Effects of recycling neutral on density shoulder formation in tokamak plasmas

Abstract

Towards the physical understanding on 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 core plasma and SOL region at tokamak midplane. For the SOL region, the effective density and temperature profiles for the ionization process are obtained by weighted averaging of the upstream and downstream profiles, which can distinguish the open target operation (OTO) from the closed target operation (CTO) by a weighting factor. Compared with the enhanced turbulent convective transport, it is complementary for the model to study the competition between the effective source Seff and the parallel particle loss LSOL . It indicates that: (1) an appropriate Seff intensity controlled by the neutral pressure due to divertor or wall recycling and (2) an appropriate Seff peak position in a far SOL region adjusted by the plasma current as well as the weighting factor could offset the damping effect of LSOL on density profile. Then Seff over LSOL in a far SOL region could be the sole process involved in bringing about SOL density shoulders.

The influence of full drifts on density shoulder formation at the midplane by numerical modeling

“…x and L D + y represent the contributions to the loss of D + particles by poloidal and radial transport, respectively, and V reg is the volume of the selected region. The negative value of L represents a net loss of particles from the region, and is consistent with the convention established in [12]. The total loss of D + source, L D+ SOL , is equal to the source provided by the plasma-neutral interactions.…”

“…Meanwhile, the enhanced ion sources increase the local plasma density to feed the shoulder [19] when n D+,CEI is high. However, it has been reported that the 'density shoulder is caused by the competition between SOL particle loss term L SOL and effective ionization source term' [12]. L SOL depends strongly on particle transport, which is influenced significantly by E × B drift.…”

“…According to equation ( 1 and the ion source S total [12]. The D + flux near the separatrix at the midplane in the SOL is affected by E × B drift in both poloidal and radial directions (see figures 5(a) and (b)).…”

“…It is often called the 'density shoulder'. It can be affected by particle transport and ionization in the SOL [12,13]. Chen et al observed that the density shoulder at the low-field side (LFS) was formed in upper single null H-mode discharges with a reversed toroidal magnetic field (B × ∇B point away from the X-point) in EAST [14].…”

“…SOLPS modeling demonstrated that increasing the radial transport and enhancing ionization at the midplane can influence the density shoulder [19]. Wu et al, using a one-dimensional radial transport model analysis, found that effective ionization sources and parallel particle transport loss can affect shoulder formation [12]. Meanwhile, the density shoulder was observed not only in the detached regime but also in the attached plasma [15].…”