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 a lign their magnetic drifts with the flux surfaces if, and only if, the line s of force have zero geodesic curvature (omnigeneous configuration). In thi s condition, the neoclassical transport is zero; unfortunately, standard to kamak devices are far from meeting such a condition. Nevertheless, a minimi zation of the geodesic curvature of the lines of force in tokamaks minimize s 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-Schluter current density j(parallel to PS), as wel l as Hirshman factor (q) over cap [Nucl. Fusion 18, 917 (1978)], which incr eases the moment of inertia of the magnetic configuration. The improved con finement observed in the VH-mode (very high confinement) and in reversed ma gnetic shear discharges of the DIII-D tokamak [Plasma Physics Controlled Nu clear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 198 7), Vol. I, p. 159] and the high plasma performances of the Joint European Torus (JET) [Plasma Physics Controlled Nuclear Fusion Research 1990 (Intern ational Atomic Energy Agency, Vienna, 1991), Vol. I, p. 27] can be correlat ed with the reduction of geodesic curvature. This reduction influences also the absolute value and the profile of the v boolean AND B part of the radi al electric field E-r, which are invoked by many authors as main ingredient s in reducing the anomalous transport in tokamak plasmas. (C) 1999 American Institute of Physics. [S1070-664X(99)02206-5].