Overview of density pedestal structure: role of fueling versus transport

How much of the density pedestal structure is determined by fueling versus transport is still an open question. In most current tokamaks, the scrape-off layer is not opaque enough to screen the neutrals and limit their penetration past the separatrix. As such, recycled neutrals and additional gas puffing can directly affect the pedestal structure through ionization. However, in ITER and other future burning plasma magnetic confinement devices, the ionization source will be pushed further out into the scrape-off layer. This poses the question: how much of the density pedestal structure is governed by this edge ionization source versus plasma transport effects? Theoretically, several turbulent modes have been identified which could provide a 'pinch-like' up-gradient transport mechanism. Up-gradient transport is necessary in a source-free region to obtain a peaked density profile, which is often observed in the core of tokamaks. In an opaque scrape-off layer, without the existence of such a pinch, the density pedestal structure would eventually disappear. Not only would this limit our ability to fuel a fusion reactor, it would also affect the pedestal stability. In this paper we will give an overview of the state-of-the-art on what sets the density pedestal structure based on experimental observations as well as theoretical models. We will complement these studies with new results from Alcator C-Mod and DIII-D where the opaqueness is increased to reach values similar to those in ITER.