Lasers for Fusion Energy

Nuclear fusion may one day become one of the main sources of energy for mankind. Mimicking the fusion processes that power stars like our Sun, a virtually unlimited amount of energy can be produced using sea water as a fuel source. Creating the extreme temperatures and densities required to get the fusion process going, however, remains a rather challenging task. As an alternative to the so-called magnetic confinement approach to fusion energy (pursued in the proof-of-principle plant ITER, currently under construction in the south of France), several laser-based approaches to a competing technique, Inertial Confinement Fusion, have been proposed. This focus section shows how Laserlab-Europe scientists are involved in investigating how the current limits of this revolutionary approach can be overcome, and how Laserlab facilities are of fundamental importance to this quest.

EUROfusion project Towards Inertial Fusion

Energy Within the Horizon 2020 programme, fusion research activities are supported and funded by EUROfusion on behalf of the European Atomic Energy Community (EURATOM). Although mainly focused on magnetic confinement fusion, EUROfusion is also supporting Inertial Fusion Energy (IFE) within its enabling research grant programme through the Towards Inertial Fusion Energy (ToIFE) project.

The ToIFE project ran from 2014 to 2018, and was initi- ated in order to capitalise on the integration reached and lessons learned within the HiPER conceptual design and preparatory phase (2006–2013), which shaped many aspects of laser fusion research within Europe ( see also the contribution by Michael Tatarakis ).

ToIFE aimed at achieving the fundamental understanding required to demonstrate the viability of laser-driven fusion as an alternative road towards a sustainable, clean and secure energy source. It hinged on four missions: (1) understanding underlying obstacles to central hot-spot ignition on megajoule scale laser facilities; (2) advancing physics towards demonstration of shock ignition (SI); (3) testing the viability of other alternative ignition schemes (fast ignition, impact ignition or aneutronic fusion); and (4) developing key IFE technologies (high-repetition-rate laser drivers, innovative diagnostics or measures against electromagnetic perturbations or novel materials).

Based on a comprehensive and coherent programme of experiments and numerical simulations, and on fostered collaborations between the fourteen partners in nine different countries, it allowed significant progress in the field of laser plasma physics and in the understanding of the challenges that the IFE community has to face.

The ToIFE achievements manifested themselves in the acceptance, for 2017–2018, of three new EUROfusion projects investigating specific issues. Preparation and Realization of European Shock Ignition Experiments ( see also the contribution by Dimitri Batani ) is taking up and extending mission #2, on all aspects except those related to the LMJ- PETAL SI experiment. Non-local thermal transport in inertial and magnetic confinement fusion plasmas , led by Christopher Ridgers (University of York, UK), is investigating – from the theoretical and numerical points of view, experiments performed within mission #1. Towards a universal Stark- Zeeman code for spectroscopic diagnostics and for integration in transport codes , led by Joao Jorge Santos (CELIA, France), is capitalising on the successful development of the capacitor-coil targets within mission #3.

The scientific and technological advances made within the project would not have been reached without the EUROfusion-supported structuring of the community and without access to high-energy research infrastructures, namely LULI2000 (LULI, France), PALS (IoP Prague, Czech Republic), PHELIX (GSI, Germany) and VULCAN (STFC/RAL, UK).

Sylvie Jacquemot, LULI, coordinator of ToIFE

HiPER: building laser-driven fusion research in Europe

The HiPER (High Power Energy Research) has been a European ESFRI Roadmap project aiming to explore laser- driven fusion schemes and, at a second phase, to develop a laser research infrastructure for the assessment of the possibility of commercial power production based on laser-driven fusion of deuterium and tritium. The Preparatory Phase of the HiPER project from 2008 to 2013 was coordinated by Laserlab-Europe partner the Science and Technology Facilities Council (STFC) in the UK.

