PhD opportunities

Laboratory Astrophysics and High Energy Density Plasma experiments: magnetised shocks, transition to turbulence and magnetic reconnection

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 experiments at the Z facility at Sandia National Lab in the USA and collaborate with groups at Cornell University, UCSD and the University of Rochester. The main focus of the possible PhD project will be on the understanding of the evolution of magnetic fields in magnetised high energy density plasmas, the relative importance of the field advection by the plasma flow versus resistive diffusion and the non-MHD effects. 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).

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

J.D. Hare et al., “Anomalous Heating and Plasmoid Formation in a Driven Magnetic Reconnection Experiment”, Physical Review Letters 118, 085001 (2017)

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

Modelling the effects of radiation and magnetic fields on shocks and turbulent flows in laboratory astrophysics experiments

 
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 laser to produce radiative blast waves and 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 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

Using Laser Wakefield Accelerators Probing strong field QED

When charged particles accelerate they radiate, and so must lose energy and momentum. The force on a charged particle in an electromagnetic field, the Lorentz force does not include this “radiation reaction”.  Radiation reaction can be added ad hoc into classical electromagnetism, however as the force on the electrons increases it becomes necessary to include quantum effects. Various approaches to modelling this quantum radiation reaction have been proposed, but they are yet to be tested experimentally.

Quantum radiation reaction plays a role anywhere there are extreme electromagnetic fields, including the extreme magnetic fields of magnetars and spinning blackholes.  

In the lab one way to reach the quantum radiation reaction regime is to collide a high energy electron bunch with a high intensity laser as in the rest frame of the electron bunch the electric fields can approach the critical field of quantum electrodynamics. 

In 2018 our group published some of the first measurements of radiation reaction in the collision between a high intensity laser and a high energy electron beam, using a Laser Wakefield accelerator.  However, the measurement was not able to clearly discriminate between different models of quantum radiation reaction. We are actively pursuing further experiments in this area to make the first definitive measurement of quantum radiation reaction and are looking to recruit a PhD student in 2021 to join this program.

For further information please contact Dr Stuart Mangles.

Using Laser Wakefield Accelerators to Make Matter From Light

When two photons collide normally nothing happens, however if the photons have enough energy they can collide creating an electron-positron pair.  This process was first predicted by Breit and Wheeler in the 1930s, but has not been observed in the laboratory using real photons. This is because it is a major challenge to produce bright enough photon sources to make a sufficient number of positrons to be detected.  

The Breit Wheeler process is important in astrophysics - it prevents very high energy gamma rays travelling long distances across the universe and also leads to production of vast amounts matter in the accretion disks near black holes. 

 We are developing  a new platform for making this observation for the  first time, based on the very bright photon sources that can be produced using laser-plasma interactions. By generating high-energy gamma rays using the electron beam from a Laser Wakefield accelerator and colliding them with the X rays generated by a rapidly heated plasma we expect to be able to produce up to 1 pair per shot. 

Our first run at the Gemini laser facility in 2018 successfully showed that the background levels are sufficiently low to make a measurement, and by making a number of improvements to the experiment we hope to be able to make a clear observation. We are looking to recruit a student in 2021 to develop the improvements necessary to make a clear observation of light turning into matter.   

For more information please contact Stuart Mangles.

Using Laser Wakefield Accelerators to Probe Matter in Extreme Conditions

Much of the visible universe exists in extreme conditions, for example at high pressures and  temperatures inside stars and gas giant planets or in the presence of intense x-ray fluxes (eg the low density gas near black holes). Much of our understanding of these systems comes from detailed atomic physics calculations.  However testing these models experimentally is very challenging -- such extreme conditions can now be created in the lab but only for very short periods of time (~ 1ps).

One of the ideal ways to study these highly transient lab systems under extreme conditions is to use X-ray absorption spectroscopy, and the ideal X-ray source would have a broad spectrum (to allow absorption features to be observed) and femtosecond duration (to freeze the transient behaviour). Our group has pioneered the development of X-ray radiation from laser wakefield accelerators which uniquely has both these properties. 

We have recently developed an experimental platform for making X ray absorption measurements on the Gemini laser system (Kettle PRL 2019) and we are interested in recruiting students to start in 2021 who are interested in experimental or theoretical aspects of the study of matter in extreme conditions.

For more information please contact Stuart Mangles.