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Electromagnetic filamentary turbulent structures are found to characterize the edge region of different magnetic configurations [1-3] including Reversed Field Pinch, stellarator and tokamak, where strong currents are associated also to ELM filamentary structures. The study of those phenomena in W7-X stellarator is of particular interest as the electromagnetic features of filaments are expected to become more relevant with the increase of the local plasma beta.
For the study of electrostatic and magnetic fluctuations characterizing the edge region of the stellarator experiment W7-X, a specifically designed insertable probe head [4,5] was constructed within the framework of EUROfusion in collaboration between Consorzio RFX, IPP Greifswald and FZJ Julich. The probe head, named High Resolution Probe (HRP), was conceived to be installed on the mid-plane multi-purpose fast reciprocating manipulator on W7-X. In its upgraded version the diagnostics, accounting for an optimized orientation of the probe head tailored for the Standard and High-Mirror magnetic configurations [6], was used during the OP2.3 experimental campaign. The probe was installed on MPM manipulator and collected measurements in a variety of magnetic configurations and scenarios. The possibility, granted by the MPM system, of regulating the duration of inserted position was exploited allowing the plasma monitoring in a stationary position up to 0,6s for each insertion plunge. This feature accompanied by multiple insertions opens the possibility to monitor transient conditions in the plasma edge, such as improved plasma performance or density ramp or steps. In particular strong indications of a change in features of magnetic fluctuations in the edge region is observed accompanying the diamagnetic energy (Wdia) transients, obtained with different scenarios combining NBI and pellet injection, reaching values up to 17 10^2 kJ. The multiple plunges allowed the monitoring of the time evolution of edge parameters accompanying the Wdia different phases. The paper illustrates the experimental features of magnetic fluctuations evolution in the described scenarios. In addition, exploiting the combination of magnetic fluctuations measured by two tri-axial coils located at the same radial position, the indication of propagating structures is investigated.
[1] M. Spolaore et al, Phys. Rev. Lett. 102, 165001 (2009)
[2] N. Vianello et al. Nucl. Fusion 50, 042002 (2010).
[3] Furno I. et al., Phys. Rev. Lett. 106, 245001 (2011).
[4] P. Agostinetti et al. IEEE Transactions on Plasma Science 46.5 (2018): 1306-1311.
[5] M. Spolaore, et al. Journal of Instrumentation 14.09 (2019): C09035.
[6] C. Killer et al., Journal of Instrumentation 17.03 (2022): P03018.