Plasma density fluctuations in the edge plasma of the RFX-mod device are measured through the gas puffing imaging diagnostics. Statistical features of the signal are quantified in terms of the probability distribution function (PDF), and computed for several kinds of discharges. The PDFs from discharges without particular control methods are found to be adequately described by a Gamma function, consistently with the recent results by Graves et al (J P Graves et al2005 Plasma Phys. Control. Fusion47 L1). On the other hand, pulses with external methods for the plasma control feature modified the PDFs. A first empirical analysis suggests that they may be interpolated through a linear combination of simple functions. An inspection of the literature shows that this kind of PDF is common to other devices as well and it has been suggested that it is due to the simultaneous presence of different mechanisms driving, respectively, coherent bursts and Gaussian background turbulence. An attempt is made to relate differences in the PDFs to plasma conditions such as the local shift of the plasma column. A simple phenomenological model to interpret the nature of the PDF and assign a meaning to its parameters is also developed.
Electrostatic turbulence in the edge of TPE-RX and driving mechanisms