Indirect drive inertial confinement fusion experiments with convergence ratios below 17 have been previously shown to be less susceptible to Rayleigh–Taylor hydrodynamic instabilities, making this regime highly interesting for fusion science. Additional limitations imposed on the implosion velocity, in-flight aspect ratio and applied laser power aim to further reduce instability growth, resulting in a new regime where performance can be well represented by one-dimensional (1D) hydrodynamic simulations. A simulation campaign was performed using the 1D radiation-hydrodynamics code HYADES to investigate the performance that could be achieved using direct-drive implosions of liquid layer capsules, over a range of relevant energies. Results include potential gains of 0.19 on LMJ-scale systems and 0.75 on NIF-scale systems, and a reactor-level gain of 54 for an 8.5 MJ implosion. While the use of 1D simulations limits the accuracy of these results, they indicate a sufficiently high level of performance to warrant further investigations and verification of this new low-instability regime. This potentially suggests an attractive new approach to fusion energy. This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 2)’.

Experiments were performed on the Omega EP laser facility to study laser pulse propagation, channeling phenomena and electron acceleration from high-intensity, high-power laser interactions with underdense plasma. A CH plasma plume was used as the underdense target and the interaction of the laser pulse channeling through the plasma was imaged using proton radiography. High-energy electron spectra were measured for different experimental laser parameters. Structures observed along the channel walls are interpreted as having developed from surface waves, which are likely to serve as an injection mechanism of electrons into the cavitated channel for acceleration via direct laser acceleration mechanisms. Two-dimensional particle-in-cell simulations give good agreement with these channeling and electron acceleration phenomena. IntroductionLaser-based plasma accelerators have become a highly promising alternative to conventional accelerators in recent years. Wakefield acceleration can be driven by a laser pulse or particle beam propagating through an underdense plasma, which produces a plasma wave with a phase velocity close to the speed of light, and can transfer energy to 'surfing' electrons trapped in the waves [1][2][3]. With the reduction of the laser pulse duration, a regime where the laser pulse duration matched the plasma frequency was achieved and this allowed significant advances in controlling and producing narrow energy spread electron beams [4][5][6]. Furthermore, transverse oscillations of the high-energy electron beams within the plasma wave structure leads to a very bright, directional x-ray source [7]. Using laser pulses of longer pulse duration produce a more complicated interaction, with the leading edge of the pulse producing plasma waves. However, if the laser pulse is intense enough, the ponderomotive force of the laser pulse expels the electrons from the regions of highest intensity to form a cavitated channel. Once the channel has formed, plasma waves are no longer present, but electrons are able to gain energy through direct laser acceleration (DLA) mechanisms [8,9].The study of this channel formation, the energy exchange from the laser pulse to electrons and the subsequent transport and dissipation of the energy is of specific relevance to the hole boring fast ignition inertial confinement fusion scheme [10]. A high-intensity laser pulse is used to form a channel though the low-density coronal plasma of the compressed fuel, so that a second laser pulse can be guided to the dense fuel and strongly heat the electrons in this region to ignite the system. The aim of this study is to gain a better understanding of the energy transfer and electron heating mechanisms in such systems.Several DLA mechanisms have been identified using particle-in-cell (PIC) simulations to accelerate electrons to energies exceeding the ponderomotive potential. The transfer of laser energy to the electrons can occur either through a stochastic acceleration mechanism [11,12], or via the coupling of quasi-static electric o...

Experiments were performed using the Omega EP laser, operating at 740 J of energy in 8 ps (90 TW), which provides extreme conditions relevant to fast ignition studies. A carbon and hydrogen plasma plume was used as the underdense target and the interaction of the laser pulse propagating and channeling through the plasma was imaged using proton radiography. The early time expansion, channel evolution, filamentation, and self-correction of the channel was measured on a single shot via this method. A channel wall modulation was observed and attributed to surface waves. After around 50 ps, the channel had evolved to show bubblelike structures, which may be due to postsoliton remnants.

The expansion of electromagnetic post-solitons emerging from the interaction of a 30 ps, 3 × 10 18 W cm −2 laser pulse with an underdense deuterium plasma has been observed up to 100 ps after the pulse propagation, when large numbers of post-solitons were seen to remain in the plasma. The temporal evolution of the post-solitons has been accurately characterized with a high spatial and temporal resolution. The observed expansion is compared to analytical models and three dimensional particle-in-cell results providing indication of the polarisation dependence of the postsoliton dynamics.