Europe has a unique opportunity to use its knowledge in plasma physics and high-power laser technology to take the lead in promoting fusion energy driven by lasers. The recent achievement of ignition of fusion reactions and energy release in experiments at the National Ignition Facility (NIF) in the U.S. is a historical breakthrough that establishes inertial confinement fusion as a valid approach to energy production. At NIF, a laser energy of 2 MJ was used to induce the compression and heating of a spherical capsule containing 1 mg of a mixture of deuterium and tritium, thereby triggering nuclear fusion reactions which produce neutrons and alpha-particles releasing more than 3.5 MJ of fusion energy.

Scientists at NIF use the indirect drive approach in which the laser beams are converted into soft X-rays inside a cavity that contains the capsule. These X-rays enable an efficient and symmetric implosion of the capsule, leading to the ignition of fusion reactions and energy release. A conversion of optical energy into X-rays provides a high degree of uniformity of the radiation and enables high-quality spherical implosions. However, such a two-step approach is energy inefficient and hardly compatible with the requirements of future fusion reactors, which need to provide a much higher energy gain and operate at a repetition rate of one shot per second or more. European scientists, in collaboration with colleagues at the Laboratory for Laser Energetics at Rochester University in the U.S., are developing an alternative approach based on direct irradiation of the spherical capsule. The success of the indirect drive calls for creating a realistic roadmap for progressing beyond NIF results and aiming at research and development of a fusion power plant. This paper describes a roadmap to inertial fusion energy in Europe proposed by an international group of scientists from several European countries.

This initiative stems from the legacy of the HiPER (High Power Energy Research) project created fifteen years ago by a consortium of ten European countries and included in 2007 in the European Strategic Forum for Research Infrastructures (ESFRI). The HiPER project lasted seven years and contributed enormously to consolidating the European inertial fusion community, creating a solid basis for laser-plasma interaction physics and inertial confinement fusion research and technology. The figure shows an original HiPER concept of the inertial fusion power plant. It consists of three modules: the laser amplifier and focusing system, the reactor chamber, and the system of energy recovery and conversion to electricity. Unfortunately, HiPER was ahead of its time. Due to the delay in achieving ignition at the NIF, HiPER finished its preparatory phase in 2013.

The new roadmap, called HiPER+, consists of three successive steps extended over thirty years: (i) creation of the European Laser Fusion Research Centre and demonstration of ignition in the direct drive scheme, (ii) construction of the pilot inertial fusion reactor and demonstration of energy production; (iii) construction of a DEMO power plant including units of energy conversion and fuel production and compatible with the market requirements. This large-scale project of high societal value needs the engagement of many countries, including research laboratories, universities, industry, and private companies. The proposed roadmap is based on four complementary axes: (i) development of the physics of laser-plasma interaction and burning plasmas; (ii) development of the high-energy high repetition rate laser technology; (iii) development of the fusion reactor technology and materials; and (iv) reinforcement of the laser fusion community by international education and training programs.

The research program proposed by the initiative group is focused on the direct drive approach, which is scalable to a high repetition rate operation, offers more efficient use of laser energy, and is compatible with many alternative options. The common basis of the direct drive scheme consists of direct irradiation of the spherical capsule with many laser beams, which can accommodate different methods of fuel ignition either as a finite stage of implosion or using supplementary energy vectors such as strong shock or beams of charged ions or electrons. These options can be considered for the layout of the European Laser Fusion Research Centre. The main scheme proposed in the roadmap is the shock ignition and its recent version, augmented shock ignition. Today, it offers the most efficient and flexible use of laser energy and is compatible with diode-pumped solid-state laser technology (DPSSL). The major challenges in physics laser fusion are: control of efficient laser energy absorption and transport in the capsule, the choice of the laser wavelength, realization of symmetric and homogeneous laser irradiation, and control of stability and symmetry of capsule implosion.

The second axis addresses technological challenges related to the future fusion reactors: the development of high-energy laser systems, which have an efficiency of 10% or more and can operate at a high repetition rate of 1 Hz or more. The primary candidate is the DPSSL technology, which should be more energy and cost-efficient, reliable, and compatible with the wavelength choice dictated by the physics. The first step is developing a demo unit delivering a nanosecond pulse of kilojoule energy and a kilowatt average power, which will be of common interest for many other applications in industry, medicine, and technology. The third axis aims at the technology of materials for fusion reactors and capsule fabrication. The development of materials resistant to high radiation fluxes, thermal loads, and mechanical stresses is common with magnetic confinement fusion. The research in this domain needs an innovative neutron source capable of imitating conditions at the first reactor wall. The DPSSL technology could be a game changer in this domain. Other challenges include the mass fabrication of the capsules corresponding to the physics requirement and their safe and robust delivery to the reactor chamber, tritium breeding and recovery, operation safety, and many other high-importance issues.

Such a large-scale, long-term project needs partner coordination and extended high-education and personnel training programs. The roadmap proposes actions to gather the scientific community and the international private initiatives around a European Laser Fusion Facility dedicated to the direct drive inertial fusion research, development, and training. It will deliver a laser facility and address all the technological challenges related to the development of future fusion reactors. The first important step in this program is highlighting the urgency to re-establish a European Laser Fusion Facility in the ESFRI roadmap through the HiPER+ project, which will acknowledge the importance of inertial fusion energy for the future European energy budget and provide a common platform for the participation of governmental and private profit and non-profit enterprises.

Publication:

Dimitri Batani, Arnaud Colaïtis, Fabrizio Consoli, Colin N. Danson, Leonida Antonio Gizzi, Javier Honrubia, Thomas Kühl, Sebastien Le Pape, Jean-Luc Miquel, Jose Manuel Perlado, R. H. H. Scott, Michael Tatarakis, Vladimir Tikhonchuk, Luca Volpe. Future for inertial-fusion energy in Europe: a roadmap[J]. High Power Laser Science and Engineering, 2023, 11(6): 06000e83. DOI: 10.1017/hpl.2023.80