Speaker
Description
Plasma flows are fundamental in understanding various aspects of the power exhaust in fusion devices, including helium exhaust, impurity transport, detachment processes, and distribution of the diverted heat flux [1-3]. A comprehensive and validated understanding of the plasma flows is required for evaluating and optimizing power exhaust strategies [4,5].
A multi-spectral coherence imaging spectroscopy (CIS) setup has been developed to simultaneously acquire the plasma flows of several impurity charge states in TCV (C$^+$, He$^+$,C$^{2+}$), together with an achromatic CIS system on Magnum-PSI (H, Ar$^+$). CIS is a camera-based interferometric technique to obtain 2D integrated plasma velocity profiles from the spectral emissions’ Doppler shift. The spatial resolution (1232x1028 lines-of-sight) and velocity precision (±200 m/s) of legacy CIS systems were improved in a new diagnostic configuration, consisting of a temperature-stabilized field-widened polarization interferometer with a full quadrant polarization sensitive camera, allowing for the observation of steep velocity gradients in 2-dimensions with a single ex-situ calibration. The diagnostic was validated against high-resolution spectroscopy in the Upgraded Pilot-PSI with excellent agreement.
Measurements on Magnum-PSI and TCV characterize the flows from the near-surface plasma region to the full scrape-off layer, providing new insights into the performance of alternative divertor configurations and detachment strategies for future fusion reactors. In Magnum-PSI, hydrogen and impurity argon flow measurements indicate collisional coupling between the main plasma and impurities that extends into the pre-sheath. Initial TCV observations indicate strong gradients in the velocity profile (shear) across the last-closed flux surface, with strong collisional coupling between He$^+$ and C$^{2+}$, whereas C$^+$ exhibits a lower velocity, indicating imperfect thermalization, in quantitative agreement with previous spectroscopic studies [6]. Investigations of the impact of plasma flows on detachment will be presented.
[1] P.C. Stangeby and D. Moulton, Nuclear Fusion (2020), 60(10):106005
[2] A. Zito et al., Nuclear Fusion (2025), 65(4):046022
[3] A.V. Chankin et al., Journal of Nuclear Materials (2001), 290-293:518-524
[4] Y. Wang et al., Nuclear Fusion (2024), 64(5):056040
[5] A.V. Chankin et al., Nuclear Fusion (2007), 47(8):762
[6] Lorenzo Martinelli, EPFL PhD thesis (2023), 10158