Description
High density plasmas in Wendelstein 7-X stellarator (W7-X) can be achieved via a combination of different heating and fueling systems. One scenario is via the injection of ice Hydrogen pellets into an electron-cyclotron radiation (ECR) heated-plasma, another scenario combines the use of neutral beam injection with ECR-heating, which leads to additional density build-up [Thomsen EPS 2025, Bozhenkov EPS 2025].
Fast MHD events are observed in various magnetic diagnostic signals as well as soft-X ray data and edge impurity spectroscopy. The crashes are associated with a loss of diamagnetic energy in the range of a few percent. The event is first visible in the Mirnov signal, followed by an activity in the soft-X ray signal intensity approximately 1 ms later and a rise in C-III impurity line radiation from the edge, reaching a first maximum after another 1 ms. The event propagation from core towards the plasma edge can also be identified in the ECE system. The characteristics of these MHD events resemble the core density collapses observed in LHD, which are reported to be driven by resistive ballooning instability [S. Ohdachi et al 2017 Nucl. Fusion 57].
The tomographic inversion of the X-ray multi-camera system data shows an inversion radius at approximately rho = 0.5, where also the steepest pressure gradient is located. As a secondary effect, there is also a rotating m=1 mode forming (stable for ~ 1 ms), which rotates with the local ExB-velocity. A similar m=1 mode in SX radiation can also be triggered by an impurity injection into the plasma by means of LBO (in this case: no Mirnov pre-trigger [Thomsen EPS 2023]). In this contribution we discuss the present understanding of the MHD events in W7-X.