PhD opportunities

Absorption physics of intense twisted light with solid targets

Supervisor Dr Robert Kingham
Type Computational & Theoretical
Funding DTA (group or Dept/Faculty)

This project will explore how intense, picosecond-duration laser beams possessing orbital-angular momentum (OAM) interact with solid density plasma. Laser beams with OAM have ‘spiral’ phase-fronts (hence the term ‘twisted’ light) and each photon carries ±ℏ of angular momentum.  Such beams, and their interaction with matter, are well understood in conventional optics, where the intensity is low.  However, the study of what happens at the ultra-high, “relativistic” laser intensities ( I ≥ 1022 W/m2 ) used in laser-plasma interactions is still in its infancy. Most research focuses on the interaction of OAM pulses with under-dense plasma. This project will focus on their interaction with solid-density plasma. The idea is to explore how angular momentum in the laser affects the laser absorption efficiency, the characteristics of the energized electrons and magnetic-field generation.  These are fundamental processes that underpin a range of applications such as proton acceleration and advanced ICF schemes.  The investigation would be carried out using a combination of HPC simulations (using the particle-in-cell code EPOCH) and analytical theory. There may be opportunities to engage with experiments.

Magnetically Driven Inertial Confinement Fusion

A PhD project for October 2017 : Supervisor Prof. J. Chittenden

An essential part of energy generation in inertial confinement fusion (ICF) is the process of ‘ignition’ where alpha particles heat the plasma and enhance the energy yield. There are currently three main approaches to studying the ignition process in an ICF plasma. These are the direct drive approach, where a laser is used to heat and compress the fusion fuel, indirect drive where the fusion fuel is instead compressed using X-rays and magnetic drive where the fuel is compressed using a high power electrical current pulse. The only experimental facility that is currently large enough to study the ignition process is the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory which has so far concentrated on the indirect drive approach. Plasma pressures reaching half the value required for ignition have been achieved along with significant levels of alpha particle heating. Reaching ignition can potentially be achieved through improvements in the design of the fusion capsule but may ultimately be limited by how much of the energy from NIF can coupled into the fusion fuel. Both the direct drive and magnetic drive approaches offer intrinsically higher energy coupling efficiency than the indirect drive approach and thus are being explored as alternative routes to ignition.

Image showing magnetization, laser heating and compressionThe main magnetic drive approach to ICF is the MagLIF concept being studied at Sandia National Laboratory. Here the fusion fuel is placed inside a cm sized metallic cylinder (or liner) to which a current of 20MA is applied over a pulse duration of 100ns. The electromechanical Lorentz force associated with this current is sufficient to rapidly compress the liner and hence the fuel. A laser heating pulse is applied in order to raise the fuel temperature prior to compression as well as a magnetic field which helps to thermally isolate the fuel from the liner and maintains a high temperature. Current experiments are exploring how to maximise the energy coupling from the laser, how to minimise contamination of the fuel and how to improve the symmetry of the compression as it reaches the axis.

The project will involve large scale high performance computing simulations of MagLIF experiments using the 3D magneto-hydrodynamics code ‘Gorgon’ developed at Imperial College. Of particular emphasis will be the multi-dimensional nature of the high temperature and density fuel region obtained after implosion. The simulation data will be used to calculate the anticipated response from a broad range of experimental diagnostics in order to help improve our understanding of the processes controlling the plasma confinement and fusion performance. This will include analysis of the behaviour of fusion products, such as the use of neutrons as a diagnostic of the plasma conditions and the behaviour of energetic alpha particles and tritons within the dense fuel. The project will be based within the Centre for Inertial Fusion Studies at Imperial College and will involve close collaborative work with Sandia National Laboratory. In addition the project will benefit from exploring the common physical issues governing performance in indirect and direct drive approaches through collaborations with Lawrence Livermore National Laboratory, the University of Rochester and MIT.

