We present an extended experimental study of the absolute yield of Kα x-ray source (17.48 keV) produced by interaction of an ultrahigh intensity femtosecond laser with solid Mo target for temporal contrast ratios in the range of 1.7 × 107–3.3 × 109 and on three decades of intensity 1016–1019  W/cm². We demonstrate that for intensity I ≥ 2 × 1018  W/cm² Kα x-ray emission is independent of the value of contrast ratio. In addition, no saturation of the Kα photon number is measured and a value of ~2 × 1010 photons/sr/s is obtained at 10 Hz and I ~1019  W/cm². Furthermore, Kα energy conversion efficiency reaches the same high plateau equal to ~2 × 10−4 at I = 1019  W/cm² for all the studied contrast ratios. This original result suggests that relativistic J × B heating becomes dominant in these operating conditions which is supposed to be insensitive to the electron density gradient scale length L/λ. Finally, an additional experimental study performed by changing the angle of incidence of the laser beam onto the solid target highlights a clear signature of the interplay between collisionless absorption mechanisms depending on the contrast ratio and intensity.

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Amplification of ultrashort laser pulses is demonstrated in a gas medium of the photolytical XeF(C–A) laser. A gas mixture of XeF2:N2:Ar was excited by vacuum-ultraviolet radiation produced by a high-current multichannel sliding discharge, which was initiated along one side of dielectric laser cavity. A small-signal gain of 1.6×10−3 cm−1 is observed for a seed pulse at 488 nm with ∼150 fs duration. The use of a second optical source together with the device optimization promise a multiple increase in the gain and the obtaining of high-contrast TW femtosecond pulses as a result of direct amplification, without application of a stretching/compression technique.