Energy content of target and electron flow in femtosecond laser target interactions

Abstract

The heating of the titanium foil in a recent femtosecond laser plasma experiment is investigated theoretically in two different ways. In the first, the energy content and thus the heating efficiency of the central volume of the foil is derived by integrating the transverse temperature profiles obtained in this experiment, using specific heats based on the average atom model. In the second approach target heating by the fast electrons, both by direct energy deposition and by resistive heating is investigated. The latter approach makes use of a specially devised electron flow model which includes a simplified quantitative treatment of multi-refluxing as a crucial component. In all, the calculated results of electron beam heating are consistent with experiment within the limitations of the modeling. Finally, a prediction for the temporal dependence of the Kα pulse from the central volume of the foil based on our electron flow model is given.

Kα emission and secondary electrons in femtosecond laser target interactions

“…In the present calculations as also presented in Nardi et al (2015), l s = 2 μm, based on measurements of Romagnani et al (2005) and is of the order of the "local" electron Debye length (Passoni & Lontano, 2004). E 0 can be varied and determines the value of the restoring potential.…”

“…We will use this method here, in our calculations as outlined below. For the conditions encountered in these types of experiments plasma effects on the stopping of electrons are neglected (Nardi & Zinamon, 1978).…”

“…The Maxwellian temperature in the Nilson et al (2012) experiment was obtained by comparing the measured Kα temporal shape to that calculated by the Class I scheme, assuming that the time spent by the electrons outside the target during recirculation is negligible. Based on our reflux modeling in a very recent paper (Nardi et al, 2015), we examined the time spent outside the target by a 1 MeV electron, penetrating a 25 μm micron foil, as it refluxes back into the target. We denote by z the distance from the edge of the foil into the vacuum in the direction of the axis of symmetry of the foil, which is also the direction of the beam propagation.…”

“…The additional foil thickness, of a factor of 1.6 times the foil thickness in the experiment was added in order to account for multi-refluxing. This number is based on our simplified but quantitative modeling of multirefluxing (Nardi et al, 2015), where the energy deposition of the first six refluxing steps, within the central 160 μm area of the foil gave an increase in energy deposition of 1.6 times that of the primary beam. Thus the path length of 32 μm for the beam transit is assumed.…”

Generation of a strong reverse shock wave in the interaction of a high-contrast high-intensity femtosecond laser pulse with a silicon target

“…The cool dense material sitting closest to the corona is rapidly heated by return current setup by the forward streaming hot electrons produced from the relativistic laser-plasma interaction. 20,21 This rapidly reverses the pressure gradient across the density ramp between the corona and the dense plasma, driving a shock wave out into the lower density plasma with which the probe is interacting, resulting in the trends in probe reflectivity and the Doppler shift observed at around 9 ps in the experiment and at around 10 ps in the simulation. A shock wave is also driven inward, into the cold dense solid which is clearly observed in the simulations.…”