starting in October 2023

Application deadline: 6 January 2023

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, Dr Archie Bott, Dr Juan Ruiz Ruiz, Prof Alexander Schekochihin, Prof Gianluca Gregori

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.


 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, 2 in 2021, and 1 in 2022; we expect to take at least 2 in 2023.

Projects in Fusion Plasmas


Some of our magnetic-confinement-fusion theory projects are offered jointly with researchers at the U.K.A.E.A. Culham Centre for Fusion Energy.

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.

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. Plasma turbulence in 3D magnetic fields
Supervisor: Prof Michael Barnes

Turbulent transport limits the confinement of tokamak plasmas, i.e., plasmas immersed in axisymmetric---and thus 2D---magnetic fields. Consequently much effort has gone into understanding the properties of turbulence and transport in tokamaks. However, perfect axisymmetry of the confining magnetic field is marred by design constraints and by large-scale plasma instabilities. Furthermore, non-axisymmetric fields are potentially preferable for fusion reactors because they do not require a current through the plasma for confinement and thus allow for steady-state operation. There is thus considerable interest in stellarators, which use 3D magnetic fields to confine the plasma. Despite the importance of the problem, there has been little work done on turbulence in 3D magnetic fields. The aim of this project is to understand how breaking axisymmetry affects plasma turbulence. In particular, can 3D effects on turbulence explain some of the mysterious transport phenomena observed in tokamaks, and how much turbulent transport can we expect in optimised stellarators? The student would address these questions with a combination of analytical theory and high performance computing.

3. Energetic particles and plasma turbulence
Supervisors: Dr Juan Ruiz Ruiz & Prof Michael Barnes
UKAEA co-supervisor: Dr Michael Fitzgerald

Future fusion power plants will operate with a Deuterium-Tritium mixture that will produce highly energetic alpha particles and neutrons as products from the DT fusion reaction. The high-energy alpha particles (3.5 MeV) will heat the rest of the plasma and lead to a self-heating state, a burning plasma. Such a state is the "holy grail" of controlled thermonuclear fusion energy, but has not been experimentally achieved to date. Confinement of highly energetic particles is the first concern, both from the point of view of maintaining a burning plasma and because they could damage the reactor vessel's walls of magnetic confinement devices. Highly energetic particles can destabilise certain types of Alfven waves [1] (Alfven eigenmodes), which can in turn produce high transport of the energetic particles out of the device [2]. This is detrimental to their confinement and therefore to the overall power balance of a fusion power plant. Highly energetic particles can also interact with the background turbulence [3-6], which will then affect the overall plasma transport and confinement of a power plant. Little is known about the interactions between energetic particles, Alfven eigenmodes and turbulence, however these are critical for predicting and designing burning plasmas. This project seeks to combine experimental measurements of turbulence and Alfven eigenmodes with state-of-the-art turbulence simulations (gyrokinetic simulations) to develop a theoretical understanding of these interactions.

1. M. N. Rosenbluth & P. H. Rutherford, "Excitation of Alfven waves by high-energy ions in a tokamak," Phys. Rev. Lett. 34, 1428 (1975)
2. E. M. Bass & R. E. Waltz, "Gyrokinetic simulations of mesoscale energetic particle-driven Alfvenic turbulent transport embedded in microturbulence", Phys. Plasmas 17, 112319 (2010); "Nonlinear verification of a linear critical gradient model for energetic particle transport by Alfven eigenmodes", Phys. Plasmas 24, 122302 (2017)
3. J. Citrin et al., "Nonlinear stabilization of tokamak microturbulence by fast ions", Phys. Rev. Lett. 111, 155001 (2013)
4. G. Wilkie et al., "First principles of modelling the stabilization of microturbulence by fast ions", Nuclear Fusion 58, 082024 (2018)
5. A. Di Siena et al., " Electromagnetic turbulence suppression by energetic particle driven modes," Nuclear Fusion 59, 124001 (2019)
6. S. Mazzi et al., "Enhanced performance in fusion plasmas through turbulence suppression by megaelectronvolt ions," Nature Phys. 18, 776 (2022)

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).  

4. 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 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)
6. R. Meyrand, J. Squire,  A. A. Schekochihin, and W. Dorland, "On the violation of the zeroth law of turbulence in space plasmas," J. Plasma Phys. 87, 535870301 (2021) 

5. Anomalous laser heating and thermal conduction in magnetised, weakly collisional plasmas
Supervisors: Dr Archie Bott & Prof Gianluca Gregori
(for this project, apply for a DPhil in Atomic and Laser Physics)

One notable achievement of early 20th century physics was the development and experimental validation of theories of the material properties (such as thermal conductivity or viscosity) of everyday gases. Today, there is a pressing need to establish analogous theories for the material properties of the magnetised, weakly collisional plasmas found in inertial-confinement fusion (ICF) experiments [1,2] and also in many extreme astrophysical environments [3]---for example, the intracluster medium (ICM) of galaxy clusters or black hole accretion flows. In ICF research, accurate modelling of heat transport is crucial for realising high-yield target designs. In astrophysics, material properties are thought to play a key role in long-standing conundrums such as the ICM's anomalous temperature profile. Correctly interpreting high-quality astronomical observations of astrophysical systems (such as the now-famous EHT image of the M87 black hole) also requires a robust understanding of the underlying physics of the plasma composing them. Yet astronomical observations and measurements from laser-plasma experiments have shown that classical models for the transport properties of magnetised, weakly collisional plasmas often fail dramatically [4]. Thanks to recent technological advances in both high-performance computing and high-energy laser facilities, now is ideal for studying this problem systematically, combining theory, simulations, and experiments. During a project on this topic, a student would focus on the design and subsequent delivery of a new experiment that will characterise laser heating and thermal conductivity in magnetised, weakly collisional plasma. Measurements will then be directly compared to newly derived models [5-7]. While it is intended that the project will be experimentally focused, there is scope for the student to pursue different aspects of the experiment depending on their interests, including data analysis techniques and simulations.

