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Reduced particle losses have been observed during the operation of an m/n = 6/2 resonant magnetic perturbation in the TEXTOR tokamak. The influence of the perturbation field on multi-scale turbulence and turbulent transport has been surveyed in detail across the perturbation-induced reduction of edge transport. The results indicate that with magnetic perturbation (MP), both the large-scale zonal flows and small-scale drift-wave turbulence are significantly reduced. At high MP currents, a reduction of edge transport can be realized due to primarily the decline of small-scale ambient turbulence and turbulent transport in the ergodic zone, where the turbulence eddy size is largely decreased. Investigation on the dynamic process of fluctuation quantities during the perturbation current ramp-up phase further shows that geodesic acoustic mode zonal flows and their nonlinear interaction with background turbulence decrease incessantly with increasing perturbation current. The transport reduction takes place only after small-scale turbulence starts to be strongly reduced by the MP when the MP strength reaches a certain threshold value. [ABSTRACT FROM AUTHOR]

Gas puff imaging (GPI) [S. J. Zweben, D. P. Stotler et al., Phys. Plasmas 9, 1981 (2002); R. J. Maqueda, G. A. Wurden et al., Rev. Sci. Instrum. 74, 2020 (2003)] is a powerful diagnostic that permits a two-dimensional measurement of turbulence in the edge region of a fusion plasma and is based on the observation of the local emission of a neutral gas, actively puffed into the periphery of the plasma. The developed in-vessel GPI telescope observes the emission from the puffed gas along local (at the puff) magnetic field lines. The GPI telescope is specially designed to operate in severe TEXTOR conditions and can be treated as a prototype for the GPI systems on next generation machines. Also, the gas puff nozzle is designed to have a lower divergence of the gas flow than previous GPI diagnostics. The resulting images show poloidally and radially propagating structures, which are associated with plasma blobs. We demonstrate that the local gas puff does not disturb plasma properties. Our results indicate also that the neutral gas emission intensity is more sensitive to the electron density than the electron temperature. Here, we present implementation details of the GPI system on TEXTOR and discuss some design and diagnostic issues related to the development of GPI systems in general. [ABSTRACT FROM AUTHOR]

Abstract: We discuss the modeling of turbulence by means of a simulation code and compare these simulations to experimental results recently obtained in TEXTOR using a new Reynolds stress probe. The probe, reciprocating on a fast manipulator, allows to track poloidal and radial electric field and density fluctuations, which permits us to compute the (E×B)-velocity fluctuations as well as the Reynolds stress, the fluctuation driven particle fluxes and correlation functions. The simulations are based on the Hasegawa–Wakatani model, completed with curvature and neutral drag terms. We further investigate the potential role Reynolds stresses play in driving or damping the background flows, with the aid of a one dimensional fluid model in which the toroidal geometry is correctly taken into account, while various sources and sinks like viscosity, interaction with neutrals and Reynolds stresses are included. [Copyright &y& Elsevier]

Abstract: The radial profiles of electrostatic Reynolds stress and fluctuation-driven particle flux have been measured in the plasma boundary using a multi-array of fast reciprocating Langmuir probes during the static 6/2 and 3/1 mode dynamic ergodic divertor (DED) operation on TEXTOR. In the ohmic discharge phase before DED, a large radial gradient of Reynolds stress is observed around the flow shear region, suggesting the importance of turbulence-driven flows in the plasma edge. With DED, it is shown that the magnetic ergodization may suppress the Reynolds stress at the plasma boundary and thus rearrange the profile of poloidal momentum. Before DED, the local turbulent flux displays a reduction in the sheared flow layer. During the static DED phase, it is generally seen that the flux reverses direction from radially outwards to inwards in the ergodic zone. At the high DED current operation in the 3/1 configuration, a strong modification on the radial profile of the turbulent flux is observed. The results may have significant implications for the understanding of the electrostatic turbulence, fluctuation-induced flows and associated transport with magnetic stochastization. [Copyright &y& Elsevier]