Edge turbulence measurements in L-mode and I-mode at ASDEX Upgrade

Abstract

The I-mode confinement regime is promising for future reactor operation due to high energy confinement without high particle confinement. However, the role of edge turbulence in creating I-mode's beneficial transport properties is still unknown. New measurements of edge turbulence ([Formula: see text]) in L-modes and I-modes at low and high densities at ASDEX Upgrade are presented in this paper. A high radial resolution correlation electron cyclotron emission radiometer measures the broadband turbulence throughout the L-mode and I-mode edge and pedestal. The weakly coherent mode (WCM) is measured in both L-mode and I-mode near the last closed flux surface with Te fluctuation levels of 2.3%–4.2%, with a frequency shift between the two phases related to a deeper Er well in I-mode. An [Formula: see text] phase diagnostic captures a change of the WCM [Formula: see text] phase between L-mode and I-mode from [Formula: see text] to [Formula: see text]. The thermal He beam diagnostic measures a WCM wavenumber range of −0.5 to −1.0 cm−1. A low-frequency edge oscillation (LFEO) appears in the I-mode phase of these discharges and displays coupling to the WCM, but the LFEO does not appear in the L-mode phase. Linear gyrokinetic simulations of the outer core and pedestal top turbulence indicate that while the dominant turbulent modes in the outer core are ion directed and electrostatic, the turbulence becomes increasingly electron directed and electromagnetic with increasing radius. Collisionality is not found to impact characteristics of the L-mode and I-mode edge turbulence with respect to the presence of the WCM; however, the quality of global confinement decreases with collisionality.

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

“…The CECE channel and reflectometer are separated by approximately 4 mm. This corresponds to a poloidal velocity of approximately 3 km/s, which is similar to previously reported values at the L-mode and Imode WCM location [8]. This estimation is performed in the laboratory frame, and the background plasma velocity would be needed to calculate the WCM's phase velocity in the plasma frame.…”

“…the Quasi-Coherent Mode in Enhanced D-alpha Hmode and the Edge Harmonic Oscillation in Quiescent Hmode [6,7]). The WCM has been observed in the density and electron temperature fluctuation signals of AUG Lmodes as well [5,8]. A Low Frequency Edge Oscillation (LFEO) is also observed as a coherent and low-frequency mode in the pedestal of I-modes, and may be a Geodesic Acoustic Mode (GAM) [9].…”

Current status of ECE system on EAST tokamak

“…CECE diagnostic provides a powerful technique for overcoming the noise limitations of stand ECE in measuring small amplitude temperature fluctuations. Recently CECE dianostic has been used to measure the electron temperature fluctuation in the improved energy confinement mode (I-mode) discharge in Asdex-upgrade, and interesting phenomenon have been observed [14].…”

Database study of turbulent electron temperature fluctuation measurements at ASDEX Upgrade

“…These temperature fluctuations are among the key parameters driving the turbulent electrostatic electron heat flux in tokamak plasmas [4]. Since the original implementation of the CECE diagnostic on W7-AS [5], follow-on studies on devices around the world have reported on important, transport-relevant experimental trends in turbulent electron temperature fluctuations using CECE data (see, e.g., ref's [6][7][8][9]). Additionally, data from the CECE diagnostic has and continues to serve as a key component in the validation of advanced gyrokinetic and gyrofluid models of plasma turbulence [10][11].…”