Since its inception, HiPER has been designed to allow a substantial, long‐term science programme in a wide range of associated science and applications to complement its mission of fusion research. During the Preparatory Phase of the Project, scientists had been working on a plan to investigate the physics of various fusion schemes, e.g., fast ignition and shock ignition. A critical advantage of shock ignition is that it is amenable to demonstration, on a single shot basis, using the Laser MégaJoule (LMJ) facility in France ( see also the contribution by Jean-Luc Miquel ).

Key outputs from the project include hundreds of peer reviewed publications in the scientific literature, covering all related aspects of the physics and technology of laser- driven fusion as well as aspects of the fundamental science programme of the project, and many high profile invited lectures at international conferences.

But above all, and maybe the most important outcome of the HiPER project is its influence on the laser fusion community, which experienced an impressive expansion in Europe. This expansion has also been driven by a dedicated HiPER physics training and networking programme via specially realised actions. Such actions are continued today and provide opportunities to discuss future actions and HiPER-related physics and technology.

Michael Tatarakis, TEI of Crete/CPPL, coordinator of HiPER’s Fundamental Science Programme

EUROfusion project on shock ignition

The physics of the shock ignition approach to Inertial Confinement Fusion is currently being studied in the EUROfusion project Preparation and Realization of European Shock Ignition Experiments , which is coordinated by Laserlab-Europe partner CELIA (Bordeaux) and involves researchers from groups in seven European countries.

In the so-called ‘indirect-drive’ approach of Inertial Confinement Fusion (as employed at the National Ignition Facility at Livermore Lab California), laser beams delivering almost two megajoule of laser energy are focused inside a cavity ( hohlraum ) and converted to X-rays which should compress the capsule containing the thermonuclear fuel in a very symmetric way. This approach is very expensive in terms of energy, but it has long been thought the only way to provide the irradiation uniformity needed to avoid hydrodynamic instabilities, which cause deformations of the target and ultimately may even break it, preventing achieving ignition.

Shock ignition is an alternative approach to laser fusion, promising to couple energy efficiency to the required compression uniformity. In this approach the laser beams at typical intensities of a few times 10 14 W/cm 2 directly compress a thicker capsule at lower velocity, which is much less affected by Rayleigh-Taylor instability. Towards the end of compression, a more intense laser spike (intensity up to about 10 16 W/cm 2 , duration few hundred picoseconds) launches a strong spherical shock wave, which converges to the centre, further heating and compressing the fuel and providing the conditions needed to trigger the nu- clear fusion reactions.

Very encouraging, although preliminary, results on shock ignition have been obtained in experiments conducted at the Omega laser facility in Rochester, USA. Nevertheless, the physics of shock ignition is still largely unexplored.

In addition to studying the physics relevant to shock ignition, the EUROfusion project aims to promote collaborations with researchers outside Europe (in particular with the University of Rochester, USA, the birthplace of shock ignition) and to prepare future experiments to be conducted on the LMJ/PETAL laser facility in Bordeaux ( see also the contribution by Jean-Luc Miquel ).

The consortium started in 2017 and has conducted several joint experimental campaigns in particular at the PALS laser facility in Prague ( see also the contribution by Gabriele Cristoforetti et al. ), as well as at the Omega facility in the US.

Dimitri Batani, CELIA (University of Bordeaux), coordinator of the EUROfusion project on shock ignition

Laser plasma interaction experiments relevant for shock ignition at PALS

In recent years, the unique capabilities of Czech Laserlab-Europe partner Prague Asterix Laser System (PALS) have been used to investigate the conditions required for the shock ignition approach of Inertial Confinement Fusion. Among the institutes involved were Laserlab partner CELIA (Bordeaux), associate partner IPPLM (Warsaw) and subcontractor INO-CNR (Pisa).

In the shock ignition approach, fuel ignition is produced by a strong converging shock, driven at the end of the compression stage by an intense laser pulse. In this scheme, understanding the laser plasma interaction (LPI) of the igniting laser spike, and the ability to control the process, are particularly tricky and critical.

