Electron temperature fluctuation measurements with Correlation Electron Cyclotron Emission in L-mode and I-mode plasmas at ASDEX Upgrade

¹ Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
² Max Plank Institute for Plasma Physics, 85748 Garching, Germany
³ Department E28, Technical University of Munich, 85748 Garching, Germany
⁴ École Polytechnique Fédérale de Lausanne, Swiss Plasma Center, CH-1015 Lausanne, Switzerland

^* e-mail: bielajew@mit.edu
⁵ See author list of U. Stroth et al. 2022 Nucl. Fusion 62 042006

Published online: 23 February 2023

Abstract

The Correlation Electron Cyclotron Emission (CECE) diagnostic at ASDEX Upgrade (AUG) is used to investigate the features of outer core and pedestal (ρ_pol = 0.85-1.0) turbulence across confinement regime transitions. The I-mode confinement regime is a promising operational scenario for future fusion reactors because it features high energy confinement without high particle confinement, but the nature of the edge and pedestal turbulence in I-mode plasmas is still under investigation. The edge Weakly Coherent Mode (WCM) appears in the I-mode pedestal and may play a role in transport. In this work we explore electron temperature (T_e) fluctuations in the plasma outer core and pedestal using a 24-channel high radial resolution CECE radiometer. CECE measurements provide turbulence information including the T_e fluctuation amplitude, turbulent spectra, and radial localization of turbulent features. With CECE measurements we show that the WCM is localized in the pedestal region in both L-mode and I-mode and is measured in optically thick plasmas with a T_e fluctuation amplitude of 2.3%. Broadband drift wave turbulence is measured in the outer core with a T_e fluctuation amplitude of <1%. A second CECE system recently installed at AUG allowed for non-standard fluctuation measurements during L-mode and I-mode experiments. The second CECE system was toroidally separated from the primary system, allowing measurements of the long-range toroidal correlation of the WCM indicating its low toroidal mode number. A reflectometer sharing a line of sight with the second CECE system enabled density-temperature cross-phase (α_ne Te ) measurements. The WCM α_ne Te changes between L-mode and I-mode as the T_e gradient steepens.