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

Cross-field particle transport increases sharply with distance into the SOL and plays a dominant role in the 'main-chamber recycling' regime in Alcato r C-Mod, a regime in which most of the plasma particle efflux recycles on t he main-chamber walls rather than floss into the divertor volume. This obse rvation has potentially important implications for a reactor: contrary to t he ideal picture of divertor operation, a tightly baffled divertor may not offer control of the neutral density in the main-chamber such that charge e xchange 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 fl ux surface averaged neutral density exceeds a critical value, flows to the divertor can no longer compete with the ionization source and particle flux es 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 fl ux crossing a given flux surface exceeds a critical value. Thus, the existe nce of the main-chamber recycling regime is intrinsically tied to the level of anomalous cross-field particle transport. Direct measurement of the eff ective cross-field particle diffusivities D-eff in a number of ohmic L mode discharges indicates that D-eff near the separatrix strongly increases as plasma collisionality increases. Convected heat fluxes correspondingly incr ease, implying that there exists a critical plasma density (or perhaps coll isionality) beyond which no steady state plasma can be maintained, even in the absence of radiation.