Dependence of divertor turbulence on plasma density and current in TCV

Abstract

To reliably predict the distribution of heat and particle fluxes at the target plates of tokamaks, a comprehensive understanding of turbulence throughout the entire Scrape-Off-Layer (SOL) is imperative. This study examines divertor turbulence systematically across a broad parameter range on the TCV tokamak, including variations in magnetic field direction, plasma current $I_{\mathrm{p}} \in [140, 320]$ kA, edge safety factor $q_{95} \in [2.6,4.7]$ and Greenwald fraction $f_{\mathrm{G}}\in [0.18,0.6]$ . The TCV X-point Gas Puff Imaging (GPI) system is used to measure 2D filament properties in the inner and outer divertor region. The fluctuation levels in the divertor are found to strongly increase with density (to 80% over most of the SOL) while remaining insensitive to $I_{\mathrm{p}}$ . The previously identified divertor-localized filaments (DLF), located on the bad curvature side of the outer divertor leg, are found to be a common feature on TCV, while no filaments are observed in the PFR. DLFs are present over most of the parameter space and in both field directions. However, they are absent, or appear only closer to the target, for sufficiently large $\Lambda_{\mathrm{div}}\gtrsim 10$ or $q_{95}\gtrsim3.7$ . Across both $I_{\mathrm{p}}$ and $f_{\mathrm{G}}$ scans, some clear trends with $\Lambda_{\mathrm{div}}$ are found for divertor filament sizes and velocities, and with target fall-off lengths of density and heat flux profiles at the outer target. This study provides important experimental insights to turbulent transport in the divertor also for comparison with self-consistent, turbulence simulations and extrapolation to future reactor conditions.

Keywords

• tokamak
• divertor
• turbulence
• filament
• scrape-off layer (SOL)
• particle and heat fluxes