OXFORD THEORETICAL ASTROPHYSICS AND PLASMA PHYSICS GROUP

   

DPhil PROJECTS IN PLASMA PHYSICS

  starting in October 2024

 Application deadline: 5 January 2024

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, Prof Gianluca Gregori, Prof Alexander Schekochihin, Prof Dmitri Uzdensky (moving to Oxford in 2024)

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.

*** NEW PROJECTS MAY APPEAR ON THIS PAGE IN DUE COURSE. COME BACK AND CHECK! ***

 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, 1 in 2022, and 4 in 2023; we expect to take at least 3 in 2024.

Projects in Fusion Plasmas

MAST

  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. Rotation in high-beta spherical tokamaks
Supervisor: Prof Michael Barnes
UKAEA co-supervisor: Dr Daniel Kennedy
Many current fusion experiments rely on externally-applied, differential plasma rotation to stabilise macroscopic instabilities and to improve plasma confinement via the reduction of turbulent transport. Sheared rotation may be even more important in high-beta plasmas, where it appears to be crucial for suppressing transport from kinetic-ballooning-mode turbulence. However, in the large, dense plasmas envisioned for next-generation experiments and reactors, the usual techniques for rotating the plasma are unlikely to be effective. It is thus critical to predict the level of rotation in such plasmas and, likely, to develop alternative methods of generating rotation. We propose to work with a DPhil student to provide rotation predictions for high-beta spherical tokamaks and to explore alternative strategies for generating rotation. This work will build on previous studies of intrinsic rotation in conventional tokamaks at low beta. In particular, we will determine the amount of rotation that could be generated with the modest applied torques expected for a STEP-like device and then explore how much rotation can be generated via use of up-down asymmetric flux surface shapes, edge momentum input and other "intrinsic" rotation mechanisms.

2. Electromagnetic gyrokinetic turbulence in tokamaks
Supervisor: Prof Alex Schekochihin
UKAEA co-supervisors: Dr Bhavin Patel & Dr Daniel Kennedy
Electromagnetic (EM) gyrokinetic (GK) turbulence in tokamaks, and the associated transport of energy, particles, and momentum are not well understood physically and are even less well parametrised quantitatively, compared to the better known case of electrostatic (ITG and ETG) turbulence. Even the nature of the linear modes in this regime is an area where significant contributions can still be made, and indeed have been made recently [1-5]. Nonlinearly, a complex picture is emerging whereby a transition from electrostatic to electromagnetic regime occurs above a certain value of beta (ratio of thermal to magnetic energy density) and leads to significant enhancement of transport. This transition may be the culprit for the unbounded growth of heat fluxes observed in GK simulations of high-beta scenarios, in particular ones envisioned for the operation of STEP. It is clear that, with STEP and a number of other high-beta devices being considered promising, and likely to be built, the EM turbulence and transport must urgently be understood and quantified. The student will join the existing collaborative effort in this area between Oxford and Culham, and will focus in particular on: (1) a comparative theory of MTM vs. KBM/TAI/interchange transport in fusion plasma regimes characteristic of MAST-U and STEP; (2) whether there exists a (nonlinear) critical manifold in the parameter space that separates high-transport EM regimes from low-transport ("Dimits-shifted") states: are there definite no-go zones? are there sweet spots nearby? We shall also look at strategies for testing theory and modelling prescriptions using experimental output from MAST-U or NSTX---this would present a unique opportunity to link the theory of EM transport to the MAST-U programme and inform the design of devices such as STEP.

1. D. Kennedy et al., "Electromagnetic gyrokinetic instabilities in the Spherical Tokamak for Energy Production (STEP), Part I: Linear physics and sensitivity," arXiv:2307.01670
2. M. Giacomin et al., "Electromagnetic gyrokinetic instabilities in the Spherical Tokamak for Energy Production (STEP), Part II: Transport and turbulence," arXiv:2307.01669
3. B. D. G. Chandran and A. A. Schekochihin, "The gyrokinetic dispersion relation of microtearing modes in collisionless toroidal plasmas," arXiv:2211.02103
4. T. Adkins et al., "Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient," J. Plasma Phys. 88, 905880410 (2022)
5. B. S. Patel et al., "Linear gyrokinetic stability of a high beta non-inductive spherical tokamak," Nucl. Fusion 62, 016009 (2022)

