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 theSOL and plays a dominant role in the `main-chamberrecycling' regime in Alcator C-Mod, a regime in which most of the plasmaparticle efflux recycles on the main-chamber walls rather than flowsinto the divertor volume. This observation has potentially importantimplications for a reactor: contrary to the ideal picture of divertoroperation, a tightly baffled divertor may not offer control of theneutral density in the main-chamber such that charge exchange heatlosses and sputtering of the main-chamber walls can be reduced. Theconditions that give rise to the main-chamber recycling regime canbe understood by considering the plasma-neutral particle balance: whenthe flux surface averaged neutral density exceeds a critical value,flows to the divertor can no longer compete with the ionizationsource and particle fluxes must increase with distance into theSOL. This critical neutral density condition can be recast into acritical cross-field plasma flux condition: particle fluxes mustincrease with distance into the SOL when the plasma flux crossing agiven flux surface exceeds a critical value. Thus, the existence ofthe main-chamber recycling regime is intrinsically tied to thelevel of anomalous cross-field particle transport. Directmeasurement of the effective cross-field particle diffusivitiesDeff in a number of ohmic L mode discharges indicates that Deffnear the separatrix strongly increases as plasma collisionalityincreases. Convected heat fluxes correspondingly increase, implyingthat there exists a critical plasma density (or perhaps collisionality) beyondwhich no steady state plasma can be maintained, even in the absenceof radiation.