Electromagnetic gyrokinetic turbulence in tokamaks

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)

Brief statement on how the project aligns with EPSRC strategy/priority research areas: Magnetic confinement fusion is a major component of UK's long-term investment in energy security and efficiency, and thus a key element for EPSRC's "Resilient Nation" outcome. CCFE is one of the world's leading centres for fusion research, hosting the MAST-U and JET projects, and is the centre of the UK

Fusion Programme. The collaborative project with CCFE described above addresses directly a number of key challenges in fusion science. It falls within EPSRC's "Plasma and lasers" research theme ('maintain' status) through which the contribution to energy security is emphasised. EPSRC's 2016 review of Fission and Fusion flagged the effectiveness of university-CCFE interactions and the UK's strengths in modelling.

The project also provides training in high-level and mutually complementary theoretical and advanced computing skills, contributing to UK capabilities in data science more widely and thus EPSRC's "Connected Nation" ambition.