In-depth Plasma-Wave Heating of Dense Plasma Irradiated by Short Laser Pulses

We investigate the mechanism by which relativistic electron bunches created at the surface of a target irradiated by a very short and intense laser pulse transfer energy to the deeper parts of the target. In existing theories, the dominant heating mechanism is that of resistive heating by the neutralizing return current. In addition to this, we find that large amplitude plasma waves are induced in the plasma in the wake of relativistic electron bunches. The subsequent collisional damping of these waves represents a source of heating that can exceed the resistive heating rate. As a result, solid targets heat significantly faster than has been previously considered. A new hybrid model, capable of reproducing these results, is described.

Figure 1

The background electron temperature as calculated by the PIC simulation and the SRC approach, at $t=90\mathrm{fs}$. The simple model discussed in the text predicts the blue curve. Also shown is the initial electron density profile.

Figure 2

(a) The electric field ($E_x$) from a 2D simulation. Only the solid-density part of the target is shown, with the laser incident from below. (b) $E_x$ inside the target from a 1D simulation, along with the electric field given by the simple theoretical model (which is derived from the fast electron current).

Figure 3

Fast electron phase space at $t=40\mathrm{fs}$, demonstrating the initial generation of “smooth” bunches (at twice the laser frequency) and the subsequent development of modulations as the bunch moves through the dense plasma and interacts with the induced wake field. Electrons travel from left to right. Note that this is within the solid density region (the $x$ axis does not correspond to that in Fig. 1).

Figure 4

A phase-space snapshot of one of the fast electron bunches with the corresponding wake field (longitudinal electric field strength) shown below.