Recent Results from Electron Cyclotron Emission (ECE) Radiometer diagnostics in the presence of Electron Cyclotron Resonance Heating (ECRH)

¹ Institute for Plasma Research, Bhat, Gandhinagar – 382 428
² Homi Bhabha National Institute (HBNI), Anushaktinagar, Mumbai- 400094, India

^* Corresponding author: varsha@ipr.res.in

Published online: 15 November 2024

Abstract

Besides furnishing localized spatial and temporal electron temperature measurements, Electron Cyclotron Emission (ECE) diagnostics are routinely employed across various tokamaks for diverse physics investigations, encompassing MHD localizations, transport studies, and fluctuation measurements. Variations in the bulk of the electron distribution function manifest as alterations in ECE signatures, often attributable to the presence of fast electrons induced by auxiliary heating mechanisms such as Electron Cyclotron Resonance Heating (ECRH). This manuscript delves into the scrutiny of such interactions on ECE spectra leveraging a newly devised 16-channel ECE radiometer diagnostic. The manuscript delineates the impact of ECRH application, including pre-ionization and heating, resulting in ECE signal saturation and an expected rise in electron temperature (T_e) respectively. Furthermore, the discourse explores instabilities potentially induced by high-energy electrons stemming from ECRH, focusing initial observations on runaway discharges characterized by relaxation oscillations in ECE, Soft X-ray (SXR), CIII, and H_α emissions. The backdrop of weakly ionized plasma with low electron density (n_e) and temperature (T_e), coupled with ECRH, fosters the generation of runaway electrons and corresponding relaxation oscillations in bulk plasma parameters. Additionally, a distinct surge in the ECE signature emerges upon cessation of the ECRH pulse, with no discernible variation in other bulk plasma parameters. Given that ECE signatures are susceptible to changes in both energy and pitch angle, this abrupt amplitude rise likely arises from Pitch Angle Scattering (PAS), potentially prompted by the sudden de-acceleration of fast electrons engendered by the ECRH pulse.