Identifying the Key Factors Currently Preventing Ignition in Inertial Confinement Fusion Experiments.

Thermonuclear fusion is the mechanism by which energy is generated in the Sun. For decades scientists have been attempting to harness fusion for electrical power production because of the huge advantages it offers as a safe, clean and almost inexhaustible supply of energy. In laboratory experiments, fusion is normally studied by heating the heavy isotopes of hydrogen to very high temperatures forming a plasma, in which the rapid motion of the positively charged ions is sufficient to overcome their electrostatic repulsion and allow them to undergo nuclear reactions. One of the main approaches to extracting energy from these reactions is Inertial Confinement Fusion. This involves assembly of the thermonuclear fuel to ultra-high density (over 1000 times the density of water) inside a mm-scale capsule through a spherical implosion driven by high-power lasers. Central to this method is the process of ignition in which the energetic alpha particles emerging from the reactions are themselves used to further heat the fuel, resulting in a self-sustaining burn wave which releases copious amounts of energy. This is a very exciting time for fusion research because with the completion of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL), the first laboratory facility with the capacity to demonstrate ignition is now operational.
Early results from the NIF however have highlighted differences between the predictions of computer models and the behaviour observed. Most importantly the number of nuclear reactions has remained too low to initiate ignition. The PI and Co-I on this grant have worked extensively with scientists at LLNL to understand the origins of these discrepancies and participated in the Science of Ignition workshop which identified priority research directions to address these issues.
For this proposal we wish to capitalise upon our experience with plasmas of extremely high density and temperature to address key uncertainties in the design of inertial confinement fusion experiments and the physics of ignition and work to provide an explanation of why the current design does not achieve ignition and burn. Key areas of research will include understanding the way in which the radiation used to drive the implosion is absorbed in the surface of the capsule, the susceptibility of the imploding capsule to hydrodynamic instabilities which cause the fuel to disintegrate before it is fully compressed and the tendency of the high temperature and low temperature regions of the fuel to stir and mix together which quenches the burn. We will also investigate the physics of the ignition process itself, evaluating whether the energetic alpha particles are able to escape the fuel before depositing their energy and the role of spontaneously generated magnetic fields which provide a form of thermal insulation and serve to keep the heat within the fuel.
Part of the work will involve developing a number of advanced computer modelling capabilities. In addition to their use in fusion research, these capabilities can also be used to exploit large scale laser facilities for fundamental research in plasma physics, nuclear physics and laboratory astrophysics. Using these computer models to simulate a fusion capsule in which we deliberately introduce an imperfection, we can calculate what characteristic signatures of this defect are embedded within the flux of energetic neutrons and X-rays emanating from the reacting fuel. Comparing synthetic diagnostic data with that obtained in experiment then allows us to isolate which physical processes are responsible for limiting fusion performance. The same computer models can then be used to design improvements which mitigate these effects and allow us to make progress towards achieving ignition. The work described in this proposal therefore represents an opportunity for UK science to make a significant contribution to what would be a major scientific achievement.

For decades the prospect of a clean, almost inexhaustible supply of energy from thermonuclear fusion has fascinated research scientists and the public alike. Within recent years, the HiPER design project did much to raise public awareness of inertial fusion energy. The key scientific challenge of ignition is being addressed right now at the National Ignition Facility. This proposal represents an opportunity for UK science to be directly involved in what would be a major scientific achievement. If ignition is achieved then the short term impact would be significant in terms of the public engagement in fusion science, the stimulation of academic and industrial research and policy decisions over investment in fusion energy. The longer term societal and environmental impact, should inertial fusion energy be realised would be substantial.

The computational models developed as part of this work will enhance our capability to design other experiments in high energy density physics and laboratory astrophysics. We will make use of the Centre for Inertial Fusion Studies at Imperial College to facilitate collaborations with other academic groups and to maximise the impact of these tools on fundamental research. If ignition can be achieved and a thermonuclear burning plasma created in the laboratory this would provide access to regimes of extreme physical conditions with significant impact for the high energy density physics community providing a new platform for experiments in plasma physics, nuclear physics and laboratory astrophysics. Another principle beneficiary of the research will be the UK defence industry and particularly AWE Aldermaston, who will benefit not only from the exchange of results, but also from collaboration on model development and from staff training on the post-graduate plasma physics plasma course at Imperial College.

The research staff involved in the project will be trained in wide range of plasma, atomic, nuclear and computational physics techniques. They will also benefit from the considerable experience in these areas within the project partners. Such skills are much sought after within the academic community and national laboratories with many of the techniques involved being transferable to other areas of the economy.

Public dissemination of the work will take place through publication in high impact factor peer reviewed journals, through presentations at international conferences and specialist meetings organised by the Centre for Inertial Fusion Studies and through the schools outreach programme at Imperial College.