Background Reading:
1. R. S. Craxton et al., Phys. Plasmas 22, 110501 (2015)
2. M. A. Barrios et al., Phys. Rev. Lett. 121, 095002 (2018)
3. M. W. Kunz et al., arXiv:1903.04080 (2019) 
4. J. Meinecke et al., Science Adv. 8, eabj6799 (2022)
5. S. Komarov et al., J. Plasma Phys. 84, 905840305 (2018)
6. J. F. Drake et al., Astrophys. J. 923, 245 (2021)
7. F. Miniati & G. Gregori, Sci. Rep. 12, 11709 (2022)

6. Magnetic-field amplification by the fluctuation dynamo in astrophysical plasmas
Supervisors: Dr Archie Bott and Prof Alexander Schekochihin
(for this project, apply for a DPhil in Astrophysics)

Understanding cosmic magnetism is a long-standing problem in astrophysics. Astronomical observations of various astrophysical environments---for example, those of our own galaxy by the MeerKAT telescope [1] or of more distant galaxy clusters [2]---reveal the presence of finely structured magnetic fields of sufficient strength to affect the dynamics of such systems. A possible mechanism for explaining the presence and characteristics of these magnetic fields is the so-called fluctuation dynamo, whereby stochastic or turbulent motions of the plasma in which these magnetic fields are embedded amplify and maintain them [3]. The fluctuation dynamo has mostly been studied using simplified magnetohydrodynamic (MHD) plasma models whose key transport properties (viz., resistivity and viscosity) do not depend on macroscopic plasma properties such as density, temperature, and the magnetic field. However, the plasma often found in the astrophysical systems of interest is often weakly collisional, and current theories for the transport properties of such plasma typically do predict a dependence on macroscopic plasma properties. It is an outstanding question whether "anomalous" transport properties can explain why certain predictions of the simplified theory of the fluctuation dynamo are at odds with astronomical observations (for example, that the energy-containing scale of the magnetic field is macroscopic). In this project, a student would explore how more realistic models of transport properties affect the fluctuation dynamo using both theory and numerical simulations, and then attempt to identify key differences with simplified dynamo theories that could be detected in astronomical observations.

Background Reading:
1. I. Heywood et al., Nature 573, 235 (2019)
2. C. L. Carilli & G. B. Taylor, Annu. Rev. Astron. Astrophys. 40, 319 (2002)
3. F. Rincon, J. Plasma Phys. 85, 205850401 (2019) 
4. A. A. Schekochihin et al., Astrophys. J. 612, 276 (2004)
5. A. K. Galishnikova, M. W. Kunz, A. A. Schekochihin, arXiv:2201.07757 (2022)
6. A. A. Schekochihin, S. C. Cowley, Phys. Plasmas 13, 056501 (2006)
7. A. F. A. Bott et al., PNAS 118, e2015729118 (2021)

7. Magnetised plasma turbulence: from laser lab to galaxy clusters
Supervisors: Prof Gianluca Gregori, Dr Archie Bott, 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," Science Adv. 8, eabj6799 (2022)

8. Microphysics of gamma-ray bursts
Supervisors: Prof Gianluca Gregori, Dr Archie Bott, and Prof Alexander Schekochihin (collaborators: Prof Subir Sarkar & Prof Bob Bingham)
(for this project, apply for a DPhil in Astrophysics)

Gamma-ray bursts (GRBs) are among the most energetic events in the Universe. They occur at cosmological distances and are the result of the collapse of massive stars or neutron stars mergers, with emission of relativistic "fireballs" of electron-positron pairs. From astrophysical observations, a wealth of information has been gleaned about the mechanism that leads to such strong emission of radiation, with leading models predicting that this is due to the disruption of the beam as it blasts through the surrounding plasma. This produces shocks and hydromagnetic turbulence that generate synchrotron emission, potentially accelerating to ultra-high energies the protons which are observed on Earth as cosmic rays. However, there is no direct evidence of the generation of either magnetic fields or cosmic rays by GRBs. Estimates are often based on crude energy equipartition arguments or idealized numerical simulations that struggle to capture the extreme plasma conditions. We propose to address this lacuna by conducting laboratory experiments at large laser and accelerator facilities to mimic the jet propagation through its surrounding plasma. Such experiments will enable in situ measurement of the plasma properties, with exquisite details that cannot be achieved elsewhere. The experiments also complement numerical simulations by providing long measurement times extending into the non-linear regime where numerical simulations are not possible today. The proposed experiments will study fundamental physics processes, unveil the microphysics of GRBs, and provide a new window in high energy astrophysics using novel Earth-based laboratory tools.

Background Reading:
1. C. D. Arrowsmith et al., "Generating untradense pair beams using 400 GeV/c protons," Phys. Rev. Res. 3, 023103 (2021)