Influence of energetic particles on plasma turbulence

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.

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.