Description
Accurate and efficient turbulent transport models are essential for developing operating scenarios for ITER and DEMO. The ITER 15 MA baseline scenario is a key operating scenario for ITER and is being extensively studied, often using quasi-linear models, which combine linear instability eigenvalues with ad hoc saturation rules to estimate turbulent transport. However, existing saturation rules were largely developed for electrostatic conditions [1,2]. This project aims to develop a saturation rule applicable to high-$\beta$ ITER-relevant plasmas.
The ITER baseline scenario aims to achieve a burning plasma with a physics gain of $Q=10$, corresponding to 500MW of fusion power, at a plasma current of 15MA and $q_{95}\simeq 3$. Preliminary studies have used the HFPS integrated modelling suite, with the quasi-linear gyrofluid model TGLF and local linear analyses using the gyrokinetic code GKW. These analyses indicate electromagnetic kinetic ballooning modes (KBMs) near the plasma centre and strong $\beta$ stabilisation of the ion temperature gradient (ITG) mode at mid-radius. HFPS-TGLF simulations show good agreement with high-fidelity GENE-Tango results using newly optimised high-$\beta$ TGLF settings, improving the linear response. However, the HFPS-TGLF simulations lack reduced density peaking in the electromagnetic case compared to the electrostatic case exhibited in the GENE-Tango simulations, likely due to $\beta$-effects on electron particle transport [3]. This will be explored by examining the weights from linear and non-linear simulations.
Most existing saturation rules assume zonal-flow-mixing [1,2]. By performing simulations with and without zonal-flow coupling, we will assess the validity of this assumption under high-$\beta$ conditions. The linear analysis indicates a difference in the critical gradient for the KBMs between TGLF and GKW. Non-linear simulations will therefore be conducted both near marginal stability and over a range of safety factor values, given the strong dependence of KBM and ITG thresholds on the safety factor. These scans will also investigate the impact of subdominant KBMs on the fluxes, which may be large [4].
[1] G. Staebler Nucl. Fusion 61 116007 (2021)
[2] H.G. Dudding Nucl. Fusion 62 096005 (2022)
[3] T. Hein, Phys. Plasmas 17, 102309 (2010)
[4] P. Mulholland, Phys. Rev. Lett. 131, 185101 (2023)