Correlation among plasma rotation, magnetic configurations and improved confinement regimes on the DIII-D Tokamak

A correlation has been established between the improvement of the energy confinement time observed in some plasma regimes on the DIII-D tokamak (VH modes and shear reversed discharges) and a geometrical characteristic of the plasma column: the Pfirsch-Schluter-like factor, which multiplies the moment of inertia of the magnetic configuration. Such a quantity is generated by the compression that the flux tubes suffer going from the external to the internal part of the torus. Therefore the configurations in which the module of the total magnetic field is more constant upon the magnetic surfaces (near omnigeneous configurations) show a lower value of the moment of inertia. The geometric parameter of Pfirsch-Schluter determines the transient and steady state behaviour of the plasma rotation under the assumption that the anomalous parallel viscosity is greater that the neoclassical one. In this way, also the profile of the part of the radial electric field (and his absolute value) is influenced by the magnetic configuration. The radial electric field, or, at least, his radial derivative, is invoked by many authors as a principal factor in reducing the turbulence (and so the anomalous transport) in magnetically confined plasmas. In particular, DIII-D machine, the highly elongated and triangular plasma discharges that evolve toward the VH-mode show a lower value of the Pfirsch-Schluter quantity and a higher level of the radial electric field; also the shear reversed profiles tend to lower 1+2q2 in the central region of the plasma column, driving towards very high values of the electric field within the reversal region