Filamentary current structures in the Madison Symmetric Torus

Filamentary structures are observed during edge relaxation events associated with spontaneous enhanced confinement periods in the Madison Symmetric Torus reversed-field pinch (Dexter R.N., Kerst D.W., Lovell T.W., Prager S.C. and Sprott J.C. 1991 Fusion Technol. 19 131). The spatiotemporal shape of these structures is measured through extended toroidal and poloidal arrays of high-frequency magnetic probes at the plasma boundary. A simple model is used to interpret these structures as field-aligned current filaments, which are born at the reversal surface and propagate in the toroidal direction. The results found in MST share interesting commonalities with recent observations of filaments made during edge-localized modes in tokamaks and spherical tokamaks, as far as typical time scales, spatial localization and particle transport are concerned. Moreover, the dynamo effect produced by these events is estimated and compared with that produced during sawteeth. Though a single event has a small impact, the cumulative contribution of many of them produces a significant dynamo effect.