Determination of poloidal mode numbers of MHD modes and their radial location using a soft x-ray camera array in the Wendelstein 7-X stellarator

• Published in: Plasma physics and controlled fusion 2021-06, Vol.63 (6), p.65006
• Main Authors: Dreval, M B, Brandt, C, Schilling, J, Thomsen, H, Beletskii, A, Könies, A
• Format: Article
• Language: English
• ISSN: 0741-3335; 1361-6587
• DOI: 10.1088/1361-6587/abf449

A forward modeling technique is developed for determining the characteristic features of observed MHD modes from the line-of-sight data of the soft x-ray (SXR) tomography diagnostics in the Wendelstein 7-X (W7-X) stellarator. In particular, forward modeling is used to evaluate the poloidal mode numbers m , radial location, poloidal rotation direction and ballooning character of the MHD modes. The poloidal mode structures have been modeled by the radially localized Gaussian-shaped emission regions rotating along the magnetic surfaces. In the present study the cases of rigid-shape emission regions and flexible emission regions are modeled. Various mode phase velocity dependences on the magnetic surface position are simulated. The modeled phase dynamics of line-integrated oscillations and the distribution of oscillation amplitudes are compared with the experimental signals of the SXR cameras which observe the plasma at various viewing angles in the poloidal cross-section. Application of this technique enables describing of the 1–50 kHz modes. In particular, in the discharge W7X-PID 20180918.045 three identified branches with the poloidal mode numbers m = 8, m = 10 and m = 11 localized at ρ ≈ 0.3 are rotating in the clockwise poloidal direction. The present paper reports the first application of the forward modeling technique to the data from the SXR diagnostics in W7-X. The high m -modes are identified by forward modeling in W7-X.