Synchrotron radiation from ultrahigh-intensity laser-plasma interactions and competition with Bremsstrahlung in thin foil targets

The emission of high-energy photons is one of the major effects of relativistic laser-plasma interactions, which underpins a wide range of applications, from plasma diagnostics to radiography to nuclear physics and quantum electrodynamics studies. In the case of solid targets, such emission is usually dominated by Bremsstrahlung and radiative transitions of excited ions, yet one expects prolific synchrotron emission to kick in and eventually prevail at high enough laser intensities, such as those contemplated at various facilities under construction worldwide. In this paper, by means of advanced, self-consistent particle-in-cell numerical simulations, we present a detailed analysis of x-ray and gamma-ray radiation under previously unexplored interaction conditions, involving ultrathin targets partially transparent to the laser light, yet accessible to multipetawatt laser systems during their early operation phase. We first examine the characteristics of synchrotron radiation from laser-driven plasmas of varying density and size. In particular, we show and explain the dependence of the angular distribution of the radiated photons on the transparency or opacity of the plasma. We then study the competition of the synchrotron and Bremsstrahlung emissions in copper foil targets irradiated with 10(22) Wcm(-2), 50-fs laser pulses. Synchrotron emission is observed to be maximized for target thicknesses of a few tens of nanometers, close to the relativistic transparency threshold, and to be superseded by Bremsstrahlung in targets a few microns thick. At their best efficiency, both mechanisms are found to radiate about 1% of the laser energy into photons with energies above 10 keV. Their energy and angular spectra are thoroughly analyzed in light of the ultrafast target expansion, the influence of which has been overlooked so far. Our results demonstrate that even using solid materials of relatively high atomic number and not-so-extreme laser pulse intensities, synchrotron radiation can be a strongly dominant and efficient source of energetic photons provided the targets are thin enough.