Blob properties in L- and H-mode from gas-puff imaging in ASDEX upgrade

Abstract

Blob properties are studied in the scrape-off layer of the tokamak ASDEX Upgrade with a fast camera. The gas-puff imaging technique is used to investigate the detection rate as well as the blob size and velocity scaling. The experiments were performed in Land H-mode phases of the same discharges to study the change in blob properties after the L-H transition. In both regimes the detection rate is of the order of a few thousand blobs per second, which is compatible with the picture of blob generation by edge micro instabilities. The blob size increases in H-mode, while the radial velocity decreases slightly. The changes are, however, not indicating a drastic change in the blob dynamics in both phases. The experimentally found blob properties were compared to predictions from a novel blob model including effects due to a finite ion temperature, which should be more appropriate for the conditions in the SOL of fusion plasmas.

Invited Review Article: Gas puff imaging diagnostics of edge plasma turbulence in magnetic fusion devices

“…On the newer ASDEX-Upgrade tokamak in Germany, a Phantom camera was used to measure 2-D images of the turbulence, and a synthetic diagnostic of field-aligned filament geometry used for interpretation of GPI data. 47 The GPI results were similar to those made with a Li-BES diagnostic. 86 The Experimental Advanced Superconducting Tokamak (EAST) in China has installed a dual GPI system with two views located ±50 • poloidally with respect to the outer midplane, in order to evaluate the up/down symmetry of the turbulence.…”

“…115 Both GPI and Li-BES were used to evaluate the properties of discrete blobs in the SOL of ASDEX-Upgrade in separate papers. 47,86 The results of the two diagnostics appear to be very similar, e.g., with radial blob widths of ∼1 cm and radial blob speeds of ∼0.3 km/s; however, it is difficult to make a direct comparison since these measurements were made at different locations and on different discharges.…”

“…Figure 5 shows a time sequence of GPI images from the ASDEX Upgrade tokamak created using a 2-D conditional averaging (CA) technique based on the normalized light intensity fluctuations. 47 The GPI data were taken again with a deuterium gas puff and Balmer-α filter, here with 8 µs between frames during an inter-ELM period in an H-mode plasma, and over a region of ∼5 cm × 5 cm just below the outer midplane. The reference pixel is the small "x," the threshold for CA is two times the standard deviation, and 556 trigger events were averaged to produce these averaged images.…”

Poloidal asymmetric flow and current relaxation of ballooned transport during I-phase in ASDEX Upgrade

“…Blobs are associated with the interchange or resistive ballooning instability. Using a fast camera in AUG [25], the average blob detection rate was found to be a function of the phase of the magnetic fluctuations (measured below the inner divertor target (C09-23)). The blob detection rate is clearly correlated with the frequency of the magnetic fluctuations (Fig.…”

Origin and turbulence spreading of plasma blobs

“…The GEMR model includes, besides the standard interchange physics in tokamak geometry at the transition from closed to open field lines, also the drift-wave and the conducting wall instabilities. In sum, it includes most effects which are believed to play an important role for the plasma blob dynamics which are three-dimensional, 28 electromagnetic, 29 finite ion temperature or finite Larmor radius [30][31][32][33][34] and shear flow (amongst others zonal flow and geodesic acoustic mode) effects. 4,11,12 …”