Modelling of plasma rotation accounting for a poloidal divertor and helical perturbation coils

Abstract

The impact of helical perturbations on the rotation velocity and thus on the energy confinement is calculated on the basis of the ambipolarity constraint, the parallel momentum equation of the revisited neoclassical theory and a simplified temperature equation. The helical perturbations can act as means for ergodizing the magnetic field and/or as momentum source or sinks, whereas at the separatrix (effective radius rs) of the poloidal divertor a temperature pedestal may arise due to the strong shear flow reducing the transport to a neoclassical level. The neoclassical theory allows the prediction of the parallel and poloidal flow speeds and thus of the ‘subneoclassical’ heat conductivity χsub used in the heat conduction equation. This heat conductivity allows us to compute the temperature pedestal and to reproduce the power balance in ALCATOR if one assumes that χ = χsub in the radial sheath with the thickness of Δ ≈ 0.7 cm, centred around the inflection radius rin, and χ = χL for r < rin − Δ/2. χL is the normal L-mode heat conductivity.Source terms account for momentum deposition by neutral beam injection (NBI), by pressure anisotropization and the force density, the latter two due to Fourier components of (rotating) helical fields. Source terms for the power deposition by NBI, Ohmic heating and radiation are also included.The main results can be summarized as follows:At a dynamic ergodic divertor in TEXTOR frequency of 10 kHz, a toroidal velocity gradient of 1.2 × 106 s−1 may be achieved which is enough to suppress the ion temperature gradient and thus to generate an ITB.The poloidal divertor suppresses the neutral gas influx and thus effects a (sub)neoclassical sheath with a temperature pedestal of Tped ≈ 400 eV and an increase of the central value by roughly the same amount. In the case of edge localized mode-control with an ergodic layer of Δ ≈ 2.5 cm, generated by the helical coils, the height of the pedestal stays unaffected if in the pedestal region a transition from L-mode confinement to subneoclassical confinement is assumed.