Understanding the negative triangularity ELM trigger and ELM free state on DIII-D with ECE-imaging

Abstract

The Electron Cyclotron Emission Imaging (ECEI) diagnostic was used to observe a finite-n interchange mode structure in the edge of negative triangularity shaped plasmas on DIII-D. At a small negative triangularity (δu = −0.2), the plasma is in the H-mode with ELMs that are triggered by a low-n interchange mode. At a larger negative triangularity (δu = −0.4) and low NBI power (2 MW), a dithering oscillation is observed that is triggered by a low-n interchange mode, whereas at higher NBI power (>2 MW), the edge reverts to L-mode and the low-n interchange mode is present continuously. In all cases, the edge pressure gradient is clamped when the interchange mode is present. It is concluded that the low-n interchange mode prevents the plasma from transitioning to H-mode at a large negative triangularity. This agrees with linear BOUT++ simulations which suggest that the interchange-type MHD can be a resistive ballooning mode whereby resistivity can significantly increase the finite-n ballooning mode growth rate. The absence of H-mode at large negative triangularity can, thus, be explained by the excitation of low-n pressure driven resistive ballooning modes in the plasma edge.

Geometric dependencies of the mean E × B shearing rate in negative triangularity tokamaks

“…The loss of second stability, triggered by changing the squareness of the plasma shape, only changes low-frequency-high-amplitude ELMs to highfrequency-low-amplitude ELMs, without eliminating the Hmode [12,13]. A recent experimental study using electron cycloton emission (ECE) imaging suggests that NT edge pressure is limited by low-n interchange type magnetohydrodynamics (MHD) modes or resistive ballooning modes [14]. Gyrokinetic simulations [3,4,[15][16][17][18][19] attribute the linear stabilisation of trapped electron mode or ion temperature gradient mode to the observed reduction of turbulence and transport in the core of NT configuration.…”