Effects of Plasmoid Formation on Sawtooth Process in a Tokamak

TL;DR

This paper investigates how plasmoid formation influences the sawtooth reconnection process in tokamak plasmas through detailed resistive MHD simulations, revealing the critical role of plasmoids in magnetic island dynamics.

Contribution

It demonstrates that at high Lundquist numbers, secondary current sheets become unstable to plasmoid formation, significantly impacting the evolution of magnetic islands during sawtooth crashes.

Findings

Plasmoid instability occurs at Lundquist numbers ≥1.6×10^7.

Merging plasmoids form a monster plasmoid affecting island evolution.

Partial reconnection in experiments may be explained by plasmoid dynamics.

Abstract

For realistic values of Lundquist number in tokamak plasmas, the 1/1 magnetic island leads to the formation of secondary thin current sheet, which breaks up into a chain of small magnetic islands, called plasmoids. The role of plasmoid dynamics during the sawtooth reconnection process in fusion plasmas remains an unresolved issue. In this study, systematic simulations are performed to investigate the resistive internal kink mode using the full resistive MHD equations implemented in the NIMROD code in a simplified tokamak geometry. For Lundquist number $S\ge1.6\times10^7$, secondary current sheet is found to be unstable to plasmoids during the nonlinear resistive kink mode evolution with a critical aspect ratio of the current sheet of ~70. The merging of small plasmoids leads to the formation of a monster plasmoid that can significantly affect the primary island evolution. This may provide an explanation for the partial reconnection observed in sawtooth experiments.