Turbulent transport is intermittent in nature. This has been observed in many experiments. Intermittency means that large transport events appear more frequently than one would expect from a Gaussian distribution of the fluctuations. In the plasma edge, the large events carry a considerable fraction of particle and energy transport.
Different explanations for the origin of intermittency have been put forward. An attractive idea is that the plasma state is under self-ordered criticality (SOC). SOC would also be consistent with the observation that temperature gradients are stiff as soon as they exceed a critical value.
The intermittency of turbulent transport has been investigated on TJ-K plasmas and DALF3 simulations. It is clear that both plasmas are not in the state of SOC. In order to get a scale separation, which is important to judge intermittency, transport data have been analysed by wavelet methods in frequency space and wavelet spectra and scale-dependent probability distribution functions (PDF) have been calculated (see figure). The PDF becomes more peaked at the shorter time scales. This is a clear sign of intermittency. Hence the intermittency in turbulent transport is not due to SOC but might be due to the fact that transport results out of two fluctuating quantities, the electric field and the density.
Head of Plasma Dynamics and Diagnostics / Library officer