New efficient mechanism of laser-driven ion acceleration based on the previously described regime of relativistic self-trapping (RST) of a high-power ultrashort laser pulse in near-critical-density plasma has been presented. Upon reaching the rear target surface, a charged up to 10–30 nC cavity, which is formed under RST due to cavitated electrons, experiences a directed Coulomb explosion (DCE).  This is the dominant process in ion acceleration. DCE is also accompanied by a boost in ion energy due to the charge-separation field from the cavity-accelerated electrons leaving the target.  The latter is manifested in somewhat increase in the maximum energy of ions.  As a result, DCE produces protons with the energies up to 30–35 MeV for a 2.2 J laser pulse. This corresponds to a record maximum proton energy per joule of incoming laser energy. The total laser energy conversion efficiency into MeV protons is approximately 2%. The mechanism does not require ultrahigh laser contrast, which is an important advantage over the competitive ion acceleration from ultrathin foils.

Figure.  Illustration of a directed Coulomb explosion during relativistic self-trapping of a laser pulse.

A.J. Castillo, S.G. Bochkarev, M.G. Lobok, V.Yu. Bychenkov,
JETP Letters 124, issue 4 (2026)