsic efficiency of an HF laser can be as high as up to 10% over a wide range of the conditions of its initiation [8]. In the work under consideration, the pumping parameters have been determined that provide the highest efficiency of electric-discharge nonchain HF and DF lasers, and the processes that affect the lasing efficiency are considered. Nonchain HF and DF lasers with a radiation pulse energy over 1 J and an electrical efficiency of up to 6 and 5%, respectively, have been developed and built for the first time. For pumping the lasers, inductor or LC generators were used; the capacitance of the primary capacitive store was С0 = 13–370 nF; the store was charged to 35 kV. The design of the laser and the measurement techniques are described in detail elsewhere [8]. The following factors that mainly affect the operating efficiency of HF (DF) lasers have been revealed: 1. Uniformity of the electric field in the discharge gap of the laser. When the uniformity of the field was violated, the efficiency of the laser was not over 1% even for mixtures with pentane, and integrated photos of the discharge showed numerous spark channels. 2. Illumination. Illumination of gas mixtures of SF6 with H2 (D2) and C5H12 with uv radiation increases the lasing energy of a nonchain laser. The effect of illumination becomes especially pronounced as the pressure of the mixture is increased. 3. The pumping pulse duration should not be over ~100 ns. In our experiments, as the duration of the discharge current in mixtures with hydrogen was increased from 100 to 250 ns, the radiation energy was observed to halve because of the constriction of the discharge. A reduced stability of the discharge and a decrease in lasing efficiency with increasing the duration of the discharge current was observed earlier for mixtures with hydrocarbons [5] and deuterium [2]. 4. Composition of the working mixture and the specific input energy. It has been revealed that if the conditions of the formation of a volumetric discharge are violated, the lasing energy and efficiency of a nonchain laser are higher for mixtures with hydrocarbons. However, when the pumping conditions are optimized, the highest efficiency of HF and DF lasers is achieved for mixtures of SF6 with H2 (D2). It has been found that the maximum intrinsic efficiency of an HF laser, ηint = 9–10%, was achieved with an input energy Ein = 30–70 J/l (the specific radiation energy was Ql ~ 3–7 J/l) and a storage capacitance С0 = 13–39 nF. Further increasing Ein and С0 reduced the lasing efficiency. For a DF laser, the intrinsic