Synthetic Mirnov diagnostic for the validation of experimental observations

Abstract

A synthetic Mirnov diagnostic has been developed to investigate the capabilities and limitations of an arrangement of Mirnov coils in terms of a mode analysis. Eight test cases have been developed, with different coil arrangements and magnetic field configurations. Three of those cases are experimental configurations of the stellarator Wendelstein 7-X. It is observed that, for a high triangularity of the flux surfaces, the arrangement of the coils plays a significant role in the exact determination of the poloidal mode number. For the mode analysis, torus and magnetic coordinates have been used. In most cases, the reconstruction of the poloidal mode number of a prescribed mode was found to be more accurate in magnetic coordinates. As an application, the signal of an Alfvén eigenmode, which has been calculated with a three-dimensional magnetohydrodynamics code, is compared to experimental observations at Wendelstein 7-X. For the chosen example, the calculated and measured mode spectra agree very well and additional information on the toroidal mode number and localization of the mode has been inferred.

Localized phase contrast imaging at the Wendelstein 7-X stellarator

“…The in-vessel Mirnov coil system 11,37-39 mainly allows detection and localization of low-k modes in the outer part of W7-X plasmas. 39 Although the soft x-ray system 11,40,41 can also locate core MHD modes using ad hoc mode models, [41][42][43] it is generally limited to f < 100 kHz, 38 which is not the case for the PCI system.…”