High-efficiency γ-ray flash generation via multiple-laser scattering in ponderomotive potential well.

γ-ray flash generation in near-critical-density target irradiated by four symmetrical colliding laser pulses is numerically investigated. With peak intensities about 10^{23} W/cm^{2}, the laser pulses boost electron energy through direct laser acceleration, while pushing them inward with the ponderomotive force. After backscattering with counterpropagating laser, the accelerated electron is trapped in the electromagnetic standing waves or the ponderomotive potential well created by the coherent overlapping of the laser pulses, and emits γ-ray photons in a multiple-laser-scattering regime, where electrons act as a medium transferring energy from the laser to γ rays in the ponderomotive potential valley.