Turbulence and transport in the presence of electromagnetic fluctuations and supra-thermal particles in tokamaks.

Providing a long-term solution to the world energy problem and climate change is one of the most scientifically challenging endeavours that faces humanity on the global scale. Fusion energy is a particularly attractive solution and is poised to become a viable energy source in the coming decades by providing carbon-free, steady-state, high energy density in the absence of radioactive waste. However, confinement of the hot plasma is remarkably complicated to achieve in fusion conditions. Magnetic confinement fusion is championed by the tokamak concept, which uses strong magnetic fields in a donut-shaped device to produce the confinement. Despite the confining magnetic fields, experiments and theory have provided strong evidence that turbulent processes in the plasma produce a constant leakage of heat and particles out of the hot confining core, which impedes efficient generation of the fusion processes. This motivates understanding the plasma turbulent processes leading to heat and particle transport from a fundamental perspective, as well as its implications for real life tokamak experiments.

Recent numerical and theoretical studies have shown that fluctuations in the electromagnetic field can lead to enhanced transport losses in the spherical tokamak core at sufficiently high values of beta (ratio of plasma to magnetic pressure), leading to a paradigm shift between the traditional electrostatic description of the 'so-called' ion-scale turbulence (characteristic of the outer-core conventional tokamak) to a fully electromagnetic description and new transport processes. Electromagnetic, meso-scale instabilities (Alfvén eigenmodes) can also be driven by the presence of supra-thermal particles, and are capable of stabilising electrostatic ion-scale turbulence fluctuations. The stabilisation of ion-scale turbulence in the ST and the inner tokamak core leads to unexplored confinement regimes that are likely dominated by the electron heat losses. These are traditionally subdominant to ion heat losses in conventional core tokamak plasma scenarios. These regimes are critically relevant for future fusion reactors which are expected to have dominant electron heating and transport. The novelty of this research is to study these unexplored confinement regimes by combining experimental measurements of the electron fluctuations that are believed to be responsible for the electron heat transport (extremely scarce to date), direct numerical turbulence simulation (gyrokinetic simulation), synthetic diagnostics for the quantitative interpretation of the experimental measurements, and analytical theory to yield a fundamental understanding of the experimental and numerical findings.

Programmatically, the study of electromagnetic fluctuations and Alfvén eigenmodes driven by fast particles is important as they bridge the gap between current machine operation and future fusion reactors. The upcoming JET DT campaign (CCFE, UK), MAST-U (CCFE, UK) and NSTX-U experiments (Princeton, USA) are the missing link between present-day machines and fusion burning plasma experiments such as ITER, STEP (UK) and SPARC (US). Scientifically, this research will lead to ground-breaking discoveries such as new interaction mechanisms between supra-thermal particles and turbulence or the discovery of enhanced confinement regimes expected of future fusion reactors. This will have direct influences affecting the projections and design of the future STEP and ITER burning-plasma experiments, and will enable the UK to gain full in-house energy independence from magnetic fusion in the coming decades.