Inertial Confinement Fusion - exploring the options for ignition.

The vast quantity of energy emitted by the Sun is produced by thermonuclear fusion. Harnessing fusion power on earth has long been a goal for scientists as it would provide an almost limitless supply of safe, clean electrical power. In the laboratory, experiments to study fusion involve heating isotopes of hydrogen to very high temperatures such that a plasma is formed. If the plasma is sufficiently hot then the rapid motion of the positively charged ions can overcome the electrostatic repulsive force resulting in a nuclear fusion reaction. One of the main approaches to producing these conditions is Inertial Confinement Fusion (ICF). The thermonuclear fuel is initially contained in a spherical capsule with a 1 mm radius. This capsule is compressed using high-power lasers such that within a few billionths of a second it becomes more than 1000 times denser than water and hotter than the core of the Sun. The key process in ICF is ignition in which the energetic alpha particles emitted by nuclear reactions cause further heating of the fuel. This results in a self-sustaining burn wave of reactions which releases copious amounts of energy. This is a very exciting time for fusion research because the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) is currently operational, producing thermonuclear plasmas at temperatures and densities never before observed in the laboratory.
Recent results from the National Ignition Facility have demonstrated a landmark achievement in generating an energy release from fusion reactions that is greater than thermal energy of the fusion fuel. Nevertheless, the number of fusion reactions remains too low to initiate the ignition process and as a consequence, the energy released is still far lower than the total energy used by each experiment. Previous work by the PIs was amongst the first to highlight the detrimental effects that three-dimensional asymmetries can have on the fusion performance. Such asymmetries are now widely believed to be the principal obstacle to achieving higher energy yields. Over the last three years, the PIs and PDRAs have worked extensively with scientists at LLNL to understand the key physical processes at work and to identify diagnostic methods which can be employed to isolate the principal causes of reduced yields.
For this proposal we wish to capitalise upon the experience we have gained from studying experiments on the NIF to explore a range of future options for progressing towards ignition in Inertial Confinement Fusion. Our work will make use of a combination of advanced theoretical models of dense plasmas, large scale parallel computing using our multi-dimensional radiation hydrodynamics codes and detailed analyses of experimental data. We will extend our computer models to calculate the anticipated response from a broad range of new diagnostics currently being introduced on NIF. This will help us contribute to the optimisation of diagnostic design and the methods used to isolate the principal causes of reduced performance. We will use this data to establish how theoretical models describing how the fusion plasma is confined need to be modified to take into account asymmetry and how learning to live with asymmetry leads to different approaches to the optimisation of yield. We will use these approaches to assess the likely impact of a number of proposed changes to the design of current experiments on NIF. We also propose a different approach to the design of experiments, which avoids some of risks associated with the pursuit of high energy yields, where the objective is instead to provide a reliable means of studying the physics of the ignition process itself. Finally, by exploiting the common physical processes which govern the final stages all inertial fusion experiments, we will use insights gained from experiments on NIF to evaluate the longer term viability of alternative approaches to achieving ignition and high energy yields.

The potential for thermonuclear fusion to provide a clean, almost inexhaustible supply of energy has fascinated both research scientists and the public for decades. For inertial confinement fusion, it is the process of ignition which is the critical step to achieving significant energy yields. The first series of laboratory experiments designed to investigate the ignition process, are taking place at this time on 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. The experience gained with ICF calculations through this grant will help to guide our development of radiation transport models for the HEDP community radiation hydrodynamics code 'Odin' which we are currently developing together with the Universities of Warwick and York (EPSRC grant EP/M01102X/1).We will make use of the Centre for Inertial Fusion Studies at Imperial College and the Oxford Centre for High Energy Density Science 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 principal 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 through the Centre for Postgraduate Training in Plasma Physics and High Energy Density Science jointly run by Imperial, Oxford and Warwick.

The research staff involved in the project will be trained in a 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. Recent examples of this are the secondment of one of our PDRAs to work in the defence industry and the movement of another PDRA to a research position within the Bank of England.