Simulation and analysis of a high-k electron-scale turbulence diagnostic for MAST-U

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Speirs, David C and Ruiz Ruiz, Juan and Giacomin, Maurizio and Hall-Chen, Valerian Hongjie and Phelps, Alan D R and Vann, Roddy G L and Huggard, Peter and Wang, Hui and Field, Anthony Robert and Ronald, Kevin (2025) Simulation and analysis of a high-k electron-scale turbulence diagnostic for MAST-U. Nuclear Fusion. ISSN 1741-4326 (In Press) (https://doi.org/10.1088/1741-4326/adb761)

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Abstract

Plasma turbulence on disparate spatial and temporal scales plays a key role in defining the level of confinement achievable in tokamaks, with the development of reduced numerical models for cross-scale turbulence effects informed by experimental measurements an essential step. MAST-U is a well-equipped facility having instruments to measure ion and electron scale turbulence at the plasma edge. However, measurement of core electron scale turbulence is challenging, especially in H mode. Using a novel synthetic diagnostic approach, we present simulated measurement specifications of a proposed highly optimised mm-wave based collective scattering instrument for measuring both normal and binormal electron scale turbulence in the core and edge of MAST-U. A powerful modelling framework has been developed that combines beam-tracing techniques with gyrokinetic simulations to predict the sensitivity and spectral range of measurement, with a quasi-numerical approach used to analyse the corresponding instrument selectivity functions. For the reconstructed MAST 022769 shot, a maximum measurable normalised bi-normal wavenumber of k_⊥ ρ_e ~ 0.6 was predicted in the core and k_⊥ ρ_e ~ 0.79 near the pedestal, with localisation lengths L_FWHM ranging from ~0.4 m in the core at k_⊥ ρ_e ~ 0.1 to ~0.08m at k_⊥ ρ_e> 0.45. Synthetic diagnostic analysis for the 022769 shot using CGYRO gyrokinetic simulation spectra reveal that ETG turbulence wavenumbers of peak spectral intensity comfortably fall within the measurable / detectable range of the instrument from the core to the pedestal. The proposed diagnostic opens up opportunities to study new regimes of turbulence and confinement, particularly in association with upcoming non-inductive, microwave based current drive experiments on MAST-U and can provide insight into cross-scale turbulence effects, while having suitability to operate during burning plasma scenarios on future reactors such as STEP (Spherical Tokamak for Energy Production).

ORCID iDs

Speirs, David C ORCID logoORCID: https://orcid.org/0000-0001-5705-6126, Ruiz Ruiz, Juan, Giacomin, Maurizio, Hall-Chen, Valerian Hongjie, Phelps, Alan D R ORCID logoORCID: https://orcid.org/0000-0002-1100-1012, Vann, Roddy G L, Huggard, Peter, Wang, Hui, Field, Anthony Robert and Ronald, Kevin ORCID logoORCID: https://orcid.org/0000-0002-8585-0746;
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