Laser ionization and heating of gas targets for long-scale-length instability experiments

This paper examines the use of gas targets to create low- and mid-Z plasmas ≊3 mm in size at 5% to 10% critical density for green and blue light (ne≊2×1020 to 1021 cm−3) with an electron temperature of several keV. At sufficiently high intensities (≊1014 W/cm2) the gas is ionized and heated by a laser absorption wave propagating faster than the sound speed. For pulses under 2 ns, the bulk of the plasma remains stationary, resulting in efficient heating minimizing density and velocity gradients, which are particularly important for instability thresholds in nonuniform plasmas. The propagation of a laser absorption wave in a preionized plasma is derived analytically. Ionization resulting from multiphoton and electron avalanche processes is studied by numerical methods and dimensional analysis. This establishes the length and time scales over which an absorption wave can be observed. Computer simulations, using the lasnex code, are presented for several implementations of this concept, applicable to the relevant regime for inertial confinement fusion. These examples include a gas jet (≊3×1 mm in cross section), 3 to 4 mm diameter gas balloons, and gas puffs generated by exploding a foil with a low-energy prepulse (≊200 J).