Blob dynamics in TORPEX poloidal null configurations

TL;DR

This paper simulates three-dimensional blob dynamics in TORPEX's X-point magnetic configurations, revealing that null regions slightly accelerate filaments mainly due to developing dipoles, with minimal influence from connection length effects.

Contribution

It introduces a non-field-aligned coordinate simulation method for blob dynamics in X-point configurations, aligning numerical results with analytical models and experimental data.

Findings

Null regions cause slight filament acceleration due to developing dipoles.

Connection length effects near the null are a minor correction.

Simulation results agree with experimental measurements.

Abstract

Three dimensional blob dynamics are simulated in X-point magnetic configurations in the TORPEX device via a non-field-aligned coordinate system, using an isothermal model which evolves density, vorticity, parallel velocity and parallel current density. By modifying the parallel gradient operator to include perpendicular perturbations from poloidal field coils, numerical singularities associated with field aligned coordinates are avoided. A comparison with a previously developed analytical model is performed and an agreement is found with minimal modification. Experimental comparison determines that the null region can cause an acceleration of filaments due to increasing connection length, but this acceleration is small relative to other effects, which we quantify. Experimental measurements are reproduced, and the dominant acceleration mechanism is identified as that of a developing dipole in a moving background. Contributions from increasing connection length close to the null point are a small correction.