H-mode filament studies with reflectometry in ASDEX upgrade

Broadband swept (i.e. frequency modulation of the continuous wave; FM-CW) and fixed frequency reflectometry (FFR) were used for the first time to study plasma filamentary activity; experiments were performed in ELMy H-mode plasmas at the ASDEX Upgrade tokamak. Electron density profiles were studied with FM-CW providing a first insight into filamentary activity and enabling us to localize the density layers probed with FFR. A novel filament detection technique was developed using a threshold criterion on the phase derivative signals from FFR. This technique was applied together with conditional averaging in measurements performed in the vicinity of the separatrix. Results showed good agreement with data from Langmuir probes and it was found that the majority of filaments propagate with dominant poloidal velocity in both periods of in-between edge localized modes (ELMs) (V-theta approximate to 575 m s(-1)) and at the ELM onset (V-theta approximate to 1180 m s(-1)). A time delay between the maximum filament activity at the outer mid-plane and the ELM peak at the inner divertor currents (approximate to -461 +/- 50 mu s) agrees with expected time scales for the ELM lifetime. In inter-ELM periods we were able to estimate typical poloidal and toroidal sizes of filaments (S-theta approximate to [5.75-11.50] cm and S-phi approximate to [33-66] cm) and a magnetohydrodynamic mode structure emerged from the measurements with poloidal and toroidal mode numbers (m approximate to [8-12] and n approximate to [2, 3]) in the range of possible peeling-ballooning modes.