Pedestal origin and extrapolation of small-ELM peak energy flux to the divertor in future plasma scenarios

Experimental analysis at DIII-D shows that small/filamentary edge-localized-modes (ELMs) in high-density regimes originate in a region near the separatrix, at the pedestal foot, providing unique direct evidence of recent theoretical predictions. The measured divertor peak ELM energy flux for a database of small ELMs (or quasi-continuous exhaust regime, QCE) is reproduced, within 35% accuracy, when an ad-hoc modification of the Eich ELM energy flux model is applied to account for the filaments’ pedestal birth location. While future devices will not tolerate sustained Type-I ELMing conditions, this study provides the first projections of small-ELM parallel energy fluxes to the divertor in ITER and SPARC, resulting in values of 2.17 MJ/m² and 1.21 MJ/m² respectively i.e. well below the melting threshold of Tungsten monoblocks of 12 MJ/m².

Conditional averaging analysis of inter- and intra-ELM electron pressure profiles reveals that, during small ELMs, the peak relative change—located at the pedestal foot—remains below 20%, compared to approximately 60% for Type-I ELMs, where the peak occurs near the pedestal top.

The results presented in this work provide quantitative assessments that support small/ filamentary ELMs as a promising regime leading to tolerable energy fluxes to the divertor in future plasma scenarios.