THE INTERACTION OF ULTRASHORT, POWERFUL LASER-PULSES WITH A SOLID TARGET - ION EXPANSION AND ACCELERATION WITH TIME-DEPENDENT AMBIPOLAR FIELD

The problem of ion expansion when a powerful subpicosecond laser pulse is incident upon a solid target is considered in conditions when the mean electron energy in the plasma is the function of time. It is shown that the problem can be solved for the simple case in which the electron has a power-law dependence with time during the laser pulse. The solutions which are obtained are significantly different from the well-known case of isothermal expansion. The ion density profile is much steeper and consequently the ion energy distribution has a very steep gradient in the high-energy tail. For the case when v(te)/v(os) < 1 (v(te) and v(os) are the thermal and oscillation velocities of electrons, respectively) profile steepening due to the dynamics of the expansion dominates over that caused by the ponderomotive force. The problem considered is relevant to laser-matter interaction experiments where the electron energy has a strong dependence on time during the laser pulse. Some comparisons with recent experiments are presented.