Investigation of Micro-Tearing Modes on MAST-U

Micro-tearing modes (MTMs) driven by electron-temperature gradients could account for a significant fraction of the anomalous heat transport in the plasma edge. They limit the achievable electron temperature gradient in the pedestal limiting confinement and the overall plasma performance. They are primarily electromagnetic phenomena due to their high-amplitude magnetic fluctuation and are often visible in magnetic probe measurements. MTMs have been identified in the pedestal of conventional aspect ratio tokamaks such as JET and DIII-D but not yet in spherical tokamaks. Theoretical and experimental work showed that MTMs get destabilized in plasmas with high plasma beta and low collisionality, and where a rational magnetic surface aligns with the peak of the drift frequency profile (omega∗).

MTMs were investigated in the previous two measurement campaigns in dedicated experiments on the MAST-U spherical tokamak. Rapid vertical displacements were introduced to high beta type 1 ELMy H-mode plasmas to align a rational magnetic surface with the peak of the omega* profile. Collisionality was kept low by heating the plasma with all available neutral beam heating power. The spectrograms of magnetic and density fluctuations were calculated and the origins of the coherent frequency bands were investigated to identify MTMs amongst other coherent fluctuations. The growth rate and the expected frequency of the MTMs were calculated with gyrokinetic simulations and were compared to the experimental findings.