Speaker
Description
Tungsten (W) has emerged as a favourable material for plasma-facing components (PFCs) in nuclear fusion devices. It has been incorporated in several tokamaks, however, in the stellarator, with 3D geometry, its suitability as PFCs, in terms of erosion, redeposition, ionization, transportation and accumulation of impurity particles in the plasma core, is yet to be demonstrated. In the stellarator Wendelstein 7-X (W7-X), tungsten PFCs are being introduced stepwise by increasing the surface area over the last few plasma campaigns [1]. During the recent OP2.2 + OP2.3 campaigns, tungsten PFCs surface areas of about 2 m2 in the heat shield (out of 47 m2) and about 0.8 m2 in the baffle (out of 33 m2), were provided.
Dedicated tungsten sputtering experiments were conducted using island control coils to drive higher heat/particle fluxes onto the tungsten baffle areas while simultaneously introducing impurity gas N2/Ne/Ar seeding. In addition, tungsten samples were exposed to the plasma edge island via a multi-purpose manipulator at defined exposure positions and durations. Tungsten tracer injection was carried out using the Tracer-Encapsulated Solid Pellets (TESPEL) and Laser blow-off (LBO) methods. Plasma discharge conditions were varied during two different magnetic configurations, i.e., standard and high mirror [2].
Observations were made at the edge with UV spectroscopy, Langmuir probes and thermal He-beam diagnostic and in the plasma core with high-resolution X-ray imaging spectroscopy (HR-XIS), a high-efficiency XUV overview spectrometer (HEXOS) and a pulse height analysis system (PHA). W I and W II line signals were detected by the UV edge spectrometers having a line of sight on the tungsten baffle tiles. Tungsten signals from erosion caused by plasma-wall interactions, TESPEL/LBO injection and intrinsic background were followed using multiple core viewing spectrometers and showed variations in the intensities when varying the island geometry and impurity seeding. The Langmuir probe measurements at the divertor surface have shown plasma temperatures of about 10 eV and asymmetry in the edge density for the upper and lower island divertors. A detailed analysis will be presented at the conference.
[1] D. Naujoks et al., Nuclear Material Energy 37 (2023) 101514.
[2] O. Grulke et al., IAEA-FEC (2025) Chengdu.