Description
Wendelstein 7-X (W7-X) is a modular advanced stellarator [1], situated in Greifswald, Germany. The machine has operated since 2015, and the latest experimental campaign was finished at the end of May 2025. The W7-X coil system allows a flexible variation of magnetic configurations. The configuration space of the machine covers configurations with boundary iota values from 5/6 to 5/4, different mirror ratios and various magnetic shears [2]. Configuration scans were performed in all W7-X experimental campaigns, varying either the rotational transform between some reference magnetic configurations or their mirror ratio. The investigated configuration space included magnetic configurations between Standard and High iota magnetic configurations (variation of the boundary iota value from 5/5 to 5/4), between Standard and Low iota configurations (variation of the boundary iota value from 5/5 to 5/6) as well as between Standard and High Mirror Configurations (variation of the mirror ratio from 0.043 to 0.104). Several intermediate limiter configurations, where the plasma volume is restricted by divertor plates intersecting nested flux surfaces, demonstrated an increase of the plasma confinement time [3]. In these configurations a chain of magnetic islands is located inside and very close to the last closed magnetic surface. The highest values of the diamagnetic energy, measured in W7 X, were reached explicitly in the configurations with 5/5-islands, positioned near the plasma boundary. This contribution discusses a systematic plasma profile analysis, based on Thomson scattering diagnostic data, in different configuration scans and compares experiments with improved confinement in different configurations. Edge density evaluation from Alkali beam measurements complement this study. VMEC-equilibrium calculations with discussed plasma profiles are presented and can be used for data mapping of different diagnostics.
References:
[1] Grieger G. et al 1992 Fusion Technol. 21 1767-1778
[2] Geiger J. et al 2015 Plasma Phys. Control. Fusion 57 014004
[3] Andreeva T. et al 2022 Nucl. Fusion. 62 026032