CHARACTERISTICS OF ASYMMETRIC (LOW-FIELD-SIDE AND HIGH-FIELD SIDE) DIVERTOR DETACHMENT IN KSTAR L-MODE PLASMAS

Abstract

The in/out detachment asymmetry in KSTAR L-mode plasmas with deuterium (D) fuelling has been investigated with the SOLPS-ITER code to understand its mechanism and identify important atomic processes in the divertor region. The simulations show that the neutral accumulation towards the low-field-side (outer) target, driving outer target detachment at lower upstream density than is required for high field side (inner) target detachment. This is consistent with available Langmuir probe measurements at both target plates. The pressure and power loss factors defined in the two-point model [1-4] of the divertor scrape-off layer (SOL) and the sources contributing to the loss factors are calculated through post-processing of the SOLPS-ITER results. The momentum losses are mainly driven by plasma-neutral interaction and the power losses by lasmaneutral interaction and the carbon radiation emission. The presence of carbon impurities in the simulation enhances pressure and power dissipation compared to the pure deuterium case. Carbon radiation is a strong power loss channel that cools the plasma, but its effect on the pressure balance is indirect. As a result, the addition of carbon saturates the momentum and power losses in the flux tube at lower upstream densities, reducing the rollover threshold of upstream density. The most important contribution to the pressure and power balance depends on the radial distance of the SOL flux tubes from the separatrix (near/far SOL) and the target (inner/outer target). This is related to the strong D2 molecule accumulation near the outer strike point. A large portion of the recycled neutral particles from both targets reach and accumulate in the outer SOL, which is attributed in strong part to the target inclination and gap structure between the central and outboard divertors and hence to the impact of geometry. The accumulated neutrals enhance the reactions involving D2 which cause momentum and power loss.