Cavity mirrors of high power lasers are easily damaged by heat absorption of laser beam, so there is an urgent need to detect the absorption coefficient of the coating layers on them. High power combustion drived lasers, which operate with large size cavity mirrors, high gas consumption and short running time, cannot offer a convenient source of radiation for studies of absorption coefficient. Other kinds of lasers cannot achieve the high power density required easily. To solve this problem, in this paper, a small-scale and cost-effective laser source is described to detect the absorption coefficient under high power density with a Quasi-Closed Cavity. This laser source is rebuilt on the basis of a 1000W-class Direct Current (DC)-discharge drived continuous-wave (CW) HF/DF chemical laser. At first, the structure of the laser source is introduced. Then, some performance parameters are measured and the experiment results are analyzed. The laser operated with a (He+NF3)+D2 gas mixture, and output of about 126W with a transmission of 3% was achieved through the experiment, corresponding to an electrical efficiency of about 3%. Power density on the surfaces of mirrors under test reached 3.74 kW/cm2 . It satisfies the requirement of the Quasi-Closed Cavity test well. Experimental results show that this improved DC-discharge drived CW HF/DF chemical laser is applicable as the laser source to detect the absorption coefficient under high power density.
Combustion-driven continuous wave (CW) DF/HF chemical lasers cannot be inflamed successfully sometimes because the spark-plug-igniter is intolerant of ablation especially after long-time operation which deeply affected the reliability of the lasers. In this paper, a pre-igniter is designed as a new igniter system to produce F2 to solve the problem. Based on the engineering practices and the principle that high-intensity spontaneous combustion will happen when mixing F2 and H2. The results of NF3 and H2 reacting with different mole ratios were calculated by CEA software. The operation reliability of the pre-igniter, the mole concentration of F2 in the mixing gas, and the equilibrium temperature were validated by a series of experiments. The experimental results were consistent with the calculated data: with the mole ratio of NF3 to H2 increasing, the equilibrium temperature decreased gradually and finally leveled off; the mole concentration of F2 in the mixing gas first increased and then decreased, achieving the maximum of about 40% when the mole ratio of NF3 to H2 was about 3.2. Experimental results outlined that the pre-igniter performed reliability and could produce high output of F2. The ignition system with a pre-igniter and a spark plug could provide a new alternative for combustion-driven CW DF/HF chemical lasers.