Parametric decay of microwave beams in the edge island of Wendelstein 7-X: Theoretical modelling and correlation with edge fluctuations

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The beam geometry and the power time-traces for experiments in high iota MC are analogous to those shown in fig.2a,b (with , ≈ 3.32 × 1019?m−2) where, however, the ECRH beams cross a thinner magnetic island in the edge. Even though no density bump is generally measured in this MC, the absence of ABES data for the relevant discharge (20180822.013) prevents firm conclusions. In high iota MC, an additional down-shifted component is excited, termed down-weak, for its lower intensity compared to the symmetric side-bands. In fig.4a,b, time-traces of the normalized SPDs of both down-shifted PDI components are compared with time-traces of the edge SXR chords (? here unavailable). Here cyan-shaded regions highlight a strong correlation between crashes in the down-weak signal and SXR from the edge chords, suggesting excitation of this component in the plasma edge. However, since the SPD of the down-strong band does not display the oscillations of interest, this component may be generated in an inner region. A potential explanation involves PDI in the inner 10/9 island chain, crossed by the gyrotron beams in proximity of the overlap with the receiver line-of-sight, thus also providing a consistent explanation for the different intensity between the two components. Summary – Crashes in the PDI-related side-bands correlated with quasi-continuous fluctuations in the plasma edge can shed light on the location of excitation of PDI in W7-X. In standard MC, the flattening of the density profile in the island during edge density fluctuations suggests suppression of the trapping mechanism, predicted in [1], and could explain the observed crashes in the PDI-related side-bands. In high iota MC, correlation of the down-weak component with edge SXR signals suggests PDI in the plasma edge whereas the stronger components could come from the inner 10/9 island chain, in proximity of the overlap between the gyrotron beams and the receiver sight-line. Acknowledgements – This work has been supported by research grant 15483 from VILLUM FONDEN and the Enabling Research grant ENR-MFE19.DTU-03 from the EUROfusion Consortium. This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. References [1] A. Tancetti et al., Nuclear Fusion., 62(7), 074003, 2022 [2] S.K. Hansen et al., PPCF, 63(9), 095002, 2021 [3] D. Moseev et al., Rev. Sci. Instrum. 90, 013503, 2019 [4] G. Anda et al., Fus. Eng. Design, 146:1814-1819, 2019 [5] A. Yu. Popov and E. Z. Gusakov, PPCF 57 025022, 2015 [6] R. Takacs et al., 47th EPS Conf. on Plasma Phys., 2021 [7] G.A. Wurden, et al., 46th EPS Conf. on Plasma Phys., 2019 [8] C. Brandt et al., PPCF, 62(3), 035010, 2020 [9] L. Stephey et al., Rev. Sci. Instr., 87(11):11D606, 2016