AbstractIn the fast ignitor scheme, an ultra intense laser is applied to accelerate an electron in a solid density plasma. When the laser photon pressure exceeds the plasma pressure, the ultra intense laser starts to accelerate the ion. In a relatively low intensity region, it is found that a shock wave is formed and high energy ions are produced due to the reflection at the shock front, in contrast with the high intensity case where the laser energy is directly converted to the ion kinetic energy. This transition of the ion acceleration mechanism results from the excess of the Mach number of the shock over its critical number. In this paper, the transition mechanism of ion acceleration and energy conversion efficiency are discussed.