# Internal disruptions and sawtooth like activity in LHD

## TL;DR

This study simulates and analyzes internal disruptions and sawtooth-like activity in LHD to understand their causes and effects, aiming to optimize plasma confinement and operational stability.

## Contribution

It provides a detailed simulation of resistive MHD modes causing sawtooth and disruption events in LHD, highlighting the roles of specific modes and their impact on plasma stability.

## Key findings

- Resonant $(n=1, m=3)$ mode drives sawtooth-like events.
- Internal disruptions are mainly caused by the $(n=1, m=2)$ mode.
- Disruptions do not occur during resonant sawtooth events.

## Abstract

LHD inward-shifted configurations are unstable to resistive MHD pressure-gradient-driven modes. These modes drive sawtooth like events during LHD operation. In this work, we simulate sawtooth like activity and internal disruptions in order to improve the understanding of these relaxation events and their effect over the device efficiency to confine the plasma, with the aim to improve the LHD present and future operation scenarios minimizing or avoiding the disadvantageous MHD soft and hard limits. By solving a set of reduced non-linear resistive MHD equations, we have studied the evolution of perturbations to equilibria obtained before and after a sawtooth like event in LHD. The equilibrium $\beta$ value is gradually increased during the simulation until it reaches the experimental value. Sawtooth like events and internal disruption events take place in the simulation for $\beta_{0}$ values between $1\%$ and $1.48\%$. The main driver of the sawtooth like events is the resonant and non-resonant effect of the $(n=1, m=3)$ mode. The instability is stronger for resonant events, and they only appear when $\beta_{0} = 1.48 \%$. Internal disruptions are mainly driven by the $(n=1, m=2)$ mode, and they extend throughout the whole plasma core. Internal disruption events do not show up when resonant sawtooth like events are triggered.