Turbulence reduction and poloidal shear steepening in reversed shear plasmas investigated by light scattering on Tore Supra

Abstract

Infrared light scattering is used to characterize turbulence during reversed shear scenario obtained by a fast current ramp-up on Tore Supra [Hoang et al., Phys. Rev. Lett. 84, 4593 (2000)]. The preformed hollow profile is freezed using ion cyclotron resonance minority heating (ICRH). Turbulence intensity is recorded to dramatically decrease during the ramp-up and to recover smoothly as the discharge is ended. Simultaneously, a confinement improvement of about 40% is achieved for about 2 s. Although the diagnostic performs a line average and the measurement is dominated by turbulence at the edge, we show that the core behavior can be deduced. Core turbulence decrease is in agreement with the effective heat diffusivity behavior. With the same optical device, the radial electric shear is deduced from the frequency spectra Doppler shift. Turbulence reduction is thus demonstrated to be correlated with the E×B shear steepening.

Characterization of core and edge turbulence in L- and enhanced Dα H-mode Alcator C-Mod plasmas

“…Localized small-angle scattering measurements have previously been used in conjunction with Eq. ͑14͒ to calculate turbulence profiles in both the Tore Supra tokamak 29,46,47 and the W7-AS stellarator. 26,35,48 To model EDA H-mode turbulence profiles we would add another term describing the position, width, and amplitude of the QC mode to the right-hand side of Eq.…”

Spatial distribution of turbulence in the Wendelstein 7-AS stellarator

“…In section 3, we present the discharges analysed. Section 4 contains a description of spatially localized measurements of fluctuations in the electron density of W7-AS plasmas using collective scattering of infrared light [2,3] and a comparison of these to an empirical model of the spatial distribution of density fluctuations [4,5]. Finally, in section 5 we relate the results to similar findings, both in W7-AS and the Tore Supra and DIII-D tokamaks.…”

“…We have in this paper presented an analysis where we arrived at density fluctuation profiles during two levels of confinement in the W7-AS stellarator. The procedure used is identical to the one used by the ALTAIR team at the Tore Supra tokamak to study differences between L-mode and reversed shear (RS) discharges [5,13].…”

“…We have seen that the same model can be used in Tore Supra and W7-AS to fit the measured data; however, this is not surprising in our case, since our number of data points is very small compared to the number of fit parameters. Nevertheless, a direct comparison of L-mode parameters stated in [5] and derived above yields From the above parameters it is quite difficult to make quantitative comparative remarks concerning the fluctuation profiles. Unfortunately, due to the installation of divertor modules in W7-AS (which severely limits the optical access), it is no longer possible to extend our measurement database.…”

Nonlocal k-space interaction and the rare existence of coherent structures in the tokamak edge plasma

“…In consequence, expression ͑1͒ can be re-written in the form 11,12 nЈ͑k ជ ,t ͒ϭ ͵ Ϫa ϩa dr n͑r,k,t ͒exp͑ Ϫ͑␣Ϫ͑r ͒͒ 2 k 2 w 2 /4͒, where ␣ is the angle that the analyzing wave vector makes with the poloidal plane and is the pitch angle. However, some localization is achieved because turbulent fluctuations in a tokamak are perpendicular to the magnetic field lines and the latter are twisted.…”