Toward more robust ignition of inertial fusion targets

Abstract

Following the 3.15 MJ fusion milestone at the National Ignition Facility, the further development of inertial confinement fusion, both as a source for future electricity generation and for high-energy-density physics applications, requires the development of more robust ignition concepts at current laser facility energy scales. This can potentially be achieved by auxiliary heating the hotspot of low convergence wetted foam implosions where hydrodynamic and parametric instabilities are minimized. This paper presents the first multi-dimensional Vlasov–Maxwell and particle-in-cell simulations to model this collisionless interaction, only recently made possible by access to the largest modern supercomputers. The key parameter of interest is the maximum fraction of energy that can be extracted from the electron beams into the hotspot plasma. The simulations indicate that significant coupling efficiencies are achieved over a wide range of beam parameters and spatial configurations. The implications for experimental tests on the National Ignition Facility are discussed.

Energy gain of wetted-foam implosions with auxiliary heating for inertial fusion studies

“…This process was estimated to result in an impressive 14% of the laser energy being transferred to the hotspot, which suggests that efficient transfer of energy between short-pulse lasers and plasma via electronbeams is indeed possible. This mechanism is distinct from the the process described in [18,19] (which did not observe filamentation in their simulations) and considered in this article, where the transfer of energy from beam to background plasma is due to the growth and damping of Langmuir waves. The key difference between these approaches is the electron distribution function used.…”

“…The electron distribution function in the experimental work [29] (obtained from their simulations) is not capable of driving the 'bump-on-tail' instability and thus this heating process would not be expected to feature in their experiment, while [18,19] feature profiles that are designed to generate such an effect (as stated previously, the generation of such an electron distribution from short-pulse laser interactions remains an area of active research). The conditions in the experimental work [29] are also significantly cooler and less dense than considered in [18,19] (this lower density is significant, as a high density is required to give a high electron-ion collision rate and thus rapid temperature equilibration). Despite these differences the work presented in [29] is a significant and highly relevant result, due to it's demonstration of collisionless heating of an implosion and the high efficiency they achieved.…”

“…This auxiliary heating process has been demonstrated in Vlasov and PIC simulations in [18,19], but has not yet been verified experimentally. However, it is worth noting that collisionless heating via counter-propagating electron beams was previously demonstrated in [29].…”

“…Another interesting innovation is auxiliary heating, which may be used to improve the fusion performance of such implosions. This technique (first proposed in [17], and further explored by [18,19]) uses a relativistic electron beam to provide collisionless heating of the hotspot of an implosion (while early work on this technique stated that overlapping electron beams would be required [18], subsequent work found that using a single beam would be equally effective [19]). The electron beam leads to a 'bump-on-tail' of the electron distribution function, resulting in the growth of Langmuir waves in the hotspot.…”

“…This article aims to interpret the results of [16,19,26] analytically, to further increase understanding of these two approaches (low convergence implosions and auxiliary heating). In section 2, a standard hot-spot ignition model is shown to be applicable to describing fixed convergence ratio implosions, and is then applied to model the results of [26].…”