High-energy high-average power pulsed hf/df chemical laser

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The compact, repetitively pulsed HF chemical laser built 2 years ago has been modified to increase the output laser energy. The discharge volume is now 1.4 1 instead of 0.54 1 for the previous electric discharge. Due to high voltage limitation, the discharge gap was only slightly increased. The discharge width along the flow direction was increased from 4 to 8 cm. To achieve an easily usable beam, a novel optical configuration consisting of two passes with 2 folding mirrors has been used. It gives a 35 X 35 mm2 square output beam. To date, a maximum output laser energy up to 12 J per pulse was obtained at the HF wavelength using C2H6 as the hydrogen donor. At DF wavelength the energy per pulse was about 3 J using D2. In repetitive operation, the repetition rate was limited to about 65 Hz due to the available flow rate of the gas loop. The average power obtained was 610 W.

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We describe results of experiments on optically pumped HF and DF overtone transitions that use a specially developed tunable and narrow-line Cr4+:forsterite laser. The performance of this Cr4+:forsterite laser is first reported. Cascading laser transitions were generated in DF(3–2) and DF(2–1) bands between 3.64 and 3.85 μm. The experimental results are compared with the theoretical predictions of a computer model.
High-power chemical lasers operating in high repetitive rate show a decrease of the output energy laser beam. In such lasers, the characteristic time which depends on the laser output is short in comparison with those related to the flow. Consequently, shock waves, acoustic waves and thermal perturbations, induced by the strong electric energy deposition and remaining in the laser cavity between two pulses, may explain the decrease of output energy of the laser beam. For a better understanding of the flowfields, a numerical approach is carried out using flux corrected transport algorithms (FCT methods) associated with a Riemann solver on the computational domain boundaries. Under two-dimensional assumptions, the inviscid flow in the convergent-divergent laser cavity is computed to describe the creation and propagation of the wave system and the hot gas column in both single and multidischarge operating modes. Distortions of the contact surfaces are put into evidence through the study of flowfield instabilities. Finally, the limitations of the two-dimensional modelization become apparent. The numerical resolution is extended to a 3D case in order to take into account the optical direction. This allows to study the influence of shock waves travelling between optics and being generated by a side effect developing at the electrodes. These waves have an effect of long duration on the flowfield and lead to a high residual perturbation level.
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The discharge equilibrium and the F-atom production kinetics in a phototriggered HF laser using gas mixtures containing Ne and with hydrogen or ethane have been investigated. Coupled experimental and theoretical studies have been carried out, through a 0D discharge modelling and measurements of the current, of the voltage and of the extracted laser energy, together with emission spectroscopy on F 3p excited states. A quantitative comparison of the total F-atom density produced in mixtures of with or is performed and the results are compared with the discharge pumped HF laser performance achieved with these molecules. An overall validation of the model has been obtained, both for the energy transfer to the active media and for the kinetics of F and its excited states. It is shown that the transmitted energy and electrical charge, as well as the discharge electric field, weakly depend on the type and on the concentration of the RH molecule, or , in the gas mixtures. Moreover, the F-atom production depends very slightly on the nature of RH. Therefore the fact that the laser performance achieved with hydrogen is lower than the performance achieved with ethane is not due to differences in the energy transfer and in the F-atom production kinetics in mixtures of with or .
The characteristics of the radiation emitted by an electric-discharge pulsed chemical HF laser with a discharge gap of 10 cm are studied. The discharge was stabilised by a semiconducting ferroelectric ceramic layer deposited on plane metal electrodes. The specific energy and technical efficiency were 3 J L-1 and 3.4 %, respectively, for a laser operating on a nonchain reaction in SF6 - H-2 mixture and 25 J L-1 and 26 %, respectively, for a laser operating on a chain reaction in F-2 - O-2 - SF6 - H-2 mixture.
The spatial dynamic of phototriggered discharges in SF<sub>6</sub> and Ne/SF<sub>6</sub> mixtures has been studied through gated intensified charge-coupled device camera measurements correlated to current and voltage measurements. Physical mechanisms involved in discharge instabilities, which limit performance of discharge pumped HF/DF lasers using H<sub>2</sub> or D<sub>2</sub>, are under investigation. It Is shown that the high cathodic emission intensity and the discharge bulk glow are separated by a pronounced dark space. Arcs develop at a time which depends upon parameters such as the SF<sub>6 </sub> pressure and the current pulse duration
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