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Optically pumped rare gas lasers have the potential to provide high-power output with excellent beam quality. They lase at wavelengths that are readily transmitted by the atmosphere, and their optical characteristics are closely similar to those of diode-pumped alkali vapor lasers (DPAL’s). As compared to DPAL’s they present two significant advantages. The first is that they use entirely inert reagents that are gases at ambient temperature (Helium plus a heavier rare gas). As there is no chemistry involved, these devices can be operated in a completely closed-cycle mode. A second advantage is derived from the energy level structures of metastable rare gas atoms. While the sp transitions of alkali metals will provide just one laser wavelength for a given element, each rare gas atom can offer a range of different output wavelengths from the first manifold of sp transitions. The primary technical challenge for the optically pumped rare gas laser is the requirement to generate the lasing medium (metastable Rg(3P2) where Rg=Ne, Ar, Kr or Xe) at a density of approximately 1e13 cm-3 in the presence of helium at total pressures in the range 0.5 – 1.0 atm. For this application we have developed a quasi-CW discharge driven by a high repetition rate power supply. The frequency and time duration of the pulses are tailored to exploit the non-steady high field characteristics of the pulsed breakdown, while sustaining a temporally steady argon metastable concentration in the gain medium. This system has been scaled to a 912.3 nm output of ~4 W when pumped by a 20 W diode laser. Recently, the pulsed discharge system has been improved by increasing the upper limit for the voltage available from the power supply. The present system operates at voltages up to 2200 V with a nominal pulse duration of 50 ns and pulse repetition frequency of 100 kHz. This has been used to sustain a discharge in 1 atm of a He/Ar mixture that produces an Ar* metastable density of 2e13 cm-3, a path length of 3 cm and a total volume of 1.2 cm3 (0.64x0.64 cm2 cross section). A diode pumped Ar* laser that employed this discharge has been operated for extended periods with no sign of performance degradation. As expected for a system that uses only inert gases, there was no indication of window damage or chemical activity. Computational models indicate that scaling to the 100 kW level is feasible with a discharge volume of 10 cm3 and discharge power of 200 W.
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