In recent years, a series of experimental campaigns have been carried out at PALS to investigate LPI and strong shock formation at intensities of interest for shock ignition. PALS is a unique laser facility in Europe, because it is able to provide the laser intensities (ca. 10 16 W/cm 2 ) and pulse durations (ca. 300 picoseconds) typical of shock ignition conditions with a sufficiently large focal spot (ca. 100 μm).

Measurements were dedi- cated to the onset and the relevance of parametric instabilities – such as Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS) and Two Plasmon Decay (TPD) – during the irradiation of laser pulses at PALS’ fundamental and at the tripled laser frequency and an intensity of around 10 16 W/cm 2 . These processes can significantly degrade laser-plasma coupling, by producing a strong reflection of light (SBS and SRS), and can generate high energy ‘hot’ electrons, which may affect pellet compression.

The comparison of experimental results with Particle In Cell simulations permitted also to get a deeper insight into the processes involved; notably their interplay, their timing and the effects of local values of plasma density and temperature.

Gabriele Cristoforetti, Leonida Antonio Gizzi, Federica Baffigi, Petra Koester – INO-CNR

G. Cristoforetti et al., EPL 117: 35001, 2017; G. Cristoforetti et al., PoP 25: 012702, 2018

Relativistic electron beam collimation for electron fast ignition

In the fast ignition approach to Inertial Confinement Fusion, a beam of charged particles provides the energy needed – on top of the energy from the implosion of the fuel sphere – to reach the temperature and density conditions required for nuclear fusion. In a recent experiment involving researchers from Laserlab-Europe partners LULI and CLPU, it was investigated how to control an electron beam for fast ignition using self- induced magnetic fields.

The advantage of electron fast ignition is that the electron beam can be easily generated by focusing a la- ser beam (of picosecond duration) directly on the external shell of the fuel sphere and no other targets are required. Once the electron beam is generated, it can propagate to the centre of the sphere and deposit the necessary energy to start nuclear fusion.

The success of this scheme relies on the possibility to control the divergence of the electron beam. A way to con- trol and guide electron beams is to use the magnetic field the electron beam produces itself in the plasma medium. This magnetic field could confine the beam if properly amplified and controlled.

At the LULI laboratories the team of researchers performed an experiment to investigate how to control the self-induced magnetic field. The trick is to split the laser pulse into two consecutive pulses in such a way that the first laser pulse generates a first magnetic field which can be used to guide the electron beam generated by the second laser pulse. The team demonstrated the feasibility of the scheme, and showed which are the relevant parameters to control the process.

Using the 100 terawatt laser system ELFIE at LULI, the researchers managed to investigate the relation between the magnetic field generated by the first beam and the main electron beam generated by the second beam as a function of the delay between the two beam delays and for different laser focal spots.

Luca Volpe, CLPU Laser-Plasma Chair at the University of Salamanca

Focused proton beams for fast ignition

Researchers from British Laserlab-Europe partners the University of Strathclyde and the Central Laser Facility (CLF) have used the petawatt-class Orion laser system in the UK to produce tightly focused beams of protons, using novel shaped targets, to assess physics relevant to Inertial Confinement Fusion and to develop laser- driven proton heating for high energy density physics.

High-power laser pulses are capable of accelerating protons to tens of MeV energies, in durations of the order of the laser pulse (tens of femtoseconds to several picoseconds) at the source. This high-dose, rapid burst of radiation is capable of isochorically heating matter to exotic states that, until recently, have been extremely difficult to generate.

The team of researchers performed an experiment on the UK’s Orion laser as part of the AWE’s academic access programme, where they employed novel target designs involving hemispherical targets to produce a high-flux, focused proton beam. A combination of gold hemispherical target foils and conical attachments produce a focused proton beam by focusing field lines on the target rear, in addition to strong transverse electric fields on the cone walls; the latter pushing the already focusing protons into an even tighter beam.

The experiment resulted in a clear demonstration of proton focusing using an open-tipped cone. This is observed at 21 MeV in the featured image. The beam appears annular at higher energies, due to ‘over-focusing’, which results from the higher transverse fields surrounding the cone at early times, when the highest energy protons are accelerated and traverse the cone structure.

