Turbulence in a toroidal magnetized plasma investigated by collective light scattering: plasma form factor and plasma diffusion
Résumé
On the toroidal magnetized plasma discharge ToriX, a collective light scattering device has been set to investigate plasma turbulence and transport. The light scattering intensity provides a measurement of the static form factor, at the scale of the scattering wave number k. The form factor is found to be very large, five to eight orders of magnitude above the equilibrium level. As a function of the k wave number, an exponential decay is found instead of a scaling law. This implies long range spatial correlation. The scattered light frequency spectral line shape is compared to the frequency Doppler transform (omega = k . v) of the plasma velocity in the observed volume. This is investigated by using both the classical Langmuir probes technique and the superheterodyne detection of the scattered light. The line shape is found to be mainly due to the non uniform convection velocity. To minimize the large scale convection velocity along the large radius, we added a vertical field to the horizontal toroidal B field. A significant decay of the form factor intensity is also observed. In this convection regulated regime, the scattered line profile modification as a function of k is investigated. It is interpreted as an effect of a brownian type of turbulent motion. According to this model, the signal auto-correlation function is expected to be the Ornstein function: C(t)=A exp{-k^2 D tc (t/tc -1 + exp(-t/tc))}, where tc is the correlation time of turbulent mouvement and D is the turbulent diffusion coefficient [1]. From this interpretation, the turbulent diffusion coefficient D across B is obtained for different plasma and observation conditions. As a function of the toroidal field intensity, D shows a characteristic Bohm behavior. These detailed investigations of the collective light scattering line profile validate the use of scattering device to get quantified measurements about plasma turbulent motion.