Description
The magnetic field of the five-period optimized stellarator Wendelstein 7-X (W7-X) is generated by seven types of unique superconducting coils, arranged in ten identical half-modules. By changing the ratios of the currents between these coil types, a wide magnetic configuration space can be explored. This space has been probed experimentally in the search for configurations with improved confinement. W7-X was optimized, among other things, for MHD stability; however, there are points in this configuration space that present only marginal Mercier stability, and some of these configurations have been attempted in the past experimental campaigns. The study of the MHD instabilities that arise in these configurations might be helpful in both for evaluating the effectiveness of stability proxies and understanding the undesired phenomena in high performance plasmas, such as crashes in diamagnetic energy [1].
In this work, the mode activity in these configurations is surveyed using data from magnetic diagnostics and a soft x-ray camera array.
The experimental database of W7-X is scanned and the expected stability of the magnetic configuration is assessed using Mercier's criterion [2] for the reference equilibria, as calculated with VMEC. In this way, suitable plasmas are identified. The spectrograms of magnetic fluctuations [3] and line-integrated soft x-ray emission [4] are used to detect the presence of both stable modes and crashes. These observed modes are characterized by their frequency and relative power, and their mode numbers are estimated. The effect of the MHD activity on bulk plasma parameters is studied, and tomographic reconstruction is used in suitable cases to find its radial location.
[1] H. Thomsen, this conference
[2] C. Mercier, Nucl. Fusion 1 47 (1960)
[3] S. Vaz Mendes et al., Nucl. Fusion 63 096008 (2023)
[4] C. Brandt et al., Plasma Phys. Control. Fusion 62 035010 (2020)