Ion energies and currents of type I and mitigated ELMs in the ASDEX Upgrade far scrape-off layer

Abstract

New measurements of ion energies and currents in type I and mitigated ELMs have been carried out in the ASDEX Upgrade far scrape-off layer using a retarding field analyser (RFA). The ion temperature averaged over an ELM, Ti ELM measured 35–60 mm outside the separatrix (i.e. 15–25 mm in front of the outboard limiter) is in the range 20–200 eV, which is 5–50% of the ion temperature at the pedestal top. Ti ELM decreases with the separatrix distance with the e-folding length of ∼10 mm measured in the far SOL for a particular set of conditions, and increases with the ELM energy WELM. Lowest Ti ELM is measured during mitigated type I ELMs. Likewise, the ELM-averaged ion current e-folding length increases with WELM, similar to the e-folding length of the heat flux density at the RFA probe head during an ELM, monitored by a fast IR camera. The most plausible explanation of observed trends is that on average the filaments of larger ELMs travel faster radially and have less time to dilute by parallel losses along field lines before reaching the far SOL. These observations provide further evidence that the fraction of the ELM energy deposited on the main chamber plasma-facing components increases with WELM.