Plasma mirror reflectivity and focal spot quality for glass and aluminum mirrors for laser pulses up to 30 picoseconds*

The Advanced Radiographic Capability (ARC) short pulse laser at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is capable of producing a high energy,∼4kJ , laser pulse of 30 ps in a ∼100μm  focal spot, leading to an intensity of 10¹⁸ W/cm². For many applications, including radiography and warm dense matter creation, higher laser intensities are desirable to produce the necessary x-ray and particle fluxes. Due to the complexity of the NIF target chamber, 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 placed within the chamber, close to the target in an attempt to focus the four ARC beams. The behavior of plasma mirrors at these relatively long pulse durations (>10 ps) is not well characterized. The results from the COMET laser at the Jupiter Laser Facility (JLF) carried out at similar pulse durations show the deterioration of glass plasma mirrors’ focal spots at longer pulse durations and less degradation when using aluminum plasma mirrors.