Impact of the pulse contrast ratio on molybdenum K generation by ultrahigh intensity femtosecond laser solid interaction.

We present an extended experimental study of the absolute yield of K 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 × 10-3.3 × 10 and on three decades of intensity 10-10  W/cm². We demonstrate that for intensity I ≥ 2 × 10  W/cm² K x-ray emission is independent of the value of contrast ratio. In addition, no saturation of the K photon number is measured and a value of ~2 × 10 photons/sr/s is obtained at 10 Hz and I ~10  W/cm². Furthermore, K energy conversion efficiency reaches the same high plateau equal to ~2 × 10 at I = 10  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.