Experimental evidence of very short power decay lengths in H-mode discharges in the COMPASS tokamak

Abstract

Analysis of heat fluxes in scrape-off layer (SOL) plasma is important for the prediction of divertor tile heat loads in future reactor-sized tokamak machines. Typically, the radial heat flux profile can be accurately characterized by the decay length parameter λ q. The predictions are then based on the dependence of λ q on plasma and machine parameters. In recent years, several empirical scaling models were derived from a collective database of decay lengths observed in several large tokamaks as well as two spherical tokamaks. Most recently, a report from the TCV tokamak showed a deviation from several of the proposed models (Maurizio et al (TCV team) 2021 Nucl. Fusion 61 024003). In this work, ∼ 100 edge plasma profiles of ELMy H-mode discharges were collected from the COMPASS tokamak database for heat flux analysis. The COMPASS tokamak has a similar machine size and plasma parameters to the TCV tokamak, offering a valuable comparison. The λ q was measured in both downstream and upstream SOL utilizing a divertor probe array validated by an IR camera and Thomson scattering diagnostics. A comparison with the predictions of several scaling models indicated an occurrence of anomalously short (by a factor of 2-5) inter-ELM power decay lengths in both upstream and downstream, as well as a difference in λ q between these two regions. Possible causes related to the accuracy of magnetic reconstruction were eliminated and physics-based sources of apparent compression effects were hypothesized as a motivation for future research.