Abstract: PSFC/JA-04-59

Study of intermittent small-scale turbulence in Wendelstein 7-AS plasmas during controlled confinement transitions

Basse, N.P., Zoletnik, S.¹ , Michelsen, P.K.² , W7-AS Team³

¹CAT-Science Bt., Detrekö u. 1/b, H-1022 Budapest , Hungary and Association EURATOM – KFKI-RMKI, H-1125 Budapest , Hungary
²Association EURATOM – Risø National Laboratory, DK-4000 Roskilde , Denmark
³Association EURATOM – Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany

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. Analysis using the former scheme has been reported upon previously [S. Zoletnik et al., Plasma Phys. Control. Fusion 44 , 1581 (2002)]. The electron temperature, along with the major spectral characteristics of magnetic and small-scale electron density fluctuations, changes dramatically at the transition from good to bad confinement. The fluctuation power is intermittent, and core bursts travelling in the electron diamagnetic drift (DD) direction are correlated between the bottom and top of the plasma, especially during degraded confinement. A corresponding top-bottom correlation for the edge ion DD direction turbulence feature was not found. Strong correlations are observed both between the two density fluctuation signals and between magnetic and density fluctuations in bad compared to good confinement. The correlation time of the bursts is of order 100 µs, similar to the lifetime observed during edge localized modes.

PACS numbers: 52.25.Gj, 52.25.Rv, 52.35.Ra, 52.55.Hc, 52.70.Kz