Temperature Relaxation in Dense, Reacting Plasmas

Understanding the properties of high energy density matter is both fundamental physics and one of the central problems that needs to be resolved before inertial confinement fusion can provide a clean and almost infinite energy source. Major steps toward this goal are being made at the moment: the National Ignition Facility in Livermore, USA has been completed last year and ignition (defined as larger energy output than input) is expected to be achieved this autumn. Ignition at NIF will have a similar impact on physics and society as the particle physics experiments using the LHC at CERN.The first experimental results support the positive predictions for full scale fusion experiments: they have demonstrated excellent energy coupling from the large scale laser systems with 192 beams into the millimeter-size cavity. Now the physics of a burning plasma has to be explored. This is done by a combination of experiments and large scale simulations. The latter are a key element of the project as the experiments are infrequent (maximum two shots a day) and very expensive. The simulations need to incorporate many physical processes; not all of them are fully understood. As a result, approximate models are currently used with represents a clear caveat for future progress.This project is aimed to substantially improve this situation for one important quantity: the electron-ion coupling. It will give definitive answers for the energy transfer rates for the whole range of parameters that occur during the heating of the fuel to fusion temperature of 10 million degrees. These energy transfer rates are needed to describe the propagation of a burn wave in the pre-compressed fuel, a process that allows for high gain targets. Interestingly, the first phase when the plasma is relatively cold is most difficult to describe as here the quantum nature of the electrons and strong forces between the ions play a major role. The quantum statistical model developed by the applicant will prove invaluable for this application.Dedicated to a specific problem within the fusion program (equation of state, melting or transport properties), a series of intermediate scale experiments has been performed over the last years. The rapid energy deposition into samples applied here creates systems with different electron and ion temperatures. Again, temperature equilibration is a major issue for the design and interpretation of the experiments. Moreover, recombination and ionisation processes are often driven by the energy deposition of the laser. The main part of this project aims to remove the theoretical uncertainties in the description of the relaxation processes involved. In particular, it will give a description of the full interplay between the changing species temperatures, the changing charge state of the ions and time-dependent correlations. In the dense matter under investigation, all of these energy contributions are of the same order of magnitude and neither can be neglected.It is very interesting to notice that intermediate scale laser experiments often reach conditions similar to those in astrophysical objects such as giant planets (including a rapidly growing number outside of our solar system), old stars and dim, midsize objects. The experimental investigation of such states, called laboratory astrophysics, requires that thermodynamic equilibrium is reached. Thus, the relaxation time is here of particular interest as it defines the minimum time delay between creation of the system and the probing. The theory developed here will provide these times.

The setting of the project within CFSA at Warwick will make knowledge exchange beyond the boundaries of dense plasma research a natural byproduct. Beneficiaries are the magnetic fusion, the solar physics and the high-performance computing communities. The structure of CFSA is specifically designed to foster the exchange of ideas between and across the EPSRC-supported programmes in plasma physics as it is proposed here and the STFC-supported astrophysics programme. The applicant brings an expertise in quantum statistics and quantum simulations applied to plasma physics that is unique in the UK. The training of PhD students and postdocs, that becomes even more diversified through the proposed project, has a large impact far beyond the field of dense plasmas (for instance, one of the applicant's PDRA was recently offered a permanent research position in Engineering at the University of Bristol). By supporting and building upon strong research links to UK industry (AWE, Fluid Gravity), the proposed programme will have also positive benefits to the R\&D programmes of these companies. Very direct impact can be expected on the programme for the ORION laser and its short-pulse option currently being built at AWE. This link was recently strengthened as the PI is supervising an AWE employee as a part-time PhD student since April. Direct impact on the worldwide laser-fusion efforts can also be expected. The techniques developed here describe extremely compressed matter with quantum degenerate electrons and strongly coupled ions and are well-suited to perform well for fusion targets. These results will surely influence future fusion research based on high-energy lasers in facilities such as NIF, OMEGA-EP at the Laboratory for Laser Energetics (University of Rochester, USA) as well as the future HiPER (Europe), FIREX (ILE, Osaka, Japan) and LIVE (USA) projects. The applicant has already strong collaborations, documented by common papers, with colleagues from Livermore (USA), Rochester (USA), Luli (France) and the Central Laser Facility (UK). The applicant can thus disseminate results rapidly through direct contact as well as through publications in high-impact journals. In the long term, the developed relaxation codes can also prove to be very useful in the simulation of laser processing of materials. Here, the possible parameter space is too large to be fully investigated by trials and one has to rely on simulations as well. The nonequilibrium states created during laser-modification of materials require a good understanding of the relaxation processes as investigated in the proposed work. Although such a result must be estimated as at risk, it would again strengthen an area that in EPSRC's preception is weak in the UK (materials: metals and alloys). Applications and dissemination will be explored through Warwick Ventures which provides an introduction and practical help to patents, licensing and spin-out companies.