On the probability distribution function of particle density and flux at the edge of fusion devices

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 (RFP) [Rostagni G 1995 Fusion Eng. Design 25 301], 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 result of the model is shown to fit fairly well empirical data. The PDF for the particle flux P(I) driven by electrostatic fluctuations is then computed using Pn(n) together with an ansatz for the PDF of electrostatic potential. The result is checked against experimental data and compared with previous formulations [Carreras B A et al 1996 Phys. Plasmas 3 2664].