Effect of ‘wandering’ and other features of energy transfer by fast electrons in a direct-drive inertial confinement fusion target

Abstract

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.

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

“…However, this is already a transition to fast ignition [41,42] with the inherent difficulties of heating the target with fast electrons due to the presence of a large mass of ablated matter (corona) around the compressed part of target. In addition, in this case, heating in the multi-flight mode will be significantly hindered, because the captured electrons will be reflected from the corona's edge and only a small part of them will fall again into the central compressed part of target due to the 'wandering' effect [43].…”

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

“…The fast-electron effect on shock-ignited target gain has been investigated on the basis of numerical simulations with DIANA-FE code, which is a DIANA code supplemented with the component for calculating energy transfer by fast electrons. The DIANA-FE code was successfully used in modeling the effect of energy transfer by fast electrons in conventional spark-ignition targets [41] and in shock-ignited targets under irradiation by 3rd Nd-laser harmonic pulse [40]. The calculation of the energy transfer by fast electrons is carried out on the basis of solving the Fokker-Planck kinetic equation by the method of characteristics, followed by averaging over trajectories of the electrons decelerating in the target and specularly reflected from its external surface by sheath field back into the target.…”

“…The energy spectrum of fast electrons is taken into account by calculating the average value of the energy after the electron has passed a certain distance by integrating with the initial energy spectrum. Since the simulation uses a solution of kinetic equation for the dependence of the energy of a fast electron on the distance traveled, in the case of a three-dimensional Maxwellian distribution of fast electrons, such averaging does not require an upper cut-off of the electron spectrum [41]. If it is necessary to introduce the maximum energy value, it is determined from the consideration that, in a time step, these particles must transfer all their energy to the plasma.…”

“…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].…”

Role of hot electrons in shock ignition constrained by experiment at the National Ignition Facility

“…Reflux information extracted from planar and conical targets might not be relevant to implosions due to the difference in target geometry, but the planar target experiment in Pisarczyk et al 48 demonstrates similar accumulation of charge with electrons refluxed up to MeV energy scales. The refluxing hot electrons are expected to lead to additional coronal heating 49 . The most significant experimental constraint on hot electron emission angle can be seen in Yaakobi et al 50 where the population is inferred to have a solid angle of Ω h > 2π sr. PIC simulation and LPI theory give a much narrower angle, Ω h ≃ 1.8sr 20,33 , which is also in closer agreement with the preheat values inferred by Christopherson et al 19 .…”