Bright synchrotron radiation from relativistic self-trapping of a short laser pulse in near-critical density plasma

In a dense gas plasma a short laser pulse propagates in a relativistic self-trapping mode, which enables the effective conversion of laser energy to the accelerated electrons. This regime sustains effective loading which maximizes the total charge of the accelerating electrons, that provides a large amount of betatron radiation. The three-dimensional particle-in-cell simulations demonstrate how such a regime triggers x-ray generation with 0.1-1 MeV photon energies, low divergence, and high brightness. It is shown that a 135-TW laser can be used to produce 3×1010 photons of >10 keV energy and a 1.2-PW laser makes it possible generating about 1012 photons in the same energy range. The laser-to-gamma energy conversion efficiency is up to 10-4 for the high-energy photons, ∼100 keV, while the conversion efficiency to the entire keV-range x rays is estimated to be a few tenths of a percent.