Effects of recycling neutral on density shoulder formation in tokamak plasmas

Towards a physical understanding of the formation of flattened upstream scrape-off-layer (SOL) density profiles, namely 'density shoulders', a self-consistent one-dimensional radial transport model has been developed to estimate the upstream profiles covering both the core plasma and SOL region at the tokamak midplane. For the SOL region, the effective density and temperature profiles for the ionization process are obtained by the weighted averaging of the upstream and downstream profiles, which can distinguish the open-target operation from the closed-target operation by a weighting factor. Compared with enhanced turbulent convective transport, it is complementary for the model to study the competition between the effective source S (eff) and the parallel particle loss L (SOL). It indicates that: (1) an appropriate S (eff) intensity controlled by the neutral pressure due to divertor or wall recycling and (2) an appropriate S (eff) peak position in a far SOL region adjusted by the plasma current as well as the weighting factor could offset the damping effect of L (SOL) on the density profile. Then S (eff) over L (SOL) in a far SOL region could be the sole process involved in bringing about SOL density shoulders.