The effect of plasma shaping on high density H-mode SOL profiles and fluctuations in TCV

Abstract

The impact of plasma shaping on the properties of high density H-mode scrape-off layer (SOL) profiles and transport at the outer midplane has been investigated on Tokamakà configuration variable. The experimental dataset has been acquired by evolving the upper triangularity while keeping the other parameters constant. The scan comprises $\delta_\mathrm{up}$ values between 0.0 and 0.6, excluding negative triangularity scenarios. Within this study, a transition from type-I edge localised modes to the quasi-continuous exhaust regime takes place from low to high $\delta_\mathrm{up}$ . The modification of the upstream SOL profiles has been assessed, in terms of separatrix quantities, within the $\alpha_\mathrm{t}$ turbulence control parameter theoretical framework (Eich et al 2020 Nucl. Fusion 60 056016). The target parallel heat load and the upstream near-SOL density profiles have been shown to broaden significantly for increasing $\alpha_\mathrm{t}$ . Correspondingly, in the far SOL a density shoulder formation is observed when moving from low to high $\delta_\mathrm{up}$ . These behaviours have been correlated with an enhancement of the SOL fluctuation level, as registered by wall-mounted Langmuir probes as well as the thermal helium beam diagnostic. Specifically, both the background and the filamentary-induced fluctuating parts of the first wall ion saturation current signal are larger at higher $\delta_\mathrm{up}$ , with filaments being ejected more frequently into the SOL. Comparison of two pulses at the extremes of the $\delta_\mathrm{up}$ scan range, but with otherwise same input parameters, shows that the midplane neutral pressure does not change much during the H-mode phase of the discharge. This indicates that indirect effects of the change in geometry, linked to first wall recycling sources, should not play a significant role. The total core radiation increases at high $\delta_\mathrm{up}$ , on account of a stronger plasma–wall interaction and resulting larger carbon impurity intake from the first wall. This is likely associated to the enhanced first wall fluctuations, as well as a smaller outer gap and the close-to-double-null magnetic topology at high shaping.

Keywords

• H-mode
• plasma shaping
• scrape-off layer
• power fall-off length
• density shoulder
• filaments