A comparison of low-n Mercier unstable Wendelstein stellarators and quasi-interchange modes in tokamaks

TL;DR

This paper compares the nonlinear stability mechanisms of Mercier unstable stellarators and stable tokamak modes, providing insights into surpassing traditional stability limits in plasma confinement devices.

Contribution

It offers a comparative analysis of nonlinear evolution in Mercier unstable stellarators and quasi-interchange modes in tokamaks, enhancing understanding of stability beyond classical limits.

Findings

Mercier unstable Wendelstein stellarators can exhibit nonlinear behaviors surpassing traditional limits.

Experimental reconstructions show sustained MHD signatures and partial reconnection.

Discrepancies between experiments and simulations highlight complex stability mechanisms.

Abstract

Mercier's criterion is typically enforced as a hard operational limit in stellarator design. At the same time, past experimental and numerical studies have shown that this limit may often be surpassed, though the exact mechanism behind this nonlinear stability is not well understood. This work aims to contribute to our current understanding by comparing the nonlinear evolution of Mercier unstable Wendelstein stellarators to that of nonlinearly stable quasi-interchange modes in tokamaks. A high mirror, very low $\iota$, W7-X-like configuration is first simulated. A second case is then considered using experimental reconstructions of intermediate $\beta$ W7-AS discharges, where saturated low-n modes were observed experimentally, with sustained MHD signatures over tens of milliseconds. The possible reasons for the discrepancies between experiment and simulation, and the observation of partial reconnection in contrast to flux pumping are discussed, in view of reproducing and designing for operation of stellarators beyond the Mercier stability limit.