ELM-free Enhanced D{\alpha} H-mode with Near Zero NBI Torque Injection in DIII-D Tokamak

TL;DR

This study investigates an ELM-free EDA H-mode regime in the DIII-D tokamak with near-zero NBI torque, revealing a stable, high-confinement plasma with a quasi-coherent mode likely driven by trapped electron modes, supported by experimental and simulation data.

Contribution

It provides detailed measurements and analysis of the edge quasi-coherent mode and its possible origin as a trapped electron mode in EDA H-mode plasmas without ELMs.

Findings

The QCM peaks inside the separatrix with multiple harmonics.

QCM propagates with very small phase velocity.

Simulations suggest QCM is consistent with trapped electron mode characteristics.

Abstract

Enhanced $D_{\alpha}$ H-mode (EDA H-mode), an ELM-free H-mode regime, is explored in neutral beam heated, lower single null plasmas with near zero torque injection. This regime exhibits a good energy confinement ($\mathrm{H}_{\mathrm{98y2}}$ $\sim 1$) with $\beta_N \sim 2$, high density, regime access at low input power, and no ELMs. This paper further presents the time-resolved measurements of electron and ion density, temperature, plasma rotation, and radial electric field during the EDA H-mode phase and examines the dynamics of the edge quasi-coherent mode (QCM). Measurements using multiple fluctuation diagnostics reveal the QCM to be a separatrix spanning mode, peaking just inside the separatrix, existing in a wide range of $k_{\perp}\rho_s \sim 0.1-1.2$ with multiple harmonics, and propagating with a very small phase velocity in the plasma frame, where $k_{\perp}$ is the binormal wavenumber and $\rho_s$ is the ion sound radius. Linear gyrokinetic simulations of an EDA H-mode discharge with CGYRO indicates that the trapped electron mode (TEM) and electron temperature gradient (ETG) are dominant instabilities in the region where QCM is unstable. Qualitative analysis indicates that the properties of TEM are consistent with the experimental observed characteristics of the QCM. These similarities suggest that the QCM might be a TEM instability existing in the edge region of the EDA H-mode plasmas.