Cross-field plasma transport and main-chamber recycling in diverted plasmas on Alcator C-Mod

Abstract

Cross-field particle transport increases sharply with distance into the SOL and plays a dominant role in the `main-chamber recycling' regime in Alcator C-Mod, a regime in which most of the plasma particle efflux recycles on the main-chamber walls rather than flows into the divertor volume. This observation has potentially important implications for a reactor: contrary to the ideal picture of divertor operation, a tightly baffled divertor may not offer control of the neutral density in the main-chamber such that charge exchange heat losses and sputtering of the main-chamber walls can be reduced. The conditions that give rise to the main-chamber recycling regime can be understood by considering the plasma-neutral particle balance: when the flux surface averaged neutral density exceeds a critical value, flows to the divertor can no longer compete with the ionization source and particle fluxes must increase with distance into the SOL. This critical neutral density condition can be recast into a critical cross-field plasma flux condition: particle fluxes must increase with distance into the SOL when the plasma flux crossing a given flux surface exceeds a critical value. Thus, the existence of the main-chamber recycling regime is intrinsically tied to the level of anomalous cross-field particle transport. Direct measurement of the effective cross-field particle diffusivities Deff in a number of ohmic L mode discharges indicates that Deff near the separatrix strongly increases as plasma collisionality increases. Convected heat fluxes correspondingly increase, implying that there exists a critical plasma density (or perhaps collisionality) beyond which no steady state plasma can be maintained, even in the absence of radiation.

Physics of ultimate detachment of a tokamak divertor plasma

“…It is known for a long time that in L-mode, cross-field transport is increasing at high plasma density when approaching detachment (see e.g. Umansky et al 1998;LaBombard et al 2000;Greenwald 2002;Rudakov et al 2005;Wiesen et al 2011. Usually it is related to the increased contribution of intermittent blobby plasma transport (see Zweben et al 2007;Krasheninnikov et al 2008;D'Ippolito et al 2011, and the references therein).…”

Development of the gas-puff imaging diagnostic in the TEXTOR tokamak

“…They may increase the recycling in the main chamber and hence result in a high level erosion of the first wall. 5,6 Therefore, characterization of blobby transport and understanding the underlying physical mechanisms of blob generation and evolution are extremely important topics in fusion research.…”

Transport by intermittent convection in the boundary of the DIII-D tokamak

“…The balance between parallel and perpendicular transport results in a SOL which is thin compared to its length 1 and the observed SOL profiles are mostly exponential 2 with short ͑1-3 cm͒ decay lengths. However, there is ample [3][4][5] evidence that the SOL profiles are sometimes wider than usual and nonexponential and flat far away from the last closed flux surface ͑LCFS͒, suggesting that perpendicular transport in these conditions can be larger than expected.…”