• Energy Research

The separatrix electron density is an important parameter for core-edge scenario integration in tokamak devices, as it influences plasma confinement, divertor detachment and disruption avoidance. This quantity has been measured in H-mode discharges on JET, ASDEX Upgrade and Alcator C-Mod by applying the same fitting function to Thomson scattering measurements, and by employing the same analysis technique based on scrape-off layer power balance. To estimate the power crossing the separatrix, the inter-ELM time derivative of the plasma energy dW/dt has been experimentally evaluated and found to be approximately a constant fraction of the absorbed heating power. Correlations between ne,sep and engineering parameters have been investigated, revealing that ne,sep scales with the divertor neutral pressure p0,div in a similar manner across all devices. Additionally, when ne,sep is normalized to the obtained p0,div dependency, no clear correlation with the plasma current is found. These observations are in agreement with the 2-point model, which suggests that the upstream separatrix density is mainly set by the recycling at the divertor target.

Toroidal and poloidal flows of injected N+ ions were measured in the high-field side (HFS) scrape-off layer (SOL) of ASDEX Upgrade by Doppler spectroscopy with different degrees of HFS divertor detachment. In high-recycling conditions, the results suggest reversed parallel N+ flow away from the inner divertor in the near SOL close to the separatrix, while the flow is towards the inner divertor throughout the SOL in detached conditions. The measured poloidal N+ flows were directed away from the HFS divertor in the near SOL for all density cases. Divertor plasma oscillations, characterized by momentary peaking of the HFS target ion flux and decrease of the HFS SOL density, were observed slightly before the roll-over of the ion saturation current to the HFS target and lead to an increase in the N+ flow towards the HFS divertor. SOLPS and ERO simulations of the experiment predict entrainment below 50% between the velocities of N+ and D+ ions, suggesting that N+ ions are quantitatively a limited proxy for measuring D+ flows. ERO simulations show significantly higher entrainment for higher ionization states, e.g., N2+ and N3+.