Intermittency in the scrape-off layer of the National Spherical Torus Experiment during H-mode confinement

Abstract

A gas puff imaging diagnostic is used in the National Spherical Tokamak Experiment [M. Ono, et al., Nucl. Fusion 40, 557 (2000)] to study the edge turbulence and intermittency present during H-mode discharges. In the case of low power Ohmic H-modes the suppression of turbulence/blobs is maintained through the duration of the (short lived) H-modes. Similar quiescent edges are seen during the early stages of H-modes created with the use of neutral beam injection. Nevertheless, as time progresses following the L-H transition, turbulence and blobs reappear although at a lower level than that typically seen during L-mode confinement. It is also seen that the time-averaged SOL emission profile broadens, as the power loss across the separatrix increases. These broad profiles are characterized by a large level of fluctuations and intermittent events.

Towards a quantitative prediction of the blob detection rate

“…A similar observation is reported for LHD [15] together with experimental evidence for a link between the waiting-time statistics and the instability drive (global β and edge pressure gradient) [16]. The detachment of blobs from coherent structures has even been visualized directly in basic plasma experiments like TJ-K [9] and the simple magnetized torus TORPEX [10], but also in fusion experiments like Alcator C-Mod [17], NSTX [18], and TEXTOR [19].…”

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

“…Figures 5-11 show measurements of the 2D structure of blobs in the plane perpendicular to B from several different devices using different diagnostics and different analysis techniques; namely: NSTX 128 and Alcator C-Mod 21 using GPI, DIII-D using beam emission spectroscopy (BES), 129 and LAPD, 130 VTF, 131 W7-AS, 119 and TORPEX 132 using Langmuir probes. The common feature of these measurements is a localized region of unusually high density typically $1-3 cm in size, which moves radially outward in this plane, typically at $0.5-2 km/s.…”

“…128 for NSTX. The number of blobs observed per unit time was higher in L-mode than in H-mode, but also increased with increasing power in H-mode, and decreased in low-powered Ohmic plasmas; however, a direct correlation of blob frequency with the measured shearing rate was not attempted.…”

Generalized self-similarity of intermittent plasma turbulence in space and laboratory plasmas

“…Intermittent transport (radial and poloidal) was observed using different diagnostics (see review Boedo et al. 2001; D’Ippolito, Myra & Zweben 2011; Maqueda, Stotler & Zweben 2011). The level of time-averaged flux originates not only from amplitudes of density and electric field fluctuations, but also a large contribution from the correlation between density and drift velocity (defined by electric field).…”