Background reading

  • M.R. Gomez et. al. PRL 113, 155003 (2014).
  • J.D. Pecover and J.P. Chittenden, Physics of Plasmas 22, 102701 (2015)
  • J.P. Chittenden et. al. Physics of Plasmas 23, 052708 (2016)
  • O. A. Hurricane, et. al. Nature 506, 343 (2014)

New models of the radiative opacity of stellar material

The material in the interior of stars is so hot that many of the electrons have been removed from the atoms and it is in the plasma state. Much work has been undertaken to calculate the electronic structure of the ions in such plasmas so as to understand how energy is transferred within the Sun and other stars through absorption and emission of photons (radiation transfer). Over the last few years experimental data has become available against which those models can be tested and usually the agreement is very good. However some recent experimental absorption spectra of hot, dense plasma taken on the ‘Z’-machine in the USA (a large pulsed-power device) under plasma conditions close to those expected at the convection-zone (CZ) boundary in the Sun have shown unexpected spectral features that current opacity modelling doesn’t explain. Moreover, changing the theoretical opacity to values suggested by the experiments could resolve an outstanding problem in stellar structure modelling concerning the position of the CZ boundary. One possible explanation is that two-photon absorption processes come into play that previously have been ignored.

The project will involve extending current opacity calculations to include two-photon absorption as well as exploring other shortcomings of the present modelling. The project will also involve designing and analysing new opacity experiments to be undertaken on other laboratory facilities such as the National Ignition Facility in the USA, the Laser MegaJoule in France and the ORION laser in the UK which will test the new opacity modelling. The project spans theoretical atomic physics, plasma physics and astrophysics as well as requiring an understanding of experimental possibilities on high-power lasers and pulsed-power machines. It will also have a major emphasis on state-of-the-art computing which will be needed to include the new physical processes in the opacity models that will be developed.

Prospective candidates are encouraged to contact Prof Rose for further information.

Email: Prof Steven Rose

Rad-hydro modelling of hohlraum energetics including VFP electron transport

Supervisor Dr Robert Kingham 
Type Computational & Theoretical
Funding CIFS (otherwise DTA)

The goal is to improve the treatment of electron thermal transport under the extremely non-equilibrium conditions found in indirect-drive ICF.  This could help understand the origins of the ~25% X-ray ‘drive deficit’ between experiment and modelling, seen at the National Ignition Facility in the US. This will be achieved by coupling an existing 2-D kinetic code for electrons (a Vlasov-Fokker-Planck code) to a radiation-hydrocode. The coupled code will allow  –  for the first time  –  proper assessment of the role of non-local effects and kinetic B-field dynamics in an integrated way on the hohlraum gas-fill, ablated wall plasma, and x-ray source at the wall in 1D and then 2D. The methodology to be developed here should also be applicable to aspects direct-drive ICF, and there may be opportunities to explore this too.  This project will involve close collaboration with LLNL.

Using thermal radiation fields to investigate fundamental physics

Supervisor - Professor Steven Rose

The subject of this PhD project is to explore how thermal radiation fields generated by high-power lasers can be used to investigate the fundamental physics of the interaction between photons, electrons and positrons (the subject of Quantum Electrodynamics – QED). Many experiments have been proposed and undertaken that use ultra-intense laser radiation to investigate these effects where the electrons and positrons interact directly with the laser radiation. However there is a category of experiments that use lasers to create thermal (or quasi-thermal) radiation fields that can be used to investigate QED processes which are of interest in astrophysics, cosmology and also of fundamental interest and it this topic that is the subject of the project.

The project will involve extending recent theoretical work on electron-positron pair production from a thermal radiation field generated directly by a laser1 and generated by a burning thermonuclear plasma2. It will involve calculating the electron-positron production process using new theoretical and numerical techniques. These will allow a more complete understanding of the interaction between photons in a thermal radiation field involving the production of electrons and positrons by multiple photons. These techniques should give a clearer understanding of the processes that generated electrons and positrons from the ultra-high temperature radiation field in the early Universe The project will also involve the design of new high-power laser experiments that demonstrate and test the predictions of these new techniques. 

The project spans theoretical quantum electrodynamics, atomic physics, plasma physics, astrophysics and cosmology as well as requiring an understanding of experimental possibilities using high-power lasers. It will also have a major emphasis on state-of-the-art computing which will be needed to undertake the new calculations that will be developed.

Prospective candidates are encouraged to contact Prof Rose for further information.

1. "A photon-photon collider in a vacuum hohlraum", O J Pike, F Mackenroth, E G Hill and S J Rose, Nature Photonics, 8, 434 (2014).

2. "Electron-positron pair creation in burning thermonuclear plasmas", S J Rose, High Energy Density Physics, 9, 480 (2013).