  Ion acceleration using a laser pulse irradiating a thin disk target is
examined using three-dimensional and two-dimensional particle-in-cell
simulations. A laser pulse of $620$ TW, with an intensity of $5\times 10^{21}$
W/cm$^{2}$ and a duration time of $27$ fs is irradiated on a double-layer
target. Simulations are performed by varying the ion density, i.e., electron
density, of the first layer. It is shown that the obtained proton energy jumps
at a certain density of the first layer, which is made of a "light" material
such as carbon; that is, Coulomb explosion of the target and radiation pressure
acceleration act effectively above a certain density. Moreover, even at the
same electron density, the reflection of the laser pulse from the first layer
is small for a "light" material.

Plasma Physics