ELM transport in the JET scrape-off layer

Abstract

Edge Localised Modes (ELMs) are universally recognised as one of the greatest threats to the viability of ITER and future fusion power plants based on the tokamak concept. They are plasma relaxations driven by MHD modes and are thought to originate in the steep pressure gradient region of the edge transport barrier characteristic of H-mode plasmas. In ITER, extrapolations from JET predict that Type I ELMs in the Q DT = 10 baseline scenario will expel between 3-8% of the 350 MJ plasma stored energy, depositing energy fluxes of 0.6 -3.4 MJm -2 on the divertor targets [1]. Only at the lowest values of this energy range would the subsequent target erosion be tolerable. Of late, concerns are being raised not just for the divertor targets, where most of the ELM energy is intercepted, but also for the main chamber walls to where ELM power fluxes are now known to extend.The mechanisms governing the ELM origin location and non-linear evolution within the H-mode pedestal and the subsequent cross-field propagation within the scrape-off layer (SOL) remain the subjects of keen debate. Once in the SOL, however, the thermal energy within the filament is removed predominantly by parallel losses to divertor targets, a process which is better understood but which is nevertheless complex, comprising both kinetic and fluid effects. This contribution aims to demonstrate how experiments and modelling at JET are significantly advancing our understanding of the ELM SOL parallel transport, providing many of the key elements required for an integrated, quantitative treatment of the ELM energy fluxes and their subsequent consequences for plasma-wall interaction.Infra-red thermography is extensively employed for divertor target measurements at JET and has recently been complemented by a unique new wide angle view of the main chamber wall surface. Such measurements are technically challenging due to the fast transient nature of the ELM and the presence of thin surface layers, which can differ radically from surface to surface. Taking proper account of this reveals that ELMs deposit energy preferentially (~ factor 2) in the outer target at low pedestal collisionality (ν*) but that this ratio inverts in favour of inner target energy deposition (up to ~ factor 3) with rising ν*.These target plate measurements of the ELM heat flux transient, in combination with fast triple Langmuir probe data, have provided the first known experimental evidence for

Ion temperature fluctuations in the ASDEX Upgrade scrape-off layer

“…Since the probe is located above the outboard midplane, it seems obvious that the intermittent current bursts are observed mainly by the i-side RFA which faces most filaments as they expand along the flux tubes. The same conclusion has been reached elsewhere [26,27].…”

First measurements of edge localized mode ion energies in the ASDEX Upgrade far scrape-off layer

“…These observations (also seen later on JET [7,8]), were consistent with increase by an order of magnitude of the tungsten influx from the outboard limiters in AUG during ELMs compared to inter-ELM periods [9]. Energetic ELM ions (impact energies exceeding 400 eV) in the far SOL (typically 2-3 characteristic SOL power widths outside the separatrix) were measured directly for the first time in JET [10] (and more recently in MAST [11]) using a Retarding Field Analyzer (RFA) technique.…”

Application of AXUV diode detectors at ASDEX Upgrade

“…The post-ELM filaments observed by the AXUV diagnostic might be remanent structures of the ELM filaments [20][21][22] or might be correlated to the X-point fluctuations reported in 23 , for which the radiation might be intensified by the ELMs. ELM filaments rotate toroidally 22 .…”