starting in October 2022

Application deadline: 7 January 2022

Oxford Plasma Theory Group welcomes applications for DPhil studies and research in plasma physics in the areas of magnetic confinement fusion (MCF) and plasma astrophysics (including "laboratory astrophysics")

Potential supervisors:
Prof Michael Barnes (projects 1, 3), Prof Alexander Schekochihin (projects 2, 3), Prof Gianluca Gregori (project 4), Prof Peter Norreys (project 5)

Size of intake: this depends on various hard-to-predict circumstances, in particular funding arrangements; we accepted 2 fully funded students in 2015, 4 in 2016, 3 in 2017, 3 in 2018, 1 in 2019, 3 in 2020, and 2 in 2021.


Projects in Fusion Plasmas


Our magnetic-confinement-fusion theory projects are offered jointly with researchers at the U.K.A.E.A. Culham Centre for Fusion Energy (Projects 1-2).

At the application stage, you are not required to (although you may if you wish) indicate which project you prefer --- we will consider all applicants purely on intellectual merit. If you are offered admission, we will strive to give you the opportunity to work on the project of your choice.
Note that the project descriptions given below are not set in stone and we are willing to discuss modifications and adjustments to them that might better reflect your interests and inclinations.

1. Plasma physics at the edge
Supervisor: Prof Michael Barnes
UKAEA co-supervisor: Dr John Omotani

In modern tokamak plasmas, the topology of the confining magnetic field changes abruptly near the plasma edge from nested toroidal surfaces to "open" field lines that intersect the material wall. This change in topology, combined with the presence of neutrals from the wall and with sharp changes in equilibrium plasma parameters, makes plasma dynamics in the open-field-line region---known as the scrape-off-layer, or SOL, for short---quite different from the dynamics in the plasma core; e.g., the core transport of particles, momentum and energy can be understood largely in terms of turbulent diffusion, whereas SOL transport is thought to be convective, with thin filaments (aka "blobs") responsible for much of the transport. A theoretical understanding and predictive capability for SOL dynamics is crucial because it determines, amongst other things, how heat will be distributed on the material wall. This in turn determines whether or not the walls of future fusion experiments and reactors will melt---and thus whether or not fusion is a viable energy source. The aims of this project are to develop a suitable set of model equations for edge plasma dynamics, to implement these equations numerically and to use the resultant simulations to study edge turbulence and transport.

2. Electron heat transport in tokamak edge pedestals

Supervisor: Prof Alexander Schekochihin
UKAEA co-supervisor: Dr Anthony Field

In the high-confinement regime of tokamak plasmas---essential to achieve a burning plasma---a transport barrier forms at the edge of the plasma, believed to be caused by suppression of ion-scale turbulence by pressure-gradient-driven sheared flows. The physics of the resulting edge "pedestal" region sets the confinement properties of the whole plasma and is thus pivotal for the entire device. The pedestal is a complex nonlinear system characterised by an interplay between steep equilibrium gradients (of density and temperature) and the turbulence that they trigger [1,2,3]---the turbulent particle and heat fluxes caused by this turbulence in turn decide the size of the gradients. A critical electron temperature gradient, proportional to the density gradient [4], is required for the linear instability [5] that drives electron-scale ETG turbulence and hence a finite heat flux. The formation of the electron-temperature pedestal is therefore intimately related to that of the density pedestal. Nonlinear gyrokinetic simulations [6,7] show that at significantly steeper gradients than the linear threshold, above the experimental operating point, the heat flux increases faster than linearly with the driving gradient, i.e., transport becomes "stiff", clamping the profiles to this nonlinear threshold. The existence of this threshold is thought to be related to the appearance of modes with a high parallel wavenumber, which are resonant with the parallel electron motion. This project's aim is to sort out the fundamental physics behind these phenomena and hence their quantitative dependence on the equilibrium parameters. Detailed comparisons of the outcome with experimental pedestal profile data from JET-ILW and MAST-U can be made over a range of conditions. Such information, suitably parameterised, can then be used to develop a reduced model of the pedestal, which is required to design future burning-plasma devices, e.g., STEP.

