On the nature of blob propagation and generation in the large plasma device: Global GRILLIX studies

Abstract

The appearance of blobs, filamentary structures featuring large intermittent perturbations, is characteristic for the scrape-off layer of magnetic fusion devices. Therefore, a global model, which does not rely on assumptions about the fluctuation level, is necessary to model blobs accurately. Whereas GRILLIX, a global 3D fluid turbulence code, is originally designed to handle complex geometries via the flux-coordinate independent approach, the analysis here focuses on a thorough verification, validation, and identification of basic phenomena in simplified slab geometry. As such the impact of the routinely employed Boussinesq approximation is studied systematically, revealing that not only the density amplitude itself matters, but also the blob regime that is also influenced by temperature. This points out that the validity of the Boussinesq approximation cannot generally be taken for granted. Furthermore, GRILLIX is validated against the large plasma device experiment and the formation of blobs is studied. A cross-phase evaluation suggests as candidates for the blob drive mechanism the rotational interchange instability.