High Efficiency Gamma-Ray Flash Generation via Multiple Compton   Scattering
  Gamma-ray flash generation in near critical density (NCD) 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 counter-propagating laser, the accelerated electron is trapped in the optical lattice or the electromagnetic standing waves (SW) created by the coherent overlapping of the laser pulses, and emits gamma-ray photons in Multiple Compton Scattering regime, where electrons act as a medium transferring energy from the laser to gamma-rays. The energy conversion rate from laser pulses to gamma-ray can be as high as 50\% 
Z. Gong, R.H. Hu, Y. R. Shou, B. Qiao, C. E. Chen, X. T. He, S. S.   Bulanov, T. Zh. Esirkepov, S. V. Bulanov, X. Q. Yan
physics.plasm-ph