Origin and creation of scrape-off layer filaments in ELM free regimes
With the thermal helium beam diagnostics the dynamics of plasma edge fluctuations can be observed from the confined region up to the far scrape-off layer.
The helium beam diagnostics was used to investigate quasi-coherent modes as well as transport near the separatrix in different confinement scenarios. The measurements show that the plasma dynamics in the confined region close to the separatrix is often dominated by regular high frequency modes (10–100 kHz), whereas filaments with an intermittent occurrence rate of 0.1–2 kHz dominate in particular the far scrape-off layer (SOL). The comparison among different regimes shows a similar filamentary activity between L-mode and inter-ELM H-mode phases. An increased filamentary activity in H-mode, however, is found for the ELM free EDA  and the QCE  regime. A possible explanation for the high number of SOL filaments could be based on strong modes which are characteristic for these regimes. These modes could enable continuous transport of particles and energy across the separatrix, forming the seed of filaments and suppress ELMs.
The findings are presented on the 24th PSI conference and are published in .
 L. Gil, et. al., Nucl. Fusion, 60, 054003 (2020)
 M. Griener et al., Nuclear Materials and Energy 25, 100854 (2020)