Merging dynamics of plasma blobs in the Scrape-off Layer of a tokamak

TL;DR

This paper models the merging behavior of plasma blobs in a tokamak's Scrape-off Layer, revealing dynamics similar to magnetic island coalescence with unique density effects, and provides insights into convective transport impacts.

Contribution

It introduces a detailed numerical model of plasma blob merging, highlighting differences from magnetic island coalescence due to density perturbations and revealing new dynamic behaviors.

Findings

Blobs rotate around each other during merging.

Merging acceleration is proportional to the square of the current density.

Blob separation oscillates, indicating sloshing behavior.

Abstract

The emergence and merging of high-density coherent structures - plasma blobs - is a recurrent phenomenon in the Scrape-off layer (SOL) of a tokamak plasma that has a significant impact on the rate of convective transport in that region. We report on a model study of the merging of two electromagnetically interacting blobs in a high beta plasma. Our detailed numerical simulations show that the merging process is akin to the coalescence instability between two magnetic islands but with important differences due to the density perturbation. The blobs are found to rotate about each other during merging and the merging occurs with an acceleration in the poloidal direction that is directly proportional to the square of the current density of the blobs and inversely proportional to its density. The separation distance between two high current density blobs is also seen to oscillate indicating a sloshing behavior.