We have measured angularly resolved and absolutely calibrated spectra of the multi-MeV electrons produced by relativistic self-channeling in a high-density gas jet. Using 200 fs laser pulses with $P_L=1.2\mathrm{}\mathrm{TW}$, we have investigated the electron spectrum dependence on the plasma electron density in the range of $3\times {10}^{19}–4\times {10}^{20}{\mathrm{cm}}^{-3\mathrm{}}$. The experimentally obtained results are closely reproduced by three-dimensional particle-in-cell simulations. A detailed analysis shows that the self-modulated laser wake field, although active, cannot explain the experimental energy spectrum. The bulk of the fast electrons are produced by direct laser acceleration at the channel betatron resonance.

• Received 17 June 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.4772