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

By means of particle-in-cell numerical simulations, we investigate the emission of high-energy photons in laser-plasma interactions under ultrahigh-intensity conditions relevant to multi-petawatt laser systems. 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 by $10^{22}\,\rm W\,cm^{-2}$, $50 \, \rm fs$ laser pulses. Synchrotron emission is observed to be maximized for target thicknesses of a few $10 \, \rm nm$, close to the relativistic transparency threshold, and to be superseded by Bremsstrahlung in targets a few $μ$m thick. At their best efficiency, both mechanisms are found to radiate about one percent of the laser energy into photons with energies above $10\,\rm keV$. Their energy and angular spectra are thoroughly analyzed in light of the ultrafast target dynamics.