Scrape-off Layer Transport for Large and Small Edge Localized Modes and Implications for First Wall Fluxes in Future Devices*

Small high-frequency Edge Localized Modes (ELMs) deposit their power over a larger area compared to Type-I ELMs, but they do so to the expense of enhanced flux to the first wall. Small ELMs, originating at the pedestal foot, are an attractive regime for future reactors due to the low divertor peak heat flux, relative to the much larger Type-I ELMs. In this work, we perform experimental quantification of small ELM energy and particle fluxes to the first-wall and divertor, allowing for intra-ELM power balance, and compare these with Type-I ELM scenarios with similar magnetic configuration and input power. Employing reciprocating probe data and fast infrared imaging from DIII-D, energy (λ_Q) and particle (λ_Γ) flux decay lengths in the scrape-off-layer (SOL) are found to be 3 and 5 times larger, respectively, for small ELMs with high divertor collisionality, compared to well-attached Type-I ELMs. ELM λ_Q and λ_Γ in the SOL are found to increase with divertor collisionality (Λ_DIV), highlighting the role of Λ_DIV in governing the upstream ELM radial velocity and consequent first wall fluxes. The fraction of ELM energy flux deposited to the first-wall has been estimated for various outer-wall-gaps (OWG), based on the results presented. With an OWG of 2 cm, 54 % of the small ELM energy flux reaches the first wall. Extrapolating this to a burning plasma scenario with a 250 MJ plasma and ΔW_ELM/W_MHD=1%, ELM energy loads of >0.25 MJ/m² for ~0.5 ms would be deposited to the first wall, which is above the tolerance of any current breeding blanket design. Even if a divertor-material solution is found for future machines, results show that a large enough OWG (> 5 cm) is needed to protect the first wall, when operating with small ELMs and a cold divertor.