Characterization of ELM Pacing via Vertical Jogs on DIII-D

TL;DR

This study demonstrates that vertical plasma oscillations on DIII-D can effectively increase ELM frequency, reduce divertor heat flux, and influence plasma stability by inducing edge currents, offering a new method for ELM control.

Contribution

It introduces a novel ELM pacing technique using vertical jogs and provides a toy model explaining the underlying edge current mechanism.

Findings

Vertical oscillations increased ELM frequency from ~5 Hz to 20 Hz.

ELM pacing reduced peak heat flux to the divertor by about half.

Downward jogs decreased impurity concentration and plasma volume.

Abstract

Edge localized mode (ELM) pacing via vertical plasma oscillations or jogging has been successfully demonstrated on DIII-D. Rapid vertical movement of the plasma toward the X-point has been shown to effectively trigger ELMs. By vertically oscillating the plasma at a rate of 20 Hz, the ELM frequency increased from $\sim$5~Hz, the natural ELM frequency in similar DIII-D discharges, to 20~Hz. Downward jogs have been observed to trigger multiple ELMs in one cycle. ELMs triggered at higher than natural frequencies lead to smaller decreases in stored energy, from ~10\% to as little as below 1\%. As a consequence, the peak heat flux to the divertor has been observed to be reduced by a factor of $\sim$2. In addition, a reduction in the carbon impurity concentration has been observed. During downward jogs in the lower single null (LSN) configuration, the X-point movement is slower and smaller than the top of the plasma. As a result, a reduction in the plasma cross section and hence volume has been observed. To understand the mechanism of ELM triggering by jogging, a toy model of the edge toroidal current has been built and tested with DIII-D experiment data. The experimental data and model suggest that when the plasma moves down towards the X-point, a net positive toroidal current is locally induced in the edge region. ELITE stability analysis suggests that this current pushes the plasma state across the peeling side of the peeling-ballooning stability boundary into the unstable region triggering ELMs.