Effect of fast electrons on the gain of a direct-drive laser fusion target

Abstract

The results of numerical and theoretical studies of the gain of a direct-drive inertial confinement fusion target, which includes a kinetic description of energy transfer by laser-accelerated fast electrons, are presented. The range of the initial temperature of the fast electrons and the fraction of laser energy contained in these particles were chosen based on the results of recent experiments at the National Ignition Facility. The effect of 'wandering' of fast electrons is taken into account which is due to the remoteness of the region of fast electron generation from the ablation surface of the imploded target. As a result a significant fraction of particles do not fall into the compressed part of target. The 'wandering' effect leads to a decrease in the negative effect of fast electron generation on the gain of the target.

Mass-limited plasmas heated by laser-driven fast electrons as a powerful source of neutron and hard x-ray radiation

“…To illustrate the dependencies of the thermodynamic parameters of a plasma formed when a cubic MLDP-target is heated by a laser-accelerated fast electrons, a series of numerical simulations was performed using the DIANA code, which provides the calculation of one-dimensional hydrodynamic equations taking into account all the main plasma processes, including energy transfer by fast electrons [38]. The code was successfully applied to modeling the effect of laser-accelerated fast electrons on the implosion and thermonuclear burning of direct-drive ICF targets [39]. The numerical simulations were made under the following statement.…”

Fast-electron maintaining a high shock-ignition gain with a significant decrease in the laser pulse energy

“…In [25,40,42,43], the fact was substantiated that fast electrons make a decisive contribution to the formation of ablation pressure of 500-1000 Mbar, which is necessary to generate an igniting shock wave, upon combined heating of the target when 20%-40% of the heating energy is contained in the energy of these particles. Such a powerful shock wave can reliably provide a high gain (ratio of thermonuclear energy to absorbed laser energy) of about 100 in a target of a traditional design in the form of a two-layer shell with an external inert ablator [7,21,25]. The effect of the pressure increase due to fastelectron energy transfer has been reliably substantiated by experimental results [22,[44][45][46][47].…”

“…The algorithm for modeling a fast electron source includes several assumptions, which were used in [40,41]. Just as in [21,25,40,41], it was assumed that the generation of fast electrons with 3D Maxwellian energy distribution occurs during the entire period of spike action in the target's corona region with a density equal to a quarter of critical plasma density.…”

“…The generation of fast electrons is actively studied both in experiments and in numerical simulations, however many aspects of the characterization of fast electrons under real irradiation conditions using multi-beam MJ-class laser facility, especially in shock ignition context, are not yet completely clear. For the first three Ndlaser harmonics, the literature data are grouped into the ranges of η h and T h values, which can be defined for conventional spark ignition as 0.5%-2% of laser energy and 20-50 keV [4,[19][20][21], respectively, and for shock ignition as 10%-40% of spike energy and 20-100 keV [22][23][24][25][26][27][28][29][30][31][32][33]. The spectrum of fast electrons, in accordance with the general properties of energy dissipation of plasma waves, is considered to be Maxwellian.…”

“…On the contrary, energy transfer to the region of subsonic flow of relatively dense matter of the evaporated part of target (corona) as well as to the external region of dense ablator plays a positive role, which consists in increasing the pressure that compresses the target (ablation pressure) [38][39][40]. In a conventional sparkignition target with a relatively low laser energy conversion into fast-electron energy, only the negative effect of compressible target's part preheating is manifested [4,18,21,41]. In shock-ignited targets, depending on the energy contained in fast electrons and their temperature, both the negative and the positive effects of energy transfer by these particles are manifested [40].…”