Scattering-induced downstream beam modulation by plasma scalded mirrors

Abstract

Previous work concluded that plasma scalds on laser-conditioned multilayer dielectric mirror coatings are a stable, benign damage morphology. Recent large-aperture measurements indicate that plasma scalds may lead to fratricide of down-stream optics by increasing beam contrast. This paper describes the results of measurements performed to examine the effect of quasi-periodic plasma scalds covering the entire clear aperture on downstream beam modulation. A collimated, linearly-polarized 1053-nm beamline was constructed that irradiated approximately 5 cm2 of the plasma scalded region. This beam was propagated ~8 meters and sampled with a 10-bit, megapixel CCD camera and analyzed for contrast (peak/average intensity). A lineout across the sample was built up by translating the optic across the beam. The contrast results were compared to a baseline wedged flat with surface figure of λ/100 and a contrast adder for the plasma scalds calculated. This was defined by. In all, optics with average plasma scald fractions of 0.9, 2.3, 4 and 14% were measured. Preliminary results indicate that plasma scald fractions of 4% and below contribute a contrast adder of less than 2.5%.