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

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 with that of nonlinearly stable quasi-interchange modes in tokamaks. A high mirror, very low $\iota$ , W7-X-like configuration is first simulated. Broad flow structures are observed, which produce a similar magnetohydrodynamic (MHD) dynamo term to that in hybrid tokamak discharges, leading to flux pumping. Unlike in tokamaks, there is no net toroidal current to counterbalance this dynamo, and it is unclear if it can be sustained to obtain a similar quasistationary nonlinear state. In the simulation, partial reconnection induced by the overlap of multiple interchange instabilities leads to a core temperature crash. 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. It is shown that these modes do not saturate in a benign quasistationary way in current simulations even in the presence of background equilibrium $\boldsymbol{E} \times \boldsymbol{B}$ flow shear. This leads to a burst of MHD behaviour, inconsistent with the sustained MHD signatures in the experiment. Nevertheless, the (1, 2) mode is observed at the experimental Spitzer resistivity, and its induced anomalous transport can be overcome using an experimentally relevant heat source, reproducing these aspects of the dynamics. 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.