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Description
Experimental analysis and simulations with the BOUT++ code[1] show that small edge-localized modes (ELMs) in reactor-relevant high-density regimes originate in a region close to the separatrix and only marginally perturb the pedestal structure. The measured divertor peak parallel energy flux for a database of small ELMs in DIII-D and ASDEX Upgrade can be reproduced, within 40% accuracy on average, if an ad-hoc modification of the peak parallel ELM energy flux model[2] is applied to account for the small ELMs pedestal birth location. This allows for first-order extrapolation of small ELM divertor parallel energy fluxes to ITER and SPARC, resulting in values that satisfy the nominal melting threshold of Tungsten monoblocks of 12 MJ/m2[3].
An inverse dependency is observed between the experimental ELM energy flux to the divertor and the separatrix turbulence control parameter, αt[4]. As αt increases from ∼0.2 to ∼0.8, transitioning from a type-I ELM to a small-ELM regime, the divertor peak energy flux decreases by a factor of five. The scrape-off-layer (SOL) transport associated with small ELMs is found to be related to divertor conditions, where higher divertor collisionality correlates with stronger upstream radial heat and particle fluxes. While large αt at the separatrix leads to substantial mitigation of intra-ELM divertor heat loads, the accompanying enhancement of first-wall fluxes might present challenges for first-wall integrity and/or impurity sources in future machines.
The findings reported in this study, both via modeling and experiments, constitute a step forward toward the assessment of high edge-collisionality scenarios as viable plasma regime for the operation of near-future fusion machines. The extrapolations to SPARC and ITER constitute first quantitative projections that can support future PMI-focused analyses aimed to inform reactors design.
[1] T. Y. Xia and X. Q. Xu, Nucl. Fusion, 55, 113030 (2015)
[2] T. Eich et al., Nucl. Mater. Energy 12 (2017) 84-90
[3] J. Gunn et al., Nucl. Fusion 57 (2017) 046025
[4] B. D. Scott, Phys. Plasmas 12 (2005) 062314
Work supported by US DOE under DE-FC02-04ER54698, DE-FG02- 07ER54917 and DE-AC52-07NA27344.