Disconnection of scrape off layer turbulence between the outer midplane and divertor target plate in NSTX

Abstract

The connection between scrape off layer (SOL) turbulence at the outer midplane and at the divertor target plate is characterized in the National Spherical Torus Experiment (NSTX). Divertor fluctuations due to midplane blobs are studied by imaging their helical intersection with the divertor plate. Divertor fluctuation levels gradually decrease approaching the outer strike point. Divertor radial turbulence scale lengths as well as radial and poloidal turbulence propagation velocities are comparable to those on the low field side midplane. Strong correlation between divertor and midplane turbulence is observed in the far SOL. The correlation progressively decreases towards the separatrix, along with the decrease in the measured divertor fluctuations. The reduction in correlation is consistent with an electrostatic two-region blob model (Myra et al 2006 Phys. Plasmas 13 112502) with perpendicular ion polarization currents due to X-point geometry causing filament disconnection.

A new mechanism for filament disconnection at the X-point: poloidal shear in radial E × B velocity

“…A similar phenomena has been observed in turbulence simulations of the COMPASS [7] and WEST [8] tokamaks using the flux-driven turbulence code TOKAM3X [9]. In NSTX, the disconnection of the midplane filament from the divertor results, in the proximity of the strike point, in a decrease of the fluctuation level with respect to rest of the SOL [10]. Decorrelation of turbulence across the X-point region has also been observed in simulations with both the BOUT [11] and ArbiTER [12] codes.…”

“…A similar phenomena has been observed in turbulence simulations of the COMPASS [7] and WEST [8] tokamaks using the flux-driven turbulence code TOKAM3X [9]. In NSTX, the disconnection of the midplane filament from the divertor results, in the proximity of the strike point, in a decrease of the fluctuation level with respect to rest of the SOL [10]. Decorrelation of turbulence across Snapshot of a low-field side filament terminating in the vicinity of the X-point, as detected from the 3D blob recognition and tracking algorithm [8] in TOKAM3X simulation of the WEST tokamak, color coded with the normalized density fluctuation δn/n 0 .…”

On the origin of high harmonic fast wave edge losses in NSTX

“…which is parametrized by a single parameter γ, the so-called 'intermittency parameter'. Skewness-kurtosis plots of edge density measurements [45,59] give some idea of the typical values of this parameter: according to [49], the (skewness, kurtosis) points lie on the parabola (2/ √ γ, 3 + 6/γ). A more sophisticated approach, proposed in [43], involves approximating the probability density function for both R and r f a bivariate skew-normal distribution.…”

Cross-field and parallel dynamics of SOL filaments in TCV

“…Their cross-field shape is squeezed by the magnetic field shear around the X-point [38], and their can elongate into the divertor region, with a characteristic parallel fluctuation propagation velocity of the order of the electron thermal velocity [39]. A recent study [40] explored the parallel extension and connection between midplane and divertor plate in NSTX in attached plasma conditions ( f G = 0.2-0.3), finding that filaments, estimated to be in the C S regime, were electrically connected from the outer midplane to the divertor plate in the outer SOL. Towards the separatrix, consistently with a stronger magnetic shear [41,42], filaments were not found to be connected, as well as displaying a generally weaker connection when transiting into the RX regime.…”