Characterisation of plasma mirrors on the HELEN laser infra red CPA beam

摘要

The performance of plasma mirrors has been characterised on the HELEN laser infra-red, chirped pulse amplification [CPA] beam line. This laser produced pulse energies up to 100J with pulse lengths of ～500fs. Plasma mirrors are initially low reflectance surfaces that transmit low intensity light but produce a reflecting plasma surface when exposed to high irradiance beams. Typically they are formed by transparent substrates at the laser wavelength and have been used either uncoated or with anti-reflection coatings. The coatings evaluated in these experiments were either multi-layer dielectrics or single layer sol-gel silica. Some of the fused silica substrates were coated on both faces, others were coated on the incident face only and a small number were used uncoated. The reflectance of the plasma mirrors was measured as a function of incident energy. A vacuum compatible pyro-electric sensor in conjunction with either a diffuser or neutral density filter was used to measure incident and reflected laser energy. Both the diffuser and filter could suffer laser damage at the highest incident energies available. The morphology of the damage of the different components and coating combinations was studied as a function of incident beam energy. The mirrors were being investigated to prevent pre-pulse effects in plasma physics experiments and increase the intensity contrast ratio of the laser beam incident onto solid targets. Their proximity to the laser target also allowed them to block debris and shrapnel arising from the laser matter interaction in some directions. These material emissions spread uncontrollably in the evacuated target chamber and may cause contamination of laser optics and filters or radiation diagnostic instrumentation. The plasma mirror components were operated at 45 degrees angle of incidence and an average input beam diameter of 5.5 millimetres at the mirrors with incident beam irradiances in the range 50 TW/cm~2 to 540 TW/cm~2. The reflected beams were focussed on to 10 micron thick, 10mm diameter metal foils and 3mm diameter polymer/metal targets with a thickness of 2 to 18 microns. The subsequent debris and shrapnel effects were studied using post shot microscopy and photography.