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

We present an extended experimental study of the absolute yield of K(α) 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(7)–3.3 × 10(9) and on three decades of intensity 10(16)–10(19) W/cm². We demonstrate that for intensity I ≥ 2 × 10(18) W/cm² K(α) x-ray emission is independent of the value of contrast ratio. In addition, no saturation of the K(α) photon number is measured and a value of ~2 × 10(10) photons/sr/s is obtained at 10 Hz and I ~10(19) W/cm². Furthermore, K(α) energy conversion efficiency reaches the same high plateau equal to ~2 × 10(−4) at I = 10(19) 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.