The interaction of a multipicosecond, kilojoule laser pulse with a surface of a solid target has been shown to produce electrons with energies far beyond the free-electron ponderomotive limit $m_e{c}^2{a}_0^2/2$. Particle-in-cell simulations indicate that an increase in the pulse duration from 1 to 10 ps leads to the formation of a low-density shelf (about 10% of the critical density). The shelf extends over $100\mu \mathrm{m}$ toward the vacuum side, with a nonstationary potential barrier forming in that area. Electrons reflected from the barrier gain superponderomotive energy from the potential. Some electrons experience an even greater energy gain due to ponderomotive acceleration when their “dephasing rate” $R=\gamma -p_x/m_{e}c$ drops well below unity, thus increasing acceleration by a factor of $1/R$. Both 1D and 2D simulations indicate that these mechanisms are responsible for the generation of extensive thermal distributions with $T_e>10\mathrm{MeV}$ and a high-energy cutoff of hundreds of MeV.

• Received 28 August 2014

DOI:https://doi.org/10.1103/PhysRevLett.116.155001