With their successful proof-of-principle experiment, the international collaboration, which also included researchers from UC San Diego (USA) and TU Darmstadt (Germany), plans on developing this work, by further studying the focusing capabilities and designing target geometries that make the beam suitable as a tool for proton fast ignition and to superheat samples for material studies.

Adam Higginson, University of Strathclyde (now at UCSD, USA)

The Laser Mégajoule (LMJ) facility and its contribution to fusion energy


View of the target chamber inside

One of the few laser facilities capable of creating the extreme conditions required by Inertial Confinement Fusion is the Laser Mégajoule (LMJ), located near Bordeaux, France. In December 2017, LMJ’s Petawatt laser beam PETAL became operational, opening the possi- bility to study aspects of various laser fusion schemes.

LMJ offers unique capabilities, providing an extraordi- nary instrument to study high energy density physics and basic science. The 176 beams of the facility, grouped into 22 bundles of 8 beams, will deliver a total energy of 1.4 MJ of 0.35 μm light and a maximum power of 400 TW. Using a variety of pulse shapes, it is possible to bring material to extreme conditions with temperature of hundreds of millions of degrees Celsius and pressures of hundreds of billions of bars. Among the multiple experiments planned on LMJ, Inertial Confinement Fusion (ICF) is the most exciting challenge, and sets the most stringent specifications on the facility.

LMJ is still in a ramp-up period: operational commis- sioning was established in October 2014 with the first bundle of eight beams and the first plasma diagnostics. Five bundles and ten diagnostics are now operational; with these forty beams and a maximal energy of 300 kJ, LMJ is the second largest laser in the world.

Since the operational commissioning, more than 500 laser shots have been performed, including 150 shots on target for experiments dedicated to the Simulation Program developed by the French Alternative Energies and Atom Energy Commission (CEA). To complete the experimental capabilities of LMJ, a PW beam (PETAL), has been added to the LMJ’s beams. It is a short-pulse (0.5 picoseconds), high-energy (up to 3.5 kJ), ultra-high-power beam.

LMJ-PETAL is open to the academic communities. Six experiments have already been selected among 25 proposals. Some of them are dedicated to ICF, and will use the outstanding LMJ-PETAL capabilities to investigate some aspects of laser fusion schemes (including standard ICF and shock ignition). Experiments combining LMJ and PETAL have started in December 2017, opening the possibility to address new fields in physics.

The PETAL project has been performed under the auspices of the Conseil R égional de Nouvelle Aquitaine, of the French Ministry of Research and of the European Union.

Jean-Luc Miquel, on behalf of the LMJ team

Simulating laser fusion

The coupled, non-linear nature of the physics of laser fusion means simulation models are critical for designing and interpreting the experiments. Unfortunately, current simulation models are not sufficiently accurate to predict the observed experimental behaviour. In an attempt to address this problem, which is particularly important for shock ignition, a three year, £1.3 million grant has been awarded to a team led by Dr Robbie Scott of Laserlab-Europe partner the Central Laser Facility (CLF) in collaboration with the Universities of York and Warwick.

A leading hypothesis for the causes of the inaccuracies of laser fusion simulations are kinetic laser plasma interaction instabilities (LPIs). These alter the experiments in ways that are both hard to measure experimentally, and hard to model. The issue faced, is that current simulation codes used to design the laser fusion experiments do not account for LPIs and the changes they cause. This makes it very difficult to account for their effects, precluding the design of experiments with sufficient accuracy for fusion energy gain.

Through a combination of dedicated laser-plasma interaction experiments and innovative code developments, the goal is to create a benchmarked, world-leading simulation code capability by including the effects of LPIs within the UK’s ‘Odin’ radiation-hydrodynamics code. This will enable academic users of the CLF to design and interpret their experiments with unprecedented accuracy, and ultimately enable a robust evaluation of the viability of the shock ignition approach to laser fusion, and the size of the laser this would require.

Helen Towrie, CLF