  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\%
