Numerical investigation of isolated filament motion in a realistic tokamak geometry

TL;DR

This study uses numerical simulations to analyze isolated filament dynamics in a realistic tokamak geometry, revealing how electron temperature and filament width influence their motion and transport properties.

Contribution

It provides new insights into 3D filament behavior and scaling laws in a realistic tokamak geometry, extending previous 2D models.

Findings

Bi-normal velocities scale linearly with electron temperature.

Radial velocities depend positively on electron temperature and filament width.

Background temperature has a weak effect on particle flux, while filament width has a strong effect.

Abstract

This paper presents a numerical investigation of isolated filament dynamics in a simulation geometry representative of the scrape-off layer (SOL) of the Mega Amp Spherical Tokamak (MAST) previously studied in [N.R.Walkden et.al, Plasma Phys. Control. Fusion, 55 (2013) 105005]. This paper focuses on the evolution of filament cross-sections at the outboard midplane and investigates the scaling of the centre of mass velocity of the filament cross-section with filament width and electron temperature. By decoupling the vorticity equation into even and odd parity components about the centre of the filament in the bi-normal direction parallel density gradients are shown to drive large velocities in the bi-normal (approximately poloidal) direction which scale linearly with electron temperature. In this respect increasing the electron temperature causes a departure of the filament dynamics from 2D behaviours. Despite the strong impact of 3D effects the radial motion of the filament is shown to be relatively well predicted by 2D scalings. The radial velocity is found to scale positively with both electron temperature and cross-sectional width, suggesting an inertially limited nature. Comparison with the two-region model [J. R. Myra et.al, Phys.Plasmas, 13 (2006) 112502] achieves reasonable agreement when using a corrected parallel connection length due to the neglect of diamagnetic currents driven in the divertor region of the filament. Analysis of the transport of particles due to the motion of the filament shows that the background temperature has a weak overall effect on the radial particle flux whilst the filament width has a strong effect.