In the high-confinement regime of tokamak plasmas---essential to achieve a burning plasma---a transport barrier forms at the edge of the plasma, believed to be caused by suppression of ion-scale turbulence by pressure-gradient-driven sheared flows. The physics of the resulting edge "pedestal" region sets the confinement properties of the whole plasma and is thus pivotal for the entire device. The pedestal is a complex nonlinear system characterised by an interplay between steep equilibrium gradients (of density and temperature) and the turbulence that they trigger [1,2,3]---the turbulent particle and heat fluxes caused by this turbulence in turn decide the size of the gradients. A critical electron temperature gradient, proportional to the density gradient [4], is required for the linear instability [5] that drives electron-scale ETG turbulence and hence a finite heat flux. The formation of the electron-temperature pedestal is therefore intimately related to that of the density pedestal. Nonlinear gyrokinetic simulations [6,7] show that at significantly steeper gradients than the linear threshold, above the experimental operating point, the heat flux increases faster than linearly with the driving gradient, i.e., transport becomes "stiff", clamping the profiles to this nonlinear threshold. The existence of this threshold is thought to be related to the appearance of modes with a high parallel wavenumber, which are resonant with the parallel electron motion. This project's aim is to sort out the fundamental physics behind these phenomena and hence their quantitative dependence on the equilibrium parameters. Detailed comparisons of the outcome with experimental pedestal profile data from JET-ILW and MAST-U can be made over a range of conditions. Such information, suitably parameterised, can then be used to develop a reduced model of the pedestal, which is required to design future burning-plasma devices, e.g., STEP.

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 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 experimental/data analysis/advanced computing skills, contributing to UK capabilities in data science more widely and thus EPSRC's "Connected Nation" ambition.

1. A. R. Field et al., "The dependence of exhaust power components on edge gradients in JET-C and JET-ILW H-mode plasmas," PPCF 62, 055010 (2020)
2. C. J. Ham et al., "Towards understanding reactor relevant tokamak pedestals", Nucl. Fusion 61, 096013 (2021)
3. G. J. Colyer et al., "Collisionality scaling of the electron heat flux in ETG turbulence", PPCF 59, 055002 (2017)
4. F. Jenko et al., "Critical gradient formula for toroidal electron temperature gradient modes", Phys. Plasmas 8, 4096 (2001)
5. J. Parisi et al., "Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals", Nucl. Fusion 60, 126045 (2020)
6. D. R. Hatch et al., "Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls", PPCF 59, 086056 (2019)
7. W. Guttenfelder et al., "Testing predictions of electron scale turbulent pedestal transport in two DIII-D ELMy H-modes", Nucl. Fusion 61, 056005 (2021)