Density Limit Experiments and Core-localized Kinetic MHD Activities in HL-2A Ohmic Heating Plasmas

TL;DR

This paper presents experimental observations of the density limit and core-localized kinetic MHD activities in HL-2A plasmas, revealing new instability behaviors related to high density and their role in plasma disruptions.

Contribution

First experimental identification of kinetic MHD instabilities like AITG and KBM at the density limit in HL-2A, linking them to plasma disruptions.

Findings

High density plasmas with ne/neG>1 achieved using gas-puff fuelling.

Kinetic MHD instabilities observed at ne/neG≈1.

Instabilities trigger plasma disruptions when density profile is peaked.

Abstract

The density limit is a mysterious barrier to magnetic confinement nuclear fusion, and is still an unresolved issue. In this paper, we will present the experimental results of the density limit and core-localized kinetic MHD instabilities on HL-2A. Firstly, the high density shots with $ne/ne_G>1$ have been achieved by the conventional gas-puff fuelling method in Ohmic heating plasmas, and the corresponding duration time is close to $t\sim500$ ms ($\sim$ $30\tau_E$), where $\tau_E$ is the global energy confinement time. Secondly, it is found for the first time that there are kinetic MHD instabilities in the core plasmas while $ne/ne_G\sim1$. The analysis suggests that the core-localized MHD activities belong to Alfv{\'e}nic ion temperature gradient (AITG) modes or kinetic ballooning modes (KBM), and firstly it is found on experiment that they trigger the minor or major disruption of bulk plasmas while the density profile is peaked. These new findings are of great importance to figure out and understand the origin of the density limit.