Ion acceleration by laser hole-boring into plasmas

By experiment and simulations, we study the interaction of an intense CO2 laser pulse with slightly overcritical plasmas of fully ionized helium gas. Transverse optical probing is used to show a recession of the front plasma surface with an initial velocity >106 m/s driven by hole-boring by the laser pulse and the resulting radiation pressure driven electrostatic shocks. The collisionless shock propagates through the plasma, dissipates into an ion-acoustic solitary wave, and eventually becomes collisional as it slows further. These observations are supported by PIC simulations which prove the conclusion that monoenergetic protons observed in our earlier reported experiment with a hydrogen jet result from ion trapping and reflection from a shock wave driven through the plasma.