Plasma diffusion across a magnetic field observed by collective light scattering: Experimental evidence for Lévy stable distributions

On a toroidal magnetized plasma discharge, ToriX, a collective light-scattering device has been set to investigate plasma turbulence and transport. The scattering device output signal is proportional to the space Fourier transform of the plasma density. The two-dimensional static form factor, at the scale of the scattering wave number k, is measured in absolute units as a function of k. It shows that fluctuations are large and mainly situated at large scales. By adding a small vertical component to the main toroidal magnetic field, a uniform plasma has been achieved and the form factor is significantly reduced. The time autocorrelation function of the scattered signal as a function of k is shown to be the Fourier transform of the displacement probability distribution. It is investigated at different times as a function of k and found to be consistent with a Levy walk model with a characteristic exponent alpha closed to 1. The displacement probability distribution is thus close to a Lorentz function instead of a Gaussian. (C) 2005 American Institute of Physics.