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

Laboratory astrophysics provides a mechanism to test our understanding of the behaviour of astrophysical bodies by using dimensional scaling to design laboratory experiments which behave in a similar manner. Such experiments must be carefully designed to replicate the physical processes which make the non-linear evolution of supersonic flows in dense astrophysical plasmas very different to that found in conventional fluid dynamics. The intense X-ray radiation generated within shock fronts allows the plasma to cool, making it more compressible and collapsing the shock until it becomes too thin to remain hydrodynamically stable. This process is believed to contribute to the break-up of expanding blast waves which form supernovae remnants. Transport and reabsorption of some of this X-ray radiation produces a ‘precursor’ propagating ahead of the shock discontinuity which fundamentally changes the nature of the shock front. Strong density and temperature gradients at the shock front or elsewhere in the plasma can also be the source of spontaneously generated magnetic fields which are thought to be a possible candidate for the first ‘primordial’ magnetic fields generated within the universe. Compression of these fields can lead to strongly magnetised flows which again modify the nature of the shock as is the case in the Earth’s magneto-spheric bow-shock. Magnetic fields can also be responsible for the development of instabilities within supersonic flows, leading eventually to turbulent magnetised flows which again have very different properties to turbulence in conventional fluid dynamics.

Experiments designed to replicate these processes in the laboratory, typically require the use of large scale lasers or pulsed power generators in order to achieve the high energy density plasma states required. Recent work has included the use of the Orion and Omega lasers to produce radiative blast waves or supersonic rotating plasmas as well as the use of the 1.5MA Magpie generator at Imperial College to investigate magnetic reconnection in supersonic flows. The principle computational tool which enables the design of new experiments on Magpie and other pulsed drivers is the Gorgon 3D magneto-hydrodynamics tool which was developed within the Plasma Physics group at Imperial College.

This PhD studentship will involve the adaptation of the Gorgon MHD code to new laboratory astrophysics configurations to enable the study of the effects of radiation and magnetic fields on the stability of blast waves, particle acceleration in processes in magnetised shocks, studies of magnetically decelerated supersonic flows and the compression of turbulent magnetised plasmas.  The work will involve the continued development of elements of the core MHD model as well as post-processing models which generate ‘synthetic diagnostics’ to facilitate comparison to experimental data. The student will be expected to collaborate with experimental groups at Imperial College, Cornell University, UCSD and Sandia National Laboratories as well as computational and theoretical groups at the Universities of Rochester and Paris VI.

Background reading

B.Remington, Plasma Phys. Control. Fusion 47 A191 (2005) doi:10.1088/0741-3335/47/5A/014

F. Suzuki-Vidal, et. al. Phys. Rev. Lett. 119, 055001 (2017) doi: 10.1103/PhysRevLett.119.055001

M. Bocchi, B. Ummels, J.P. Chittenden, et. al. The Astrophysical Journal 84, p767 2013 (doi:10.1088/0004-637X/767/1/84)

J. Hare Phys. Rev. Lett 118, 085001 (2017) doi: 10.1103/PhysRevLett.118.085001

Type:   Experimental, including development of diagnostics

Laboratory plasma physics experiments allow reproducing, in a scalable fashion, a variety of dynamic astrophysical phenomena relevant to the physics of astrophysical jets and outflows, radiative shock waves, plasma flows interacting with magnetized bodies, magnetic reconnection and more. We perform laboratory astrophysics experiments at our 1.4MA MAGPIE pulsed power facility at Imperial College, participating in the development of magnetic reconnection and collisionless shock experiments at the Z facility at Sandia National Lab in the USA and collaborate with groups at Cornell University, University of Michigan, UCSD and the University of Rochester. A possible PhD project would focus on the understanding of the onset and evolution of the radiative cooling instabilities in magnetised high energy density plasmas, and formation of laser-driven collisionless shocks. Work will involve modification of the already existing experimental set-ups, designing appropriate targets allowing quantitative characterisation of the plasma parameters, and designing and testing of new experimental configurations. The project will also involve the development and implementation of advanced plasma diagnostics, such as Thomson scattering, interferometry, optical and x-ray imaging and spectroscopy and X-ray radiography.

Background reading:

S.V. Lebedev, A. Frank, D.D. Ryutov, “Exploring astrophysics-relevant magnetohydrodynamics with pulsed-power laboratory facilities”, Reviews of Modern Physics 91, 025002 (2019).

J.D. Haliday et al., “Investigating radiatively driven, magnetized plasmas with a university scale pulsed-power generator”, Phys. Plasmas, 29, 042107 (2022).

D. R. Russell et al., “Perpendicular Subcritical Shock Structure in a Collisional Plasma Experiment”, Phys. Rev. Letters, 129, 225001 (2022)

L.G. Suttle et al., “Interactions of magnetized plasma flows in pulsed-power driven experiments”, Plasma Physics and Controlled Fusion, 62, 014020 (2020)

F. Suzuki-Vidal et al., “Bow Shock Fragmentation Driven by a Thermal Instability in Laboratory Astrophysics Experiments”, The Astrophysical Journal 815, 2, p. 96 (2015)

 For more information, please contact Sergey Lebedev.

Time Resolved X-Ray Spectroscopy using Laser Wakefied Accelerators

Professor Stuart Mangles

Type: experimental with computional modelling 

Funding:  JAI and Science Technology and Facilities Council - STFC studentship

Description:  This PhD project will research the use of X-Rays produced by laser wakefield accelerators for ultrafast time-resolved absorption spectroscopy.

The project will develop X-Ray absorption methods for the UK's new EPAC laser facility and will provide insights into the potential of laser wakefield accelerators for a range of of applications, including time-resolved studies of dynamic processes in material systems, detection of ultrafast electronic dynamics, and matter in extreme conditions.