Modelling of plasma rotation accounting for neutral beam injection and perturbation coils in JET and TEXTOR

Abstract

The impact of the helical perturbations, which can act as a momentum source or sink, on the rotation velocity is calculated on the basis of the ambipolarity constraint and the parallel momentum equation of the revisited neoclassical theory; this theory allows prediction of the parallel and poloidal flow speeds, v∥, and vΘ, respectively, and therefore the radial electric field Er via the usual radial momentum balance equation.Source terms account for the momentum deposition by neutral beam injection, pressure anisotropization and the force density, the latter two due to Fourier components of (rotating) helical fields.However, the neoclassical theory cannot account for the effect of the electrostatic turbulence on rotation in, e.g. TEXTOR L-modes. This is included by replacing the neoclassical viscosity η2 by an anomalous one due to turbulence. The main results can be summarized as follows.Using in the case of JET the data of shot #59316 the maximum rotation speed can be reproduced with an accuracy of 10%. A similar result is obtained for the TEXTOR shot #91269.If the angular velocity of the (m = 2, n = 1) Fourier component of the helical field is at low slip frequencies ωp − ωf (ωp is the plasma rotation frequency and ωf the rotation frequency of the helical field) gradually reduced to zero, a localized minimum appears and the gradient of the toroidal velocity becomes around 4 × 105 s−1 (JET).However, if the slip frequency is larger than a critical value, the rotation profile of the rotation velocity is not influenced as observed at JET.Although it is possible to create a large velocity shear around the singular surface, this shear is nonetheless limited by the reduction of the central velocity. Therefore, it might not be possible to trigger an ITB by plasma braking at the singular surface.