The heating of inertial confinement fusion (ICF) target by fast electrons, which are generated as a result of laser interaction with expanding plasma (corona) of a target, is investigated theoretically. It is shown that due to remoteness of the peripheral region, where electrons are accelerated, a significant portion of these particles, moving in corona and repeatedly crossing it due to reflection in a self-consistent electric field, will not hit into the compressed part of target. Using the modern models of fast electron generation, it is shown that in a typical target designed for spark ignition, the fraction of fast electrons that can pass their energy to compressed part of target is enough small. Only 12% of the total number of fast electrons can do it. Such an effect of "wandering" of fast electrons in corona leads to a significant decrease in a negative effect of fast electrons on target compression. Taking into account the wandering effect, the distribution of energy transmitted by fast electrons to different parts of target and the resulting reduction of deuterium-tritium (DT) fuel compression are established.
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.
In the previous works (Rozanov et al., 2013; 2015) we have performed one-dimensional (1D) numerical simulations of the target compression and burning at the absorbed energy of ~1.5 MJ. As a result, the target was chosen to have a low initial aspect ratio in order to be less sensitive to the influence of such parameters as laser pulse duration, total laser energy, and equations of state model. The simulation results demonstrated a higher probability of ignition and effective burning of such a system. In the present work we discuss the impact of irradiation asymmetry on this baseline target implosion. The details of the 1D compression and a possible influence of 2D and 3D effects due to the hydrodynamic instability and mixing have been described. In accordance with the 2D calculations the target is still ignited, however, the symmetry analysis of 3D ones gives reasons to further reduce the efficiency of conversion of kinetic energy into potential energy.
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