Light for controlled fusion energy: A perspective on laser-driven inertial fusion

The status of laser-driven inertial confinement fusion research is briefly reviewed. The recent major achievement of fusion energy release exceeding the energy delivered by the laser to the fuel (Hurricane O. et al., Nature, 506 (2014) 343), and the efforts towards ignition demonstration using indirect-drive are discussed. Physics model reliability is addressed. The potentials of alternative schemes, in particular direct-drive shock ignition, are also illustrated. perspective Copyright c © EPLA, 2015 Introduction. – Very recently, for the first time, a laser-driven deuterium-tritium (DT) fuel attained a fuel gain larger than unity: the energy released by fusion reactions exceeded the energy the laser had delivered to the fuel [1]. The reaction involved is D + T → α + n, with a Q-value of 17.6MeV. Prompted by this accomplishment and on the occasion of the International Year of Light, we present a brief overview and a personal perspective on laser-driven inertial confinement fusion (ICF) [2,3]. Fusion energy research began in the early 1950s. The achievement of controlled fusion energy is still one of the grand challenges for the 21st century physics [4]. Two main alternative paths —magnetic confinement fusion (MCF) [5] and inertial confinement fusion [2]— are actively investigated. In both cases temperatures of the order of 10 keV (in energy units) have to be achieved. In MCF, a low-density DT plasma is confined with the help of intense magnetic fields. In ICF, instead, a small amount of DT (milligrams at most) is compressed by a powerful pulsed energy source to a density of hundreds of g/cm [6]. No means can be used to keep it confined: matter only remains compressed for the time allowed by its own inertia (hence the name), typically a fraction of a ns. This process is intrinsically pulsed. A future fusion reactor can be envisaged burning fuel elements (targets) at a rate of a few hertz. Each target would be irradiated by driver (laser) pulses of a few MJ, and should attain an energy gain (ratio of fusion energy to delivered driver energy) G ≥ 100. More precisely, if we call ηd the electrical efficiency of the driver, the condition Gηd ≥ 10 should be satisfied [2,7]. The attainment of largeG requires the above-quoted compression as well as hot-spot ignition. Only a small portion of the fuel, a hot spot, should be heated at 5–10 keV and initiate fusion burn. The energy deposited by the α-particles should then self-heat the spot and drive a burn-wave propagating through the whole fuel. Of course, a necessary major step towards energy production by laser fusion is the laboratory demonstration of ignition and onset of burn propagation. The following steps will concern the achievement of the gain required for net energy production and, eventually, the development of concepts and technology for economic energy production. In this paper, we focus on ignition and on physics modelling. After brief remarks on laser fusion principles, we shall consider the results obtained by the US National Ignition Campaign. The goal is a brief discussion of the status of our understanding of the underlying physics, and then of the reliability of the models used by the ICF community. This in particular relates to the study of target concepts for power producing reactors. Such designs, indeed, rely on numerical simulations and involve rather large extrapolation of parameters with respect to present experiments. In the final part of the paper we discuss the potentials of the (laser-driven) shock ignition scheme, also taking advantage of the lessons learnt from the above ignition experiments.