Speaker
Description
Prompt redeposition of tungsten plays a central role in impurity transport, main-chamber erosion, and net wall evolution in full-metal fusion devices. Yet experimental quantification of prompt redeposition remains limited due to the diagnostic challenges of ultraviolet (UV) tungsten spectroscopy and the need for reliable atomic physics data. This work presents spectroscopic measurements, collisional–radiative (CR) modeling, and tungsten-sourcing experiments aimed at quantifying prompt redeposition in both the WEST divertor and in regions influenced by ion cyclotron resonance heating (ICRH).
High-resolution measurements of the W I 400.9 nm and W II 364.1 nm emission lines were used to constrain the dynamics of neutral and singly ionized tungsten. These transitions were selected because W I 400.9 nm provides a direct indicator of neutral tungsten sourcing, while the W II 364.1 nm line offers a robust measure of the earliest ionized population. Line-integrated fluxes were inferred via the S/XB method, using electron temperature and density from Langmuir probes. CR coefficients were taken from Smyth et al. [1] (W I) and Dunleavy et al. [2] (W II).
Under X-Point Radiator divertor conditions, comparison of W I- and W II-based fluxes indicates that approximately 90–97% of grossly eroded tungsten is promptly redeposited within the divertor region. Spatially resolved analysis reveals systematic asymmetries: prompt redeposition is consistently higher on the high-field side.
A dedicated ICRH tungsten-sourcing experiment was conducted by scanning the plasma vertical position to modify antenna sheath potentials. Resulting variations in W I and W II emission demonstrate clear signatures of ICRH-driven tungsten release and rapid return of ionized particles to nearby surfaces. These measurements confirm that antenna-sourced tungsten largely undergoes short-range prompt redeposition rather than long-range transport to the divertor.
Overall, this work demonstrates that coordinated W I/W II spectroscopy combined with simplified CR-based modeling provides a practical and experimentally grounded method to quantify prompt redeposition in WEST. The diagnostic robustness of the W II 364.1 nm line, in particular, enables clear separation of gross erosion and ionized-flux components. The methods developed here support future efforts to link in-situ redeposition measurements with post-mortem surface analysis under ITER-relevant conditions.
This work was funded under DE-AC05-000R22725.
References
[1] R.T. Smyth, , C.P. Ballance, C.A. Ramsbottom, C.A. Johnson, D.A. Ennis, and S.D. Loch. Physical Review A 97, no. 5 (2018)
[2] N.L. Dunleavy, C.P. Ballance, C.A. Ramsbottom, C.A. Johnson, S.D. Loch, and D.A. Ennis. Physics B: Atomic, Molecular and Optical Physics 55, no. 17 (2022)