Up-down asymmetries of density fluctuations in Tore Supra

Abstract

In Tore Supra, density fluctuations near to the plasma edge present up-down asymmetries which increase with the edge safety factor, weakly depend on density and reverse with the plasma current direction. The strongest asymmetries are observed in the particular case where the plasma is leaning on the lower limiter close to the measurement chord, with a connection length along the magnetic field line to the lower measurement region of the order of 0.6 m. For strong asymmetries, top and bottom k-spectra have different slopes. Possible theoretical explanations for these asymmetries are briefly discussed.

Non-neoclassical up/down asymmetry of impurity emission on Alcator C-Mod

“…This Figure 12: Time history demonstrating the prompt change in the up/down asymmetry due to a small change inn e is linked to the change in the direction of the core rotation 15 would rule out ballistic transport, indicating that the H-like argon ion has had time to collisionally equilibrate with the background plasma, and the radial transport is diffusive or convective. While measurement of any poloidal inhomogeneity in the turbulence was not possible for these CMod plasmas, up/down asymmetries in fluctuations have previously been observed in Ohmic plasmas on TEXT [21] and Tore-Supra [22]. Changing the current direction has been shown to reverse observed up/down asymmetries in n e fluctuation amplitude in laser scattering experiments on TEXT, and the fluctuation asymmetry is localized to the spatial region 0.8 < r/a < 1.0.…”

“…If this radial transport is not uniform on a flux surface then it will result in a poloidal asymmetry, qualitatively similar to how a poloidally varying limiter source has been shown influence the up/down asymmetry in low-Z emission [5] [20]. While measurements of such an up/down asymmetry of turbulence are currently not possible at Alcator C-Mod, observations have been made previously on TEXT [21] and Tore-Supra [22] that show not only an up/down asymmetry in n e fluctuations at large minor radii, but also show this phenomenon reverses sign, up/down goes to down/up, with the reversal of the direction of the current with respect to the toroidal field. This is consistent with present and previously reported observations [13] of the up/down asymmetry in Ar emission reversing with the ∇B drift direction since Alcator C-Mod must reverse both current and toroidal field directions to maintain the same helicity due to design of plasma facing components.…”

CO 2 laser based two-volume collective scattering instrument for spatially localized turbulence measurements

“…II D the maximum appears to be between 0.2 and 0. 35. If the correlation function ideally selected fluctuations only from the top or bottom of the plasma the amplitude of the correlation function would be 0.5.…”

“…2,4 A similar picture of the spatial distribution of fluctuations was also found in recent measurements at W7-AS using a Li-beam diagnostic. 20 An up-down asymmetry of the size observed in Tore Supra 35 has not been found in W7-AS.…”

Ion-temperature-gradient sensitivity of the hydrodynamic instability caused by shear in the magnetic-field-aligned plasma flow

“…6 These flows are deemed unstable [17][18][19] against the development of the well known hydrodynamic D'Angelo instability. 21 Schwander et al 19 showed that plasma might be unstable to the parallel-flow shear instability around limiters, as inferred from the experimental findings of Fenzi et al, 20 thereby explaining local enhancements of turbulence and showing that according to the local linear stability criterion, stability is sensitive to core parallel rotation. Their work speaks to the interest that would be given to future numerical modelling of experimentally relevant plasma conditions to assess both the properties of the transport coefficients associated with the parallel-flow shear-driven instability and the presence of freeenergy that would support turbulence that arises from that instability.…”