Confinement regimes in simple magnetized toroidal plasmas*

In the simple magnetized torus TORPEX (R = 1 m, a = 0.2 m), we explore experimentally the accessibility of a high confinement mode, which is predicted by theory for this configuration [P. Ricci, et al., Phys. Rev. Lett. 1\textbf{00}, 225002 (2008)]. We consider different gases (H$_{2}$, D, He, Ne, N$_{2})$ and we measure the dependence of the temperature and density gradients upon the ratio $\gamma $/v$^{\mbox{'}}_{ExB}$ ($\gamma $ is the interchange linear growth rate and v$^{\mbox{'}}_{ExB}$ is the velocity shear). We observe that $\gamma $/v$^{\mbox{'}}_{ExB}$ decreases from $\gamma $/v$^{\mbox{'}}_{ExB}\sim $6-7 for hydrogen to $\gamma $/v$^{\mbox{'}}_{ExB}\sim $0.3-0.4 for neon. Consistently with theory, temperature and density profiles steepen when $\gamma $/v$^{\mbox{'}}_{ExB}$ drops below unity. The perpendicular particle flux is estimated by particle balance and measured using a multi-pin flux probe. First simulation results are presented with a global fluid code that solves the drift-reduced Braginskii equations in the whole TORPEX domain.