ELECTROSTATIC TURBULENCE INTERMITTENCY DRIVEN BY MHD RELAXATION PHENOMENA IN A RFP PLASMA

It is generally accepted that electrostatic turbulence and the related anomalous transport play a major role in reducing the confinement in fusion devices [1,2]. The origin of electrostatic turbulence is currently not clear, although in some Reversed Field Pinch (RFP) experiments a possible non-linear coupling between low-frequency MHD activity and high frequency electrostatic fluctuations has been suggested [3] Recently new insight on the physics of electrostatic fluctuations has been gained from the analysis of the statistical properties of the signal. A detailed study of the floating potential [4] and magnetic fluctuations [5] at the edge of the RFX device has shown that these fluctuations show intermittency. In order to avoid possible confusion between bursty fluctuations and intermittency, according to Frisch[6], we define intermittency as a departure from pure self-similarity characterized by a change of Probability Distribution Function (PDF) of normalized fluctuations as the time scale is varied. If we introduce a scaling law for generic fluctuations h (where = (t+ )- (t) is the typical fluctuation of the signal at the scale ) a scale variation leads to a scaling law of the fluctuations a