Correlation among geodesic curvature of the magnetic field lines, plasma rotation and improved confinement regimes in present tokamak experiments

In toroidal magnetic confinement configurations all the charged particles align their magnetic drifts with the flux surfaces if, and only if, the lines of force have zero geodesic curvature (omnigeneous configuration). In this condition, the neoclassical transport is zero; unfortunately, standard tokamak devices are far from meeting such a condition. Nevertheless, a minimization of the geodesic curvature of the lines of force in tokamaks minimizes the neoclassical transport, and could as well reduce any other transport terms depending on the noncoincidence between magnetic and drift surfaces. In particular, the geodesic curvature of the lines of force generates both the neoclassical Pfirsch-Schlüter current density j_∥PS as well as Hirshman factor q̂ [Nucl. Fusion 18, 917 (1978)], which increases the moment of inertia of the magnetic configuration. The improved confinement observed in the VH-mode (very high confinement) and in reversed magnetic shear discharges of the DIII-D tokamak [Plasma Physics Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] and the high plasma performances of the Joint European Torus (JET) [Plasma Physics Controlled Nuclear Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, p. 27] can be correlated with the reduction of geodesic curvature. This reduction influences also the absolute value and the profile of the v∧B part of the radial electric field E_r, which are invoked by many authors as main ingredients in reducing the anomalous transport in tokamak plasmas.