The Influence of Filaments in the Private Flux Region on Divertor Particle and Power Deposition

TL;DR

This study investigates how filamentary structures in the private flux region influence particle and power deposition in the divertor of the MAST tokamak, revealing filament characteristics and their effects on flux fluctuations.

Contribution

It provides detailed analysis of filament properties and their role in plasma transport in the private flux region, a novel focus in divertor studies.

Findings

Filaments are approximately 1-2cm in diameter with mode number 2-3.

Filaments eject plasma deeper into the private flux region at 0.5-1.0km/s.

Particle flux fluctuations are strongest in the private flux region and insensitive to core plasma conditions.

Abstract

The transport of particles via intermittent filamentary structures in the private flux region of plasmas in the MAST tokamak has been investigated using a fast framing camera recording visible light emission from the volume of the lower divertor, as well as Langmuir probes and IR thermography monitoring particle and power fluxes to plasma-facing surfaces in the divertor. The visible camera data suggests that, in the divertor volume, fluctuations in light emission above the X-point are strongest in the scrape-off layer (SOL). Conversely, in the region below the X-point, it is found that these fluctuations are strongest in the private flux region (PFR) of the inner divertor leg. Detailed analysis of the appearance of these filaments in the camera data suggests that they are approximately circular, around 1-2cm in diameter. The most probable toroidal mode number is between 2 and 3. These filaments eject plasma deeper into the private flux region, sometimes by the production of secondary filaments, moving at a speed of 0.5-1.0km/s. Probe measurements at the inner divertor target suggest that the fluctuations in the particle flux to the inner target are strongest in the private flux region, and that the amplitude and distribution of these fluctuations are insensitive to the electron density of the core plasma, auxiliary heating and whether the plasma is single-null or double-null. It is found that the e-folding width of the time-average particle flux in the PFR decreases with increasing plasma current, but the fluctuations are unchanged. At the outer divertor target, the fluctuations in particle and power fluxes are strongest in the SOL.