Multi-diagnostic analysis of plasma filaments in the island divertor

Abstract

Filaments or blobs are well known structures in turbulence in magnetic fusion devices, they are considered to be the major cross-transport channel in the scrape off layer. They originate at the last closed magnetic flux surface and propagate out on the low field side of toroidal devices due to polarization in the curved magnetic field. The Wendelstein 7-X stellarator has a complex three-dimensional magnetic field structure and additionally the plasma is bounded by a chain of magnetic islands, forming an island divertor. After the first observation of filaments in Wendelstein 7-X with video cameras a multi-diagnostic study is presented in this paper to reveal their 3D structure and dynamics. Filaments are seen to be born at the edge and, at least in some cases, seen to extend to up to 4 toroidal turns. After moving radially out a few cm they enter the edge island. Here they disappear from the equatorial plane and about 200 microseconds later reappear on the outboard side of the island. A long-wavelength (∼20–30 cm) quasi coherent mode is observed in both regions where filaments appear. The similarities and differences between the filaments seen in W7-X and other devices are discussed. Possible explanations for this strange radial propagation are considered, together with the likely role of filaments in the edge and island density profile.

Turbulent transport in the scrape-off layer of Wendelstein 7-X

“…This observation is further solidified in the probability distribution functions of these three quantities presented in figure 4(b)) and the associated statistical moments listed in table 1. While ñ and ṽr closely resemble normal distributions (in agreement with earlier investigations in the W7-X SOL [13,14]), Γ r is characterized by positive skewness and positive (excess) kurtosis. This is due to the cross phase between ñ and ṽr and indicates a tendency towards positive (outwards) particle transport with an increased occurrence of large transport events.…”

Understanding baffle overloads observed in high-mirror configuration on Wendelstein 7-X

“…The formation of these scratches is not clear, but it seems that they all point in the direction perpendicular to the magnetic field. There are several possibilities to speculate on the origin of the scratches including electric arcs of plasma causing 'magnetized' [29] patterns in −J × B direction, field-aligned filaments drifting inside the island in E × B direction [30] or damages already done before the campaign possibly during manufacturing or assembly. Colorful traces are also observed surrounding the screw, which are considered to be deposition layers consisting of different impurities.…”

Global fluid simulation of plasma turbulence in a stellarator with an island divertor

“…In W7-X, the fluctuations are approximately normally distributed [13], hinting that the turbulent structures are the result of fluctuations that have a local origin rather than being advected from a different radial position, in agreement with their small radial velocity [12]. While a broad-band turbulent spectrum in the range 1-10kHz is observed in stellarators as in tokamaks [11,13], quasi-coherent modes with 10-50kHz and small poloidal wave-numbers (corresponding to a wavelength of approximately 30cm), propagating both in the ion and electron diamagnetic drift directions, are also observed in W7-X [11].…”

“…While boundary turbulence has been thoroughly characterized experimentally in tokamaks [7], similar investigations are still in their infancy in stellarators. In addition to the measurements carried out in small, low-temperature plasma stellarators such as TJ-K [8,9,10], experimental measurements in the edge of W7-X are recent [11,12,13]. These have shown significant differences with respect to tokamaks: field-aligned structures (dubbed filaments) are observed to be essentially bound to their flux surface, covering very small radial distances due to their slow radial velocity of the order of 100m/s [11,12], while their poloidal velocities are of the order of a few km/s [13].…”