The dynamic ergodic divertor in the TEXTOR tokamak:: plasma response to dynamic helical magnetic field perturbations

Recently, the dynamic ergodic divertor (DED) of TEXTOR has been studied in an m/n = 3/1 set-up which is characterized by a relatively deep penetration of the perturbation field. The perturbation field creates (a) a helical divertor, (b) an ergodic pattern and/or (c) excitation of tearing modes, depending on whether the DED current is static, rotating in the co-current direction or in the counter-current direction. Characteristic divertor properties such as the high recycling regime or enhanced shielding have been studied. A strong effect of the ergodization is spin up of the plasma rotation, possibly due to the electric field at the plasma edge. Tearing modes are excited in a rather reproducible way and their excitation threshold value, their motion and their reduction due to the ECRHIECCD have been studied. The different scenarios are characterized by strong modifications of the toroidal velocity profile and by a reduced or enhanced radial transport.