A PhD project for October 2023 : Supervisor Prof. J. Chittenden

Recent progress on the National Ignition Facility have resulted in the first creation of a burning fusion plasma in the laboratory.  In order to achieve this, the plasma is compressed to pressures and temperatures higher than the centre of the Sun, producing extreme conditions similar to those in the early universe. These experiments offer not only a potential path to fusion energy production, but also a platform for the study of the most extreme states of matter.

These experiments make use of the ‘ignition’ process where the energetic alpha particles released by fusion reactions provide a heat source which raises the plasma temperature further and leads to a larger energy release. Once ignited the energy yield is amplified still further as the fusion process propagates as a ‘burn wave’ from the ignition region into the surrounding reservoir of dense fuel. Measurements of the neutrons produced in the fusion reactions provide key method to determine the properties of this burning plasma. Neutron data from the highest energy producing shots is suggesting that the processes driving burn wave propagation may be more complex than originally thought, for example distribution of ion energies may be driven out of thermal equilibrium by the burn process. Understanding the physics of burn propagation is key to our ability to design experiments for high energy output from inertial fusion.

Recreating the conditions at the centre of the Sun allows us to study nuclear fusion reactions in a similar environment to where they naturally occur in the universe. In nuclear astrophysics one of the key concepts is understanding how the high density and temperature plasma conditions affect nuclear screening and reaction rates. This has potential impact for understanding the abundance of different elements in the universe. In addition, the extremely high fluxes of neutron produced within an inertial confinement fusion experiment allow us to study the conditions where the heavier elements are formed through the S or R neutron capture processes in extreme environments such as supernova explosions or even the merger of neutron stars. The fusion neutron fluxes coming from high energy fusion experiment on the National Ignition Facility now allow us to design experiments to study these nuclear astrophysics processes in the laboratory for the first time.

The project will involve a combination of theoretical study of the nuclear processes alongside computational simulation and the design of experiments. The project will make use of existing radiation-hydrodynamics and particle-based modelling tools developed at Imperial College for studies of inertial confinement fusion. The work will involve adaptation of these models for the study of burn propagation on the National Ignition Facility and on future larger scale inertial fusion experiments as well as the design of experiments to study nuclear astrophysics and other fundamental phenomena using inertial fusion plasmas.

Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment, H. Abu-Shawareb et al. Phys. Rev. Lett 129, 075001 (2022)

Efficacy of inertial confinement fusion experiments in light ion fusion cross section measurement at nucleosynthesis relevant energies, A. J. Crilly et. al. Front. Phys. 10 937972 (2022)

Thermonuclear reactions probed at stellar-core conditions with laser-based inertial-confinement fusion. D.T. Casey et al. Nat Phys 13(12) 1227 (2017)

Supervisor:      Dr Robert Kingham

Type:                  Simulation & Theory

Funding:             STFC  (Ada Lovelace Centre + Central Laser Facility)   [TBC]

This project aims to enhance the simulation tools available to the Central Laser Facility (CLF) and its users, to design and interpret experiments carried out on high-power, nanosecond-duration laser systems such as Vulcan. It is aimed at improving the modelling of complex geometry targets and/or laser beam arrangements in high energy density plasma (HEDP) and inertial confinement fusion (ICF) relevant experiments. Traditionally, hydrodynamic codes employed for HEDP tend to use fixed structured meshes (e.g. grid of rectangular or pineapple-chunk shaped cells) and use finite difference (FD) or finite volume (FD) numerical methods. These work well for targets and beam configurations with symmetry (planar, cylindrical, spherical). But real experiments invariably do not have such symmetry.

The primary objective is to develop a 3D code that uses an unstructured mesh instead: Triangular (or tetrahedral) elements that can placed in an arbitrary way to provide high resolution around complicated shapes or where multiple beams intersect. The finite element method (FEM) [4] allows for this, and modern packages such as NEKTAR++  (spectral/hp element framework) also provide increased accuracy over FD and FV methods.

The secondary objective is for the code to include magnetic fields [1,2]. The past few years have seen an increase in experiments (including at the CLF) interested in studying the effects of magnetised transport, either where strong external magnetic fields are applied or spontaneous strong magnetic fields are created. However traditional ideal (or even) resistive MHD is not valid here. Instead extended-MHD is needed (with a more complex magnetic induction) and B-fields rotate fluxes (currents, heat-flows) so that the transport coefficients (thermal conductivity, electrical resistivity, etc.) become tensors. So, the code will be designed to use the full Braginskii transport.