Background Reading:
1. A. R. Field et al., "The dependence of exhaust power components on edge gradients in JET-C and JET-ILW H-mode plasmas," PPCF 62, 055010 (2020)
2. C. J. Ham et al., "Towards understanding reactor relevant tokamak pedestals", Nucl. Fusion 61, 096013 (2021)
3. G. J. Colyer et al., "Collisionality scaling of the electron heat flux in ETG turbulence", PPCF 59, 055002 (2017)
4. F. Jenko et al., "Critical gradient formula for toroidal electron temperature gradient modes", Phys. Plasmas 8, 4096 (2001)

5. J. Parisi et al., "Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals", Nucl. Fusion 60, 126045 (2020)
6. D. R. Hatch et al., "Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls", PPCF 59, 086056 (2019)
7. W. Guttenfelder et al., "Testing predictions of electron scale turbulent pedestal transport in two DIII-D ELMy H-modes", Nucl. Fusion 61, 056005 (2021)

Projects in Plasma Astrophysics and Laser Plasmas

Candidates interested in any of these projects or generally in plasma astrophysics, astrophysical turbulence and/or dynamo theory are welcome to get in touch with prospective supervisors for further information. A more bespoke project can be designed to align with the inclinations and interests of the student (for example how much emphasis is placed on analytical vs. numerical methods or kinetic theory vs. fluid dynamics, etc., is negotiable).  

3. Free-energy flows in turbulent astrophysical plasmas
Supervisors: Prof Michael Barnes and Prof Alexander Schekochihin
for this project, you may also apply for a DPhil in Astrophysics)

In magnetised astrophysical plasmas, there is a turbulent cascade of electromagnetic fluctuations carrying free energy from large to small scales. The energy is typically extracted from large-scale sources (e.g., in the solar wind, the violent activity in the Sun's corona; in accretion discs, the Keplerian shear flow; in galaxy clusters, outbursts from active galactic nuclei) and deposited into heat---the internal energy of ions and electrons. In order for this dissipation of energy to happen, the energy must reach small scales---in weakly collisional plasmas, these are small scales in the 6D kinetic phase space, i.e., what emerges is large spatial gradients of electric and magnetic fields and large gradients of the particle distribution functions with respect to velocities. This prompts two very fundamental questions: (1) how does the energy flow through the 6D phase space and what therefore is the structure of the fluctuations in this space: their spectra, phase-space correlation functions etc. (these fluctuations are best observed in the solar wind, but we can measure density and magnetic fluctuations even in extragalactic plasmas, via X-ray and radio observations); (2) when turbulent fluctuations are dissipated into particle heat, how is their energy partitioned between various species of particles that populate the plasma: electrons, bulk ions, minority ions, fast non-thermal particles (e.g., cosmic rays). The latter question is particularly important for extragalactic plasmas because all we can observe is radiation from the particles and knowing where the internal energy of each species came from is key to constructing and verifying theories both of turbulence and of macroscale dynamics and thermodynamics. This project has an analytical and a numerical dimension (which of these will dominate depends on the student's inclinations). Analytically, we will work out a theory of phase space cascade at spatial scales between the ion and electron Larmor scales. Numerically, we will simulate this cascade using "gyrokinetic" equations---an approach in which we average over the Larmor motion and calculate the distribution function of "Larmor rings of charge" rather than particles (this reduces the dimension of phase space to 5D, making theory more tractable and numerics more affordable). 

Background Reading:
1. A. A. Schekochihin et al., "Astrophysical gyrokinetics: kinetic and fluid turbulent cascades in magnetized weakly collisional plasmas," Astrophys. J. Suppl. 182, 310 (2009)

2. A. A. Schekochihin et al., "Phase mixing vs. nonlinear advection in drift-kinetic plasma turbulence," J. Plasma Phys. 82, 905820212 (2016) (2019 IoP Payne-Gaposchkin Prize)
3. Y. Kawazura, M. Barnes, and A. A. Schekochihin,
"Thermal disequilibration of ions and electrons by collisionless plasma turbulence," PNAS 116, 771 (2019)
4. R. Meyrand, A. Kanekar, W. Dorland, and A. A. Schekochihin, "Fluidization of collisionless plasma turbulence," PNAS 116, 1185 (2019)
5. A. A. Schekochihin, Y. Kawazura, and M. A. Barnes,
"Constraints on ion vs. electron heating by plasma turbulence at low beta," J. Plasma Phys. 85, 905850303 (2019)

