Description
Bremsstrahlung emission from mildly relativistic electrons is peaked in the forward direction. This angular anisotropy depends on the energy of the electrons and of the emitted photons, and is stronger for highly energetic electrons. At the WEST tokamak, energetic electrons are generated either through absorption of radiofrequency waves of the lower-hybrid (LH) type, which accelerate electrons up to a few hundred keV, or through the production of runaway electrons in dedicated experiments, with energies up to 15 MeV.
For the first time, the toroidal anisotropy of the bremsstrahlung emission has been measured in different plasma scenarios at WEST. The experimental setup employs two multi-energy cameras equipped with pixelated detectors with adjustable lower energy thresholds for photon detection: the ME-SXR camera, with energy thresholds in the range 11–18 keV, and the ME-HXR camera, with energy thresholds in the range 20–60 keV. Thanks to the 2D pixelated detectors, both cameras image a toroidal angle of 9°, which is sufficient to observe a toroidal anisotropy of the emitted radiation.
Anisotropy ranging from 10% to 50% in the toroidal viewing angle was measured in different plasma scenarios and at different poloidal locations. Anisotropy of the thin-target bremsstrahlung emission from fast electrons was observed in the core plasma during LH-heated discharges. It was reproduced with the suite of codes C3PO/LUKE/R5-X2, which model LH wave absorption, generation of fast electrons and bremsstrahlung emission. In the same scenario, anisotropy of the thick-target bremsstrahlung emission from fast-electron losses in the scrape-off layer (SOL) was also measured. Anisotropy of the thin-target bremsstrahlung emission was observed during experiments involving the production of runaway electrons, both at startup and triggered by argon injection. The measured anisotropy is found to increase substantially from the plateau phase of the emitted radiation to burst events. These measurements may be used in future machines as a runaway-electron alarm.