Characterization of the poloidal asymmetries in the ISTTOK edge plasma

Abstract

Edge plasma in/out and up/down asymmetries have been directly investigated on ISTTOK with a probe system that simultaneously samples the plasma at four poloidal angles. An experimental investigation of the asymmetries is presented as a function of the direction of plasma current and toroidal magnetic field. The edge plasma parameters show significant in/out asymmetries in both equilibrium and fluctuating parameters. Asymmetries favoring the outboard side are observed in density, turbulent particle flux and diffusion coefficient independently of the toroidal magnetic field and plasma current directions, suggesting a ballooning-like transport. This interpretation is supported by the characteristics of the fluctuations as they are found to be more intermittent on the outboard side. In addition, smaller up/down asymmetries reversing with the reversal of the toroidal magnetic field direction and favoring the ion ∇B drift direction are found. Significant poloidal asymmetries were also measured in the parallel flow that can be attributed mainly to Pfirsch-Schluter flows.

Intrinsic toroidal rotation in the scrape-off layer of tokamaks

“…More precisely, we assume that even in the case of ballooned transport, the portion of poloidal angle in which the plasma is radially transported by turbulence is a non-negligible fraction of 3608. We remark that this has been suggested by measurements obtained on the ISTTOK 44 and Tore Supra 45 tokamaks, where the portion of poloidal angle is found to be of about 1208 and 608, respectively. Thus we assume…”

“…Therefore, if turbulence in the SOL does not have a strong poloidal dependence, as it has been revealed experimentally 44,45 and by numerical simulations of ballooningmode driven turbulence, 47 also shown in Sec. IV, the continuity equation gives r k (nV ki ) $ const and thus a poloidal profile of V jji close to linear, as taken into account in Eq.…”

Characterization of edge turbulence in different states of divertor detachment using reflectometry in the ASDEX Upgrade tokamak

“…It was previously shown [17] that during complete detachment the radial velocity of the filaments increases, leading to broad density profiles at the LFS. As intermittent radial transport is poloidally asymmetric favoring the LFS [1][2][3][4][5][6][7][8][9][10], filaments will propagate along the field lines and they are expected to reach the far SOL at the HFS. Assuming a typical radial velocity of 300 m/s characteristic of the complete detachment state [17] and propagation at sound speed along the field lines, filaments should be detected in the HFS far SOL around ρ pol ≈ 1.1.…”

“…Observations from a large number of tokamak experiments provide clear evidence that the turbulence drive is ballooning-like, favoring the outboard plasma region [1][2][3][4][5][6][7][8][9][10]. However, most of the present knowledge of turbulence comes from measurements at the low-field-side (LFS) scrape-off layer (SOL), while data from the high-field-side (HFS) is scarce and needed because the evaluation of poloidal variations is critical to draw conclusions about turbulent transport [11][12][13].…”

ISTTOK real-time control assisted by electric probes

“…The ISTTOK electric probes are used to characterize the plasma poloidal asymmetries [10] and are also used for indirectly estimating the plasma position by measuring the floating potential in four poloidal angles. There are four sets of electric probes installed in ISTTOK similar to the probe depicted in fig.…”