Comparison between measured and predicted turbulence frequency spectra in ITG and TEM regimes

TL;DR

This study compares measured and predicted turbulence frequency spectra in tokamak plasmas, demonstrating that core quasi-coherent modes are signatures of TEM turbulence, validated through gyrokinetic simulations and reflectometry data.

Contribution

It provides the first detailed comparison between experimental spectra and gyrokinetic predictions for TEM and ITG regimes, confirming core QCMs as markers of TEM turbulence.

Findings

QCMs are linked to TEM turbulence in the LOC phase.

Nonlinear simulations show distinct spectral features for ITG and TEM regimes.

Synthetic diagnostics reproduce experimental reflectometry spectra.

Abstract

The observation of distinct peaks in tokamak core reflectometry measurements - named quasi-coherent-modes (QCMs) - are identified as a signature of Trapped-Electron-Mode (TEM) turbulence [H. Arnichand et al. 2016 Plasma Phys. Control. Fusion 58 014037]. This phenomenon is investigated with detailed linear and nonlinear gyrokinetic simulations using the \gene code. A Tore-Supra density scan is studied, which traverses through a Linear (LOC) to Saturated (SOC) Ohmic Confinement transition. The LOC and SOC phases are both simulated separately. In the LOC phase, where QCMs are observed, TEMs are robustly predicted unstable in linear studies. In the later SOC phase, where QCMs are no longer observed, ITG modes are identified. In nonlinear simulations, in the ITG (SOC) phase, a broadband spectrum is seen. In the TEM (LOC) phase, a clear emergence of a peak at the TEM frequencies is seen. This is due to reduced nonlinear frequency broadening of the underlying linear modes in the TEM regime compared with the ITG regime. A synthetic diagnostic of the nonlinearly simulated frequency spectra reproduces the features observed in the reflectometry measurements. These results support the identification of core QCMs as an experimental marker for TEM turbulence