Plasma Mirror Focal Spot Quality for Glass and Aluminum Mirrors for LASER Pulses up to 20 picoseconds

High intensity short pulse lasers are being pushed further as applications continue to demand higher laser intensities. Uses such as radiography and laser-driven particle acceleration require these higher intensities to produce the necessary x-ray and particle fluxes. Achieving these intensities however is limited by the damage threshold of costly optics and the complexity of target chambers. This is evidenced by the Advanced Radiographic Capability (ARC) short pulse laser, at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory, producing four high energy ≈1 kJ laser pulses at 30 ps pulse duration being limited to an intensity of 10^18 W/cm^2 by the large focal spot size of ≈ 100μm. Due to the setup complexity of NIF, changing the location of the final focusing parabola in order to improve the focal spot size is not an option. This leads to the possible use of disposable ellipsoidal plasma mirrors (PMs) placed within the chamber, close to the target in an attempt to refocus the four ARC beams. However, the behavior of plasma mirrors at these relatively long pulse durations (tens of ps) is not well characterized. The results from the COMET laser at the Jupiter Laser Facility (JLF) carried out at 0.5 to 20 ps pulse durations on flat mirrors are presented as a necessary first step towards focusing curved mirrors. The data shows defocusing at longer pulse durations and higher intensities, with less degradation when using aluminum coated mirrors