Speaker
Description
Mitigating the heat load of type-I edge-localised modes (ELMs) via impurity seeding has been investigated in single-null plasmas of the Tokamak à Configuration Variable (TCV). The measurements exploit TCV’s fast diagnostics to resolve ELMs, from the plasma edge to the divertor targets: the ELM energy loss is measured with a diamagnetic loop (DML, acquisition frequency of f=10kHz); the radiation evolution is inferred with absolute extreme ultraviolet diodes (AXUV, f=200kHz), and both inner and outer target heat flux profiles are evaluated with infrared thermography (IR, f≥10kHz). The heating is mainly provided by neutral beam injection, while electron cyclotron heating is used to reduce core contamination by impurities. A systematic scan of neon, argon, and nitrogen seeding rates was performed, covering from unseeded discharges to ones where impurities caused a radiative collapse. Comparing DML and IR data indicates that dissipative processes lower the ELM energy by only 10-25% before the ELM crash. The findings agree with previous experiments and modelling in JET, where injecting argon in the divertor did not yield considerable radiation during ELMs [1]. Nevertheless, in TCV, strong seeding modified the pedestal profiles, reducing the ELM energy loss, which substantially decreased inner and outer peak heat fluxes—up to 40% with neon, 60% with argon, and 90% with nitrogen. In the nitrogen case, the plasma stored energy diminished by 11% during seeding. Nonetheless, the energy confinement time, the core effective ion charge $Z_{eff}$, the core electron temperature and the pedestal electron density were not affected, while the radiation around the X-point intensified, core $n_e$ rose by 30%, and the pedestal $T_e$ lowered by about 35%, transitioning the ELM regime from type-I to type-III. Furthermore, in AUG, JET, and EAST, an increase in confinement time was observed simultaneously with a reduction of the ELM energy loss [2-4], by using nitrogen (AUG) and neon (JET, EAST). The TCV, AUG, JET, and EAST results indicate that, although impurities cannot prevent the burn-through of high-energy transients, they can modify edge profiles to avoid the formation of large ELMs—a feature of direct interest for future fusion reactors.
[1] J. Rapp et al. Nucl. Fusion 44 312–319 (2004)
[2] M. Komm et al. Nucl. Fusion 63 126018 (2023)
[3] C. Giroud et al. EPS Conference on Plasma Physics (2022)
[4] K. Li et al. Nucl. Fusion 63 026025 (2023)