Discovery of Density Limit Disruption Induced by Core-localized Alfv${\'e}$nic Ion Temperature Gradient Instabilities in a Tokamak Plasma

TL;DR

This paper reports the discovery of core-localized Alfvénic ion temperature gradient instabilities in tokamak plasmas that trigger density limit disruptions, providing new insights into the longstanding challenge of plasma density limits in fusion reactors.

Contribution

It identifies and characterizes core-localized AITG modes as triggers of density limit disruptions in tokamaks, a novel finding in fusion plasma physics.

Findings

Multiple-branch MHD instabilities observed at high density ratios.

Simulation links instabilities to Alfvénic ion temperature gradient modes.

Instabilities trigger plasma disruptions during density peaks.

Abstract

To achieve a high energy gain, the fusion reactor plasma must reach a very high density. However, the tokamak plasmas ofen undergo disruption when the density exceeds the Greenwald density. The density limit disruption in tokamak plasmas is a mysterious barrier to magnetic confinement nuclear fusion, and hitherto, is still an unresolved issue. Over the past several years, the high density experiments with Greenwald density ratio $n_e/n_{eG}\sim1$ has been carried out using the conventional gas-puff fuelling method in HL-2A NBI and Ohmically heated plasmas. It is found for the first time that there are multiple-branch MHD instabilities in the core plasmas while $n_e/n_{eG}>0.85$. The simulation analysis suggests that the core-localized magnetohydrodynamics (MHD) activities belong to Alfv${\'e}$nic ion temperature gradient (AITG) modes, and on experiment firstly, it is discovered that they trigger the minor or major disruption of bulk plasmas while the density is peaked. These new findings are of great importance to figure out and understand the origin of density limit disruptions, as well as to forecast and avoid them for future fusion rectors.