X-ray backlight measurement of preformed plasma by kJ-class petawatt LFEX laser.

Foot and pedestal pulses that precede the main pulse from a high-intensity laser greatly affect laser-plasma interactions. Especially in fast ignition schemes, preceding pulses generate a plasma prior to irradiation by the main pulse. This results in a too energetic and divergent electron beam being generated in the preformed plasma, which reduces the energy coupling efficiency from the heating laser to the dense fuel core. A preformed plasma with a density scale length of 40-60 μm was observed by a time- and space-resolved x-ray backlight technique using the LFEX laser system at the Institute of Laser Engineering, Osaka University. Preceding pulses (i.e., the foot and pedestal) of the LFEX were characterized by comparing observations with calculations results obtained using a two-dimension (2D) radiation-hydrodynamic simulation code. In a separate experiment, the 2D code was benchmarked with the experimentally observed hydrodynamic behavior of a gold plasma produced by a nanosecond laser pulse that mimicked foot and pedestal pulses (intensity: 1 ×10¹¹-1×10¹²W/cm²). The preceding pulses were estimated to have an intensity of 1 ×10¹²-10¹³W/cm², a duration of 2.0 ns, and a spot diameter at the target of 200-600 μm by comparing the measured hydrodynamics of the preformed plasma with that calculated by the 2D hydrodynamic simulation code. [ABSTRACT FROM AUTHOR]