Effect of Laser Group Velocity on Maximum Attainable Ion Energy in the Radiation Pressure Dominant (RPD) Regime

The effect of group velocity of the incident laser pulse, which being smaller than the vacuum speed of light, plays an important role in laser-driven ion acceleration as it limits the value of the maximum attainable velocity of the ultrathin solid density foil target. This effect significantly alters the radiation pressure acceleration (RPA), and the maximum energy gained by the ions as well. This article presents our analytical and numerical results showing the effect of laser group velocity on the frequency of the reflected wave, and the energy transfer to the accelerated ions. The other important factor that limits the transfer of laser energy to the ions is the transverse expansion of the foil. We make a comparative study of these two limiting factors on the transfer of laser energy to the dense foil.