Statistical features of edge turbulence in RFX-mod from gas puffing imaging

Abstract

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 al, Plasma Phys. Control. Fusion 47, L1 (2005)]. On the other hand, pulses with external methods for plasma control feature modified 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 PDFs is common to other devices as well, and has been suggested to be 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.

On the statistics of edge fluctuations: comparative study between various fusion devices

“…The need for a different modelling tool with respect to existing proposals was stimulated by the recognition in RFX-mod data of a novel feature, unnoticed in other experiments, i.e., the frequent presence of a double slope in the tail of the PDF [5]. We may now confirm that a similar feature is spotted also in other devices (Alcator C-Mod and TPE-RX).…”

“…This is the approach followed in the Labit paper as well as in several earlier studies [3,4,5,7]. The coefficients A, B, C may be compared with those directly computed from the guessed PDF.…”

“…We anticipate here that some differences with those works will be found and discussed. The last part of the paper will attempt to organize these novel features within a phenomenological model previously advanced [5].…”

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

“…17 in Sec. IV A), and the shapes of the density PDFs have been fit with various statistical models 66,156,162,182,187,190,200,201 (Sec. IV D).…”

Predicting PDF tails of flux in plasma sheath region

“…We model the sheath region with a general non-linear equation describing the time evolution of the variable φ for fluctuations with exponential interaction Ke cφ [7], [27]- [28],…”