What sets the energy lost per ELM at low collisionality?
In plain words
Each ELM throws out a fraction of the edge layer's stored energy, and that fraction grows as the edge becomes hotter and less collisional, as it will be in ITER. The mechanism behind this trend, and hence the size of ITER's ELMs, is not established.
Precise statement
Multi-machine data show the type-I ELM energy loss $\Delta W_{\mathrm{ELM}}/W_{\mathrm{ped}}$ rising from a few percent to about 20 percent as pedestal collisionality $\nu*_{\mathrm{ped}}$ decreases, together with a larger conductive fraction. Identify the mechanism (parallel electron conduction along stochastic edge fields, filament convection, nonlinear ballooning-mode saturation) and predict $\Delta W_{\mathrm{ELM}}$ and the divertor wetted area for natural ELMs in ITER $Q = 10$. An answer is nonlinear MHD with validated parallel closures reproducing the measured collisionality trend on at least two devices.
What would settle it
Nonlinear MHD simulations that reproduce the measured $\Delta W_{ELM}(\nu*_{ped})$ trend and heat-load footprints on two devices, applied to ITER without retuning.
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