The third objective is for the code to include non-local electron transport, via a reduced model.  The HEDP and ICF relevant experiments at the CLF produce steep temperature and density gradients where the standard prescription of thermal conduction (and other associated transport effects) break down. Heat flow can be suppressed in some places, emerge in other places where there are vanishing temperature gradients and even flow at an angle to temperature gradients in places. The leading model for this that is tractable enough to use in-line in hydrocodes is the SNB [3] multigroup method. If there is time, this will be added.

A physics investigation using the code, based on recent CLF experiments with results that are proving difficult to interpret (due to limitations of available codes).

1. Bissell, Kingham & Ridgers, Phys. Plasmas 19, 052107 , (2012)                  [ DOI: 10.1063/1.4718639 ]

2. Walsh, Chittenden, Hill & Ridgers, Phys. Plasmas 27, 022104 (2020)         [ DOI: 10.1063/1.5124144 ]

3. Nicolai, Feugeas & Scurtz, Phys. Plasmas 13, 032701 (2006)                       [ DOI: 10.1063/1.2179392 ]

4. Hesthaven & Warburton. “Nodal Discontinuous Galerkin Methods”, Springer (2008)         [ DOI: 10.1007/978-0-387-72067-8 ]

Supervisor:         Professor Zulfikar Najmudin
Type:                     Experimental (but will include simulation work)
Funding:               JAI studentship
 

The recent announcement of positive energy gain in an inertial confinement fusion has generated great excitement around the world [1] [2]. These results suggest that we are at the dawn of being able to control fusion in the laboratory, and potentially opening it up as a new source of (carbon-free) energy. These results were made possible by meticulous improvements in capsule design and better understanding of the power balance between laser beams to ensure more uniform irradiation of the capsule being compressed. However, the results to-date still suffer from shot-to-shot fluctuation (with only one shot showing positive energy gain so far?). Most of the difficulties in the compression have been a result of lower-than-expected velocities for the laser-driven shocks that initiate the compression, which has only been inferred from the poor neutron yields in previous shots. Being able to diagnose and characterise the shock formation and velocity would be a major step in better controlling the inertial confinement process [3].

We have been developing new x-ray imaging techniques for characterising dense matter interaction with high temporal and spatial resolutions. This source is based on synchrotron radiation from laser wakefield accelerators. The same large fields that make wakefield accelerators much more compact than conventional accelerators, also make them emit synchrotron radiation strongly. The source, with its small temporal and spatial emission size, and high photon energy (> 10 keV) is ideal for diagnosing dense dynamic systems [4]. We propose to use this imaging source to better understand the coupling of laser energy to a variety of targets in direct-laser driven targets as would be found in high-gain ICF designs. We will also study methods to improve laser-matter coupling which could drive the implosions much faster making the capsules potentially much easier to ignite.

[1] [Online]. https://www.bbc.co.uk/news/science-environment-63950962
[2] [Online]. https://www.llnl.gov/news/national-ignition-facility-achieves-fusion-ignition
[3] A. Do et al “Direct Measurement of Ice-Ablator Interface Motion for Instability Mitigation in Indirect Drive ICF Implosions, Phys. Rev. Lett. 129 215003 (2022)
[4]  J. C. Wood et al, "Ultrafast Imaging of Laser Driven Shock Waves using Betatron X-rays from a Laser Wakefield Accelerator," Scientific Reports, vol. 8, p. 11010, 2018.
 

PhD project for October 2023 - supervisor Dr Grigory Kagan

When the mean-free-path is comparable to the plasma size, the particle distribution is no longer Maxwellian. In turn, for a charged particle, the mean-free-path scales as the square of this particle energy, so the tail of their distribution can be driven away from thermodynamic equilibrium even if the bulk particles are Maxwellian. It is the tail ions and electrons that are mostly responsible for fusion reactions and hard X-ray emission from HED plasmas. This project will elucidate the non-trivial connection between the kinetic and nuclear/radiation physics.

PhD project for October 2023 - supervisor Dr Grigory Kagan

Different ion species communicate through collisions (micro-level) in a peculiar way. One of the most intriguing macro-physics consequences is that strong gradients, such as occurring in shocks, can separate species in an initially homogeneous ionic mixture and perturb the relative species concentrations. Analysis of the resulting shock structure can shed light into some mysteries of relative behavior of deuteron and triton ions in laser-driven implosions.

PhD project for October 2023 - supervisor Dr Grigory Kagan

When the plasma becomes dense one can no longer distinguish the binary acts of collisions; a new approach need to be developed for understanding the micro-physics. This is the case in the warm-dense-matter regimes relevant to many astrophysical and laboratory plasmas. Of particular interest is the diffusion and thermal conduction. This project will aim at the very basic plasma physics; new many-body kinetic methods will be utilized to gain an insight into how the non-ideal plasma transport is different from the conventional, ideal case