4. Magnetised plasma turbulence: from laser lab to galaxy clusters
Supervisors: Prof Gianluca Gregori and Prof Alexander Schekochihin
(for this project, you may also apply for a DPhil in Atomic and Laser Physics or a DPhil in Astrophysics)

There are a number of possibilities within this project to design, take part in, and theorise about laboratory experiments employing laser-produced plasmas to model astrophysical phenomena and basic, fundamental physical processes in turbulent plasmas. Recent examples of our work in this field include turbulent generation of magnetic fields ("dynamo") [1,2], supersonic turbulence mimicking star-forming molecular clouds [3], diffusion and acceleration of particles by turbulence [4,5], suppression of thermal conduction in galaxy-cluster-like plasmas [6]. Our group has access to several laser facilities (including the National Ignition Facility, the largest laser system in the world). Students will also have access to a laser laboratory on campus, where initial experiments can be fielded. Depending on the student's inclinations, it is also possible to pursue a project focused on theory and/or numerical modelling of plasma phenomena in astrophysical and laboratory-astrophysical environments.

Background Reading:
1. P. Tzeferacos et al., "Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma," Nature Comm. 9, 591 (2018) (2019 APS Dawson Prize)
2. A. F. A. Bott et al.,
"Time-resolved fast turbulent dynamo in a laser plasma," PNAS 118, e2015729118 (2021) (2020 EPS PhD Award in Plasma Physics)
3. T. G. White et al.,
"Supersonic plasma turbulence in the laboratory," Nature Comm. 10, 1758 (2019)
4. A. F. A. Bott et al.,
"Proton imaging of stochastic magnetic fields," J. Plasma Phys. 83, 905830614 (2017)
5. L. E. Chen
et al., "Transport of high-energy charged particles through spatially intermittent turbulent magnetic fields," Astrophys. J. 892, 114 (2020)
J. Meinecke et al., "Strong suppression of heat conduction in a laboratory analogue of galaxy-cluster turbulent plasma," preprint arXiv:2105.08461

5. Real photon-photon scattering using ultra-intense lasers
Supervisor: Prof Peter Norreys
(for this project, you should apply for a DPhil in Atomic and Laser Physics)

Quantum electrodynamics (QED), described by Feynman as "the jewel of Physics", is an exquisite description which brings together Einstein's special theory of relativity with the non-intuitive world of quantum mechanics. It is one of the most successful models of modern physics, laying the foundations of our understanding of the interactions of matter and light. Yet, despite continuing attempts, one of its key predictions remains unproven by experiment. Classical physics states that light cannot interact with itself the absence of matter---shine two beams of light at each other and they pass through, unaffected. However, QED predicts that photons, particles of light, should be able to scatter off each other even in complete vacuum. If this can be proven experimentally, and the photon scattering patterns analysed, it would provide not only confirmation of our understanding of the Standard Model of Physics, but enable us to explore some theories of "Beyond the Standard Model" physics which are needed to completely explain physical properties of our Universe. We have a four-year project that brings together a team of academic experts from UK and international universities with experienced staff from the Central Laser Facility, the UK's national experimental laser site, first to build and test new laser optics and detection equipment required for these experiments and then use this instrumentation to perform the ultimate experiments using the world's highest power lasers at the "Super-intense Ultrafast Laser Facility" in China.  Our team also combines outstanding theorists in Oxford Physics who will create mathematical models for the photon interactions, with leading scientists who have contributed to some of the greatest physical discoveries in recent times, such as the detection of the Higgs boson at the Large Hadron Collider. Together they will enable the optimisation of our experimental procedures, provide predictions on signatures which would be generated by phenomena beyond the standard model of physics and constrain our experimental results. Please apply to join our project for your DPhil and join us in the unique physics investigation.