On the genesis and nature of Palm Tree Modes in the JET tokamak

TL;DR

This paper investigates the formation of long-lived, localized palm tree modes in the JET tokamak's edge plasma, proposing a turbulence-based creation mechanism involving magnetic field structures like blobs and holes.

Contribution

It introduces a novel turbulence-based explanation for palm tree mode genesis, linking edge plasma structures to magnetic fluctuations observed in JET.

Findings

PTMs are associated with magnetic fluctuations similar to those produced by current filaments.

Captured holes on q=3 surfaces generate PTM-like signatures.

Captured filaments on q=4 surfaces produce outer mode signatures.

Abstract

Long-lived, highly localized structures called palm tree modes (PTM) are observed in the edge plasma of the JET tokamak. Although PTMs are well documented, little is known about the mechanisms which produce these structures. In the case of the PTM, an ELM-postcursor, its genesis is usually explained by ergodisation of the magnetic field due to edge localized modes and the appearance of a seed magnetic island which evolves into a PTM later. In this study we try to invoke a creation mechanism based on the concepts and observations in edge plasma turbulence. An interesting aspect of plasma turbulence is the occurrence of coherent, long-lived structures in the scrape-off-layer (SOL). These localized and magnetic-field-aligned regions with higher or lower plasma densities are called blobs and holes. Measurements show that these filaments carry parallel currents. We thus here interpret ELM-filaments as massive blobs and the interspace between these filaments as holes. We demonstrate that a forward-modelled closed current filament on a q=3 surface produces similar magnetic fluctuations as measured by the JET in-vessel magnetic pickup coils if a PTM is present. From that we deduce that if a hole is captured on a q=3 surface after an ELM-crash, a PTM equivalent signature is generated. If the ELM-filament itself is captured on a q=4 surface, a signature equivalent to an outer mode appears.