  The Probability Distribution Function (PDF) Pn(n) of the particle density at
the edge of several magnetic fusion devices, including tokamaks, stellarators,
and linear devices, is known to be strongly non-gaussian. In this paper we
present experimental results from RFX Reversed Field Pinch [G. Rostagni, Fus.
Eng. Design 25, 301 (1995)], confirming the universal shape of Pn also for
RFP's. An explanation for the form of Pn is attempted, on the basis of simple
conservation equations. The model result is shown to fitted fairly well
empirical data in a few different experimental scenarios.
