Measurement Discrepancy in High Temperature Plasmas*

Discrepancies between Electron Cyclotron Emission (ECE) and Thomson Scattering (TS) measurements are a long-standing issue in high-temperature tokamak plasma conditions. A heuristic model, tested on a large JET dataset, shows that introducing a bipolar perturbation in the electron distribution function (EDF), affecting EC emission/absorption spectra, resolves such discrepancies. However, the underlying cause of this perturbation remain elusive.

This work explores the interaction between electrons and Kinetic Ballooning Modes (KBMs) as the underlying physical cause of the bipolar perturbation in the EDF. Gyrokinetic studies of a JET pulse show that unstable KBMs impact the EDF's velocity phase-space, forming a bipolar structure similar to the one of the heuristic model. The position in the velocity space of this structure is linked to the electron diamagnetic frequency, and so to local plasma parameters like electron density and temperature gradients, and therefore with the heating methods, as predicted by the heuristic model. Moreover, the perturbation's amplitude evaluated in nonlinear simulations of KBM-dominated plasma conditions qualitatively matches the one of the heuristic model.

This study demonstrates that KBMs destabilized by high-β plasma conditions in high-temperature cores perturb the EDF, altering the EC spectrum. ECE could, thus, be used to diagnose turbulence characteristics in the plasma core of the next-generation high-performance fusion devices.