Measurement of the time-dependent distribution of ionization states in an argon plasma from a Mach-10 jet

Results from experiments designed to measure the time-dependent distribution of ionization states in an argon plasma produced by laser irradiation of a high-Mach-number gas jet are presented. The gas valve/nozzle target system was designed for nominal Mach-10 performance and produced a jet of neutral gas several centimeters long from a nozzle exit of 1.5 mm diameter. The jet was irradiated at a position having an approximate atomic density of 1 x 10(18)/cu cm by 500 J of 351-nm laser light with a pulse width of 600 ps at the NLUF's OMEGA laser facility. It was necessary to pre-ionize the gas in order to couple-in sufficient laser energy to produce a plasma. An argon plasma was then created that was dominated by charge states of Ar VIII through Ar XI. Comparison with a theoretical model which solved the time-dependent rate equations indicates that a plasma temperature of approximately 70 eV was achieved. A subsequent development effort carried out at the Lockheed Palo Alto Research Laboratory (LPARL) to improve the gas jet target system is also described. This work indicated that by increasing the pressure in the valve by a factor of 5 increased the coupling to the laser and produced significantly larger signals and higher plasma temperatures.