Speaker
Description
Controlling heat and particle fluxes on plasma-facing components remains a major challenge on the path toward ITER and future fusion reactors. These fluxes are strongly influenced by the dynamics in the boundary region, where turbulence and plasma flows partly determine first-wall heat and particle loads, as well as impurity transport, ultimately affecting sputtering and core plasma performance [1,2]. Characterizing these processes requires diagnostics capable of providing fast and spatially resolved measurements of, among other, electron and ion temperatures, densities, electric fields, and flows. Reciprocating probes have proven effective in meeting this need, providing spatial profiles across the outer midplane scrape-off layer (SOL) and divertor for such key quantities [3, 4], motivating the development of the Fast Reciprocating Diagnostic (FReDi) for the Tokamak à Configuration Variable (TCV).
FReDi is designed to allow measurements of low-field-side SOL profiles either at the midplane or in the divertor, with interchangeable probe heads, the first of which integrates opposingly oriented Retarding Field Analyzers (RFAs), providing ion temperature and density. Inherent challenges in the design, such as transparency dependence on parallel velocity and space-charge effects [3, 5], which constrain the geometry and set the operating limits for accurate measurements, are evaluated through Monte Carlo simulations of ion trajectories. On the same probe head, a flexible array of Langmuir probes enables the simultaneous inference of electron temperatures, densities, electric fields, and parallel plasma flow.
Designing such an advanced probe is demanding due to the harsh plasma environment in which it must operate. The fast and precise horizontal motion is achieved using a servolinear motor. Other key design considerations include material selection to minimize plasma perturbation, shaping of the probe head to prevent temperature surges above material limits, and characterization of the electronics transfer function to ensure distortion-free measurements.
The finalized design meets these constraints, establishing FReDi as a powerful diagnostic for investigating SOL and divertor turbulence, flows, and heat and particle flux profiles across a wide range of operating scenarios, thereby substantially enhancing TCV’s diagnostic capabilities.
[1] Giacomin, M. & al. (2021), Nuclear Fusion
[2] Pitts, R. A. & al. (2025), Nuclear Materials and Energy
[3] Brunner, D. & al. (2013), Review of Scientific Instruments
[4] Killer, C. & al. (2022), Journal of Instrumentation
[5] Kočan, M. & al. (2008), Review of Scientific Instruments