Multiscale turbulent dynamics of tokamak plasmas

Plasma turbulence underpins a wide range of phenomena, including the formation of stars and galaxies; the properties of the solar wind, and - the focus of this programme - the confinement of plasmas in tokamaks. It is complicated by feedback mechanisms that couple space and time scales spanning several orders of magnitude. The full problem is extremely challenging, and so to make progress for real world applications we must develop reduced models that capture the essential physics. The goal of our proposed programme is to address this by advancing our understanding of these multi-scale interactions at a fundamental science level. This will be achieved by coupling analytic theory, advanced computation and experimental capabilities, including the newly upgraded MAST-U tokamak.

Plasma turbulence is complicated by the fact that there are at least two types of interacting "fluids" - electrons and ions - and these are charged. Fluctuations in density therefore drive charge separation and hence fluctuations in the electrostatic field, while fluctuations in velocity drive currents and hence fluctuations in the magnetic field. These fields then couple the relative motions of the electron and ion "fluids". The situation is further complicated by the rich variety of waves that a magnetised plasma supports, and the resonances that exist when the phase velocity of a wave matches the particle velocity. To properly treat these resonances requires knowledge of the particle velocity distribution; this, in turn, requires either a kinetic or an advanced fluid approach - a daunting task.

Turbulence, typically at the millimetre-centimetre scale in tokamaks, interacts in a complex way with the global equilibrium profiles (density, temperature and flow gradients, for example), which are on the metre-scale. To quantify the complex, multi-scale feedback mechanisms between tokamak plasma turbulence and profiles, and so provide a predictive capability for the quasi-steady final states, we will address and integrate a number of topics. We will first learn how mean flows interact with electrostatic turbulence (ie neglecting fluctuations in the magnetic field), requiring coupling between fluctuations with characteristic scales ranging from the electron Larmor radius (sub-mm) through to the ion Larmor radius (few mm) and beyond (cm), to the system length scale of the profiles (m). Our new theory and simulations will inform experiments on MAST-U, exploiting two diagnostic instruments already planned for the device (beam emission spectroscopy and doppler back-scattering). It is likely there will be gaps in the wavelength range that these instruments can measure, so we anticipate a need to develop and install a new microwave imaging system. This will be designed using knowledge gained from the early phase of the programme, and deployed for further experiments towards the end.

Understanding of electromagnetic turbulence is less developed and new theoretical models will be required. Building on the knowledge gained from the electrostatic turbulence, we will seek to again understand the multi-scale interactions and feedbacks, including flows. However, now the situation is more complicated as electromagnetic turbulence can drive large scale currents, modifying the magnetic field which confines the plasma, and coupling into large scale electromagnetic modes.

A key motivation is to optimise tokamak plasmas for fusion performance, and this requires us to understand the impact of fast particles. These can drive turbulence directly through the instabilities they excite, or influence the turbulence driven by the thermal particles. Our simulations will assess the impact of the fast particles created by the neutral beam heating systems on MAST-U, and also the impact of energetic alpha particles from fusion reactions on future devices like ITER, as well as experiments planned on JET with the deuterium-tritium mix fusion fuel.

The largest impact will be on the fusion programme. This impact will occur at several levels, as follows.
(1) A greater understanding of turbulence and confinement on MAST-U will enable us to identify a wider range of plasma scenarios on MAST-U. These, in turn, will broaden the range of plasma parameters achievable on MAST-U, further extending its flexibility for a wide variety of plasma science experiments. This will benefit many UK researchers, but will also make the device even more attractive for international participation which will increase opportunities for international funding for MAST-U operations (EUROfusion, and the wider international community).
(2) Our improved understanding will feed into the development of operational scenarios for the DT operation of JET and ITER. This will, in turn, help maintain UK influence in ITER, and contribute towards ITER meeting its fusion objectives in a timely way. This is important, as the design of the first demonstration power plants, and hence the commercialisation of fusion energy, cannot be finalised until the first results are obtained from burning fusion plasmas in ITER.
(3) The size and capital cost of a tokamak fusion power plant depend to a large extent on the effectiveness of the confinement which, in turn, depends on turbulence. We will exploit our improved understanding of turbulence to seek techniques to further enhance tokamak confinement, and feed these results into the integrated design studies of fusion reactors which are presently under way (e.g. at UKAEA). If we are successful, our research results will underpin designs for more compact fusion reactors, that could form the basis for small modular fusion power plants, or for a fusion components test facility that would be valuable for mitigating risks of delays to commercial fusion power.

The main pathway to realising the above impacts will be through our close links with the UK national fusion programme led by UKAEA. However, we also have close collaborative links with international programmes, including those in Europe, USA, S Korea and China. These collaborative links will provide us with a broader range of fusion facilities and expertise to help meet our research objectives, but also provide a pathway to international impact. We will, of course, seek to strengthen our impact via high profile presentations at international conferences and publications in high impact journals. While our research has a clear application, it is also developing fundamental plasma science. As such, there is potential for publications in journals with a wider readership, beyond the fusion community, such as in Nature and Science.

Turbulence is a feature of most magnetised plasma systems, and plays a role in many solar and astrophysical phenomena, as well as laser-produced plasmas. We have contacted key individuals from astrophysics groups from 14 UK universities, asking about their interest in a 'National Centre for Turbulence in Magnetised Plasmas' that would cut across astrophysical and laboratory plasmas. The fundamental science has similarities across the disciplines, so the different systems, when brought together, could provide opportunities to advance the field beyond the capabilities of the individual communities. We will launch this new Centre as a key part of our Programme, organising three national meetings which, we anticipate, will act as a platform on which to build a strong interdisciplinary plasma turbulence community that will last well beyond the term of the Programme Grant.

A sustainable, cost-effective source of energy is fundamental to our standard of living and the national economy. We will ensure that the general public, industrialists and policy makers are kept aware of the significant contribution that fusion energy can make as a key part of a portfolio of possible solutions through public lectures and targeted events at our universities.