PhD opportunities

Absorption physics of intense twisted light with solid targets

Supervisor Dr Robert Kingham
Type Computational & Theoretical

EPSRC DTA  or  Imperial President’s PhD Scholarship


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.

 Background reading

 A.M. Yao and M.J. Padgett, Adv. in Optics and Photonics 3, 161 (2011)

Y. Shi, et. al, Phys. Rev. Lett. 112, 235001 (2014)

Y. Shi, et. al, Phys. Rev. Lett. 121, 145002 (2018)

Magnetised ignition and burn in inertial confinement fusion plasmas

Supervisor Prof. J. Chittenden

One of the principle components of inertial confinement fusion (ICF) is the process of ‘ignition’, where energetic alpha particles released by the fusion process become the dominant heat source, driving further fusion reactions. The robust ignition of a central fusion ‘hotspot’ leads to strong heat flow into the surrounding fuel, resulting in a ‘burn wave’ propagating outwards, that leads to large amplification of the energy yield. Recent experiments on the National Ignition Facility (NIF) laser at Lawrence Livermore National Laboratory have reached the regime where alpha particle heating is dominant, but have yet to achieve sufficiently robust ignition to trigger a self-sustaining burn wave. Fusion performance on NIF is currently limited by inherent asymmetries in the radiation source and capsule structure which give rise to inhomogeneous implosions and effectively reduce the heat coupled to the hotspot. While the majority of current research efforts at NIF are directed at understanding and controlling the growth of these asymmetries, an alternate route accepts that perturbations are ubiquitous in ICF experiments and instead reduces hot-spot cooling through the application of external magnetic fields. Magnetisation of the electrons within the hotspot plasma suppresses thermal conduction losses and sustains the high temperatures for longer. Preliminary simulations suggest that applying a 50 Tesla initial field to the best performing the capsules, may be sufficient to push the hotspot over an ‘ignition cliff’ into a regime where self-sustaining burn begins, with an accompanying ten-fold increase in overall energy yield.
The physics of ignition in magnetised plasmas becomes fundamentally different to that in conventional ICF implosions. Magnetising the electrons requires fields of several tens of thousands of Tesla, which relies on effective compression of the seed field by the implosion itself. A complete treatment of the magneto-hydrodynamic (MHD) models for the plasma reveals additional terms which redistribute the magnetic flux and modify the heat flow. The field results in an inherent directional bias meaning that the heat flow, the ignition process and the burn propagation all become intrinsically anisotropic. For very large magnetic fields the alpha particles become trapped within hotspot such that burn propagation becomes driven by radiation transport rather through electrons and alphas. As part of this work, we will study the physics of magnetised burn in ICF, investigating modifications to the heat flow, alpha heating and burn propagation and the inherent asymmetries that result. Magnetisation and alpha heating also affect the burning plasma on a microscopic level, changing the electron distribution function and hence all of the collisional processes within the plasma.
This PhD project will involve large scale high performance computing simulations of magnetised ignition and burn in ICF plasmas 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 ignition in high yield NIF capsule implosions. The physical processes of interest are, however, common to all approaches to magnetised ICF. The project will therefore also explore the benefits of fuel magnetisation in direct drive designs as well as understanding the process of ignition within magnetised liner inertial fusion where fuel magnetisation is an intrinsic component of the design. The project will also explore how magnetisation affects the extrapolation of fusion performance to next generation facilities in indirect, drive and magnetically driven scenarios. The work will involve a combination of skills including the development of theoretical models for magnetised microphysics, their integration into large scale computer models and design simulations for experiments on NIF, the Omega laser and the ‘Z’ pulsed power facility.
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, the Laboratory for Laser Energetics at the University of Rochester and Sandia National Laboratory.
Background reading
L. J. Perkins, et. al. Physics of Plasmas 24, 062708 (2017).
C. Walsh et. al. Phys. Rev. Lett. 118, 155001 (2017).
J.P. Chittenden et. al. Physics of Plasmas 23, 052708 (2016).
O. A. Hurricane, et. al. Nature 506, 343 (2014).