3. Influence of energetic particles on plasma turbulence
Supervisor: Prof Dmitri Uzdensky (moving to Oxford in 2024)
UKAEA co-supervisor: Dr Michael Fitzgerald
Other Oxford collaborators: Dr Juan Ruiz Ruiz, Prof Michael Barnes, Prof Alex Schekochihin, & Dr Michael Hardman
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 copious high-energy alpha particles (3.5 MeV) will heat the rest of the plasma and lead to a self-heating state, a burning plasma. These conditions have not yet been achieved experimentally, and there are serious questions on the extrapolations of the existing models of turbulence to the burning regime, particularly for spherical tokamaks. Highly energetic particles are known to destabilise long-wavelength Alfven waves in spherical tokamaks such as MAST-U and ST40, and will certainly do so for ITER. There is growing experimental and computational evidence that these waves on an intermediate scale can mediate the small-scale turbulence that governs the plasma confinement. Candidate explanations for this could either be the generation of flows by the Alfven waves, or the wave-wave nonlinear interaction. At high plasma beta in machines such as STEP, the distinction between electromagnetic turbulence and Alfven waves is further blurred and the coupling between fast particles and turbulence is more fundamental. This DPhil project will focus on the following: (i) developing a theory of how Alfven waves mediate turbulence in fusion plasmas; (ii) estimating fast-particle population thresholds for the interaction between Alfven waves and turbulence, particularly for STEP conditions; (iii) comparing new gyrokinetic computations with fluctuation measurements on MAST-U.



 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 and universal equilibria in turbulent astrophysical plasmas
Supervisors: Prof Michael Barnes & Prof Alexander Schekochihin
(for this project, 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).

Finally, with a theory of plasma turbulence in hand, it is possible to attack what is probably the most fundamental question of the field: in the absence of collisions, are there universal equilibria, or classes of equilibria, independent of initial conditions, that a turbulent plasma will want to converge to? There is some recent progress indicating that the answer is yes and that one predict statistical-mechanically the emergence of universal power-law (in particle energy) distributions---this is exciting both on its own merits and because of the astrophysical challenge of explaining theoretically power-law distributions that are observed for, e.g., cosmic rays or solar-wind electrons. How to construct a theory of that for a magnetised, turbulent plasma is an open and exciting question. Attempting to do this will again involve kinetic theory and/or kinetic simulations.  

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 et al., "Thermal disequilibration of ions and electrons by collisionless plasma turbulence," PNAS 116, 771 (2019)
4. R. Meyrand et al., "Fluidization of collisionless plasma turbulence," PNAS 116, 1185 (2019)
5. J. Squire et al., "High-frequency heating of the solar wind triggered by low-frequency turbulence," Nature Astron. 6, 715 (2022)
6. R. J. Ewart et al., "Collisionless relaxation of a Lynden-Bell plasma,'' J. Plasma Phys. 88, 925880501 (2022)
7. R. J. Ewart et al., "Non-thermal particle acceleration and power-law tails via relaxation to universal Lynden-Bell equilibria," arXiv:2304.03715
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 & 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 et al., PRX 12, 041027 (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, & Prof Alexander Schekochihin
(for this project, apply for a DPhil in Astrophysics or a DPhil in Atomic and Laser Physics)
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)
6. 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
Supervisor: Prof Gianluca Gregori
Other Oxford collaborators: Dr Archie Bott, Prof Todd Huffman, Prof Subir Sarkar, & Prof Alexander Schekochihin
(for this project, apply for a DPhil in Astrophysics or a DPhil in Atomic and Laser Physics)
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)

9. Nonthermal particle acceleration in extreme plasmas around black holes
Supervisors: Prof Dmitri Uzdensky (moving to Oxford in 2024) & Prof Alexander Schekochihin
(for this project, apply for a DPhil in Astrophysics)
Accreting supermassive black holes (SMBHs) residing in active galactic nuclei (AGN) at the centres of many galaxies, including our own, are rightfully regarded as some of the most fascinating and enigmatic objects in the Universe. These black holes, and the powerful relativistic jets emanating from them, often exhibit spectacular, violent phenomena, such as bright high-energy flares with nonthermal X-ray and gamma-ray spectra. They are also viewed as the most likely generators of highly energetic non-electromagnetic observational "messengers": extremely relativistic cosmic rays and ultra-high-energy neutrinos, detected on Earth with dedicated ground-based observatories. All these observational signals indicate that BH environments are very efficient relativistic particle accelerators. Understanding how these cosmic accelerators work is an outstanding problem in modern high-energy astrophysics. Nonthermal particle acceleration is believed to be a natural product of collective kinetic nonlinear plasma processes, such as magnetic reconnection, shocks, and magnetised plasma turbulence taking place in weakly collisional plasmas. How these processes operate under the extreme physical conditions expected in the exotic relativistic plasma environments of accreting black holes is, however, not well understood. The situation is greatly complicated by special- and general-relativistic effects, by strong interaction of the energetic emitting plasma particles with radiation, and by quantum-electrodynamic effects such as pair creation. Elucidating the complex interplay of these effects with collective plasma processes and, consequently, their impact on particle acceleration is an active area of today's plasma astrophysics research, presenting a number of interesting, nontrivial theoretical challenges. This project will address these fundamental plasma-theoretical questions and their observational implications through a combination of analytical and computational methods, with the balance between theory and computation to be decided based on the student's preferences.