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

The understanding of turbulence in the edge region of toroidal magnetised plasmas is one of the key issues in modern fusion research. Great efforts are spend, experimentally and theoretically, to quantify and understand particle, energy and momentum transport. Blob-like transport was observed first by Zweben in 1985 during studies of edge density turbulence in the Caltech research tokamak. Zweben described coherent magnetic field-aligned structures of higher or lower plasma density and called them consistently blobs and holes. These long-lived filaments are born in the edge shear layer, where zonal flows shear off meso-scale coherent structures. The occurrence of localized filamentary blobs and holes have recently found renewed interest with the advent of fast camera diagnostics. It is now well known that Edge Localized Modes (ELMs) lead to the ejection of a number of filamentary structures into the scrape off layer (SOL). ELMs thus generate structures with excess energy and density and it can be conjectured that they leave corresponding holes behind. In contrast to blobs/filaments, holes are usually quickly filled by the edge plasma along the magnetic field and therefore exhibit a restricted lifetime and consequently they are more difficult to observe. If such a hole, however, is able to reach a resonant surface with low rational q, due to its size or low shear, it closes on itself and increases its lifetime significantly. We believe that a signature of such an event can be found in the JET tokamak, called Palm Tree Mode (PTM). The PTM is a special MHD phenomenon, which was only detected in JET type-I ELMy H-mode plasmas as long as the rational q =3 surface is in the ELM perturbed region. Fast sampling magnetic pickup coils were used to study onset, decay, dynamics and the helical structure of the mode. Comparisons with charge exchange recombination spectroscopy show that the PTM is co-rotating with the edge plasma after recovery of ELM induced momentum losses. The decay rates of the current give insight in the closing phase and the filling mechanisms, which determine the lifetime of the PTM. Based on relative signal strengths and decay rates a new method has been developed to locate the mode. Fluctuations of PTMs can also be found in the electron cyclotron emission diagnostic. The effect of the mode on the diagnostic is twofold. One is the increase in temperature measured inside the filament. The other is the change in temperature perceived through the change in geometry by the magnetic perturbation of the PTM. These two contributions have been separated for the first time with the method of empirical mode decomposition. Forward modelling suggests that PTMs are closed unipolar current filaments with negative current. The size of the current hole is of the order of the edge current density. It is demonstrated that a rotating filament can produce the same characteristic spectra like observed in FFTs of ECE and magnetics data. The indications, especially found from forward modelling, support the hypothesis. The concept of blobs and holes therefore gives insight in the genesis and nature of the PTM. The replacement of a wave-like description of the PTM by a filament or quasiparticle picture appears to be possible and also contributes to explain other edge phenomena like outer modes or edge snakes. (author)