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.

Inertial confinement fusion using laser driven and wire array Z-pinch driven hohlraums

A PhD project for October 2018 : Supervisor Prof. J. Chittenden

One of the main approaches to inertial confinement fusion (ICF) is ‘indirect drive’ where lasers or X-ray sources are used to heat a cylindrical cavity called a ‘holhraum’ which surrounds a spherical capsule containing the fusion fuel. This hohlraum is typically heated to several million degrees Centigrade at which point the inner surface acts as a black body radiation source. The X-rays emitted by this black body then bombard the capsule causing the surface to expand rapidly. The reaction force due to this surface expansion causes the interior of the capsule to implode, compressing the fusion to very high densities and heating it to fusion temperatures.
Much of the recent work investigating the use of hohlraums in indirect drive ICF has taken place on the National Ignition Facility laser at Lawrence Livermore National Laboratory. Recent experiments have concentrated on trying to achieve the process of ‘ignition’, where alpha particles heat the plasma and enhance the energy yield. Plasma pressures inside the capsule reaching half the value required for ignition have been demonstrated along with significant levels of alpha particle heating. One of the factors currently thought to be inhibiting further increases in fusion yield is a lack of symmetry in the radiation from the hohlraum reaching the capsule. There are a number of potential causes for this, such as the non-uniform expansion of the inside wall of the hohlraum that occurs as it is heated by the laser. In addition, the extreme plasma density and temperature gradients that exist within the hohlraum are thought to be a source of spontaneously generated magnetic fields which can strongly affect the uniformity of the black body temperature distribution that is obtained.
An alternative to the use of lasers to heat the hohlraum is to use X-ray sources from magnetically driven implosions. One such scheme uses a cylindrical target formed from an array of fine metallic wires which is imploded using the electro-mechanical force from a pulsed electrical driver with several mega-amperes of current. This ‘wire array Z-pinch’ plasma provides a highly efficient X-ray source which can in turn be used to heat a larger scale hohlraum and capsule than is used with laser driven indirect drive. This approach provides a potential means of realising plasma ignition on a larger scale plasma with substantially higher fusion yields.
This PhD project will involve large scale high performance computing simulations of laser driven and wire array driven hohlraums using the 3D radiation magneto-hydrodynamics code ‘Chimera’ developed at Imperial College. One of the principle objectives is to undertake the first comprehensive three-dimensional treatment of the effects of magnetic fields on radiation symmetry in laser driven hohlraums. Further simulations will investigate potential future designs for wire array Z-pinch driven hohlraums. This work will also involve modelling in-house experiments on the Magpie pulsed electrical generator designed to study the ablation of materials using X-ray pulses from imploding wire array Z-pinches.
The project will be based within the Centre for Inertial Fusion Studies at Imperial College and will involve close collaborative work with experimental groups at Lawrence Livermore National Laboratory and Sandia National Laboratory as well as the Magpie group at Imperial.
Background reading
O. A. Hurricane, et. al. Nature 506, 343 (2014).
J.P. Chittenden et. al. Physics of Plasmas 23, 052708 (2016).
C. Walsh et. al. Phys. Rev. Lett. 118, 155001 (2017).
J.H. Hammer, et. al. Physics of Plasmas 6, 2129 (1999).

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).