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We describe and demonstrate a two-volume collective scattering system for localized measurements of plasma turbulence. The finite crossfield correlation length of plasma turbulence combined with spatial variations in the magnetic field direction are used to obtain spatially localized turbulence measurements at the Wendelstein 7-AS fusion experiment. By(More)
In this paper measurements of short wavelength electron density fluctuations using collective scattering of infrared light are presented. The Wendelstein 7-AS (W7-AS) stellarator (Renner H et al 1989 Plasma Phys. Control. Fusion 31 1579) and the diagnostic are briefly described. A series of plasma discharges with reproducible confinement transitions was(More)
Spherical tokamaks (STs) provide a challenging regime for the study of turbulent transport in magnetised plasmas, providing an opportunity to test gyrokinetic and MHD fluid turbulence theory under extreme conditions. The strong field curvature at low aspect ratio has an important influence on micro-stability; both aspect ratio and flow-shear affect(More)
  • H Meyer, R J Akers, F Alladio, L C Appel, K B Axon, N Ben Ayed +104 others
  • 2008
Several improvements to the MAST plant and diagnostics have facilitated new studies advancing the physics basis for ITER and DEMO, as well as for future spherical tokamaks. Using the increased heating capabilities P NBI ≤ 3.8 MW H-mode at I p = 1.2 MA was accessed showing that the energy confinement on MAST scales more weakly with I p and more strongly with(More)
In this paper theory on collective scattering measurements of electron density fluctuations in fusion plasmas is revisited. We present the first full derivation of the expression for the photocurrent beginning at the basic scattering concepts. Thereafter we derive detailed expressions for the auto-and crosspower spectra obtained from measurements. These are(More)
At certain values of the edge rotational transform ι a , the confinement quality of plasmas in the Wendelstein 7-AS (W7-AS) stellarator is found to react very sensitively to small modifications of the edge rotational transform ι a. As ι a can be reproducibly changed either by external fields or by a small plasma current, these transitions offer a precise(More)
Beam emission spectroscopy (BES) measurements of ion-scale density fluctuations in the MAST tokamak are used to show that the turbulence correlation time, the drift time associated with ion temperature or density gradients, the particle (ion) streaming time along the magnetic field, and the magnetic drift time are consistently comparable, suggesting a(More)
  • N P Basse, S Zoletnik, P K Michelsen, W7-As Team
  • 2005
Confinement transitions in the Wendelstein 7-AS stellarator [H. Renner et al., Plasma Phys. Control. Fusion 31, 1579 (1989)] can be induced by varying either the internal plasma current or the external magnetic field. In this paper we report on experiments where closely matched confinement states (good and bad) were constructed using the latter method.(More)
  • G Pokol, G Por, S Zoletnik, N P Basse, W7-As Team
  • 2004
This work is based on the amplitude correlation analysis of the signals from a poloidal Mirnov-coil array on the Wendelstein 7-Advanced Stellarator (W7-AS). The motivation behind this work is an earlier finding, that changes in the RMS amplitude of Mirnov-coil signals are correlated with the amplitude of small scale density turbulence measured by CO2 Laser(More)
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