The Influence of the Dynamic Ergodic Divertor on the Radial Electric Field at the Tokamak TEXTOR

In this work the influence of external Resonant Magnetic Perturbations (RMPs) on the radial electric field Er in magnetically confined plasmas is investigated by Charge Exchange Recombination Spectroscopy (CXRS) at the Tokamak TEXTOR. For this purpose, an optical diagnostic was developed to perform CXRS with C6+ ions utilizing a neutral hydrogen diagnostic beam. Two systems are used to measure the poloidal rotation as well as the ion pressure and the toroidal rotation, respectively. The setup allows measurements with 1-2 cm radial resolution and a velocity resolution of typically 1 km/s for vθ and 5 km/s for vφ. For the radial calibration active beam emission spectra (Doppler shift) are used. The wavelength reference is determined via differential Doppler shift spectroscopy and a neon reference line for the poloidal and toroidal system, respectively. The radial electric field, Er, is then deduced from the radial force balance of C6+ ions. The experiments have been performed at the Tokamak TEXTOR. Here, the RMPs are produced with the Dynamic Ergodic Divertor (DED), a set of 16 helical perturbation coils located at the high field side of TEXTOR. Within this work, the base mode number of perturbations has been m/n = 6/2. With these new observation systems, we have first investigated the influence of external torque from neutral heating beams on plasma rotation and Er. The toroidal rotation in the center is proportional to the net beam torque with an offset counter-current rotation at zero net torque. At the plasma edge, the toroidal rotation is dampened by friction with neutrals and coupled to the counter-current Scrape-Off layer flow by perpendicular viscosity. The poloidal rotation follows the poloidal component of the beam torque in the plasma center (ranging from 3± 3 km/s), at the plasma edge it is dominated by drift effects and loss of fast particles of the counter beam. Under influence of the DED, a spin-up of rotation with the rise of the perturbation amplitude causes the Er to become more positive. An increase of +10 kV/m is observed. The toroidal contribution increases all across the minor radius because of the large perpendicular viscosity, while the poloidal contribution increases only locally inside the ergodic zone caused by the RMPs. The ergodic zone causes an electron loss, and subsequently a j × B force driven by the compensating ion return current. In addition, the DED changes the global confinement properties. Depending on the edge safety factor (”field line twist”) qa, either increased or decreased particle confinement is observed. In case of the increased particle confinement (IPC) the increase in density (40%) and particle confinement time τp (30%) is correlated to the connection of field lines at the q=5/2 surface to the DED target, locally changing the transport properties and the Er. Transport is reduced and the Er shear is increased locally at q=5/2 up to 1.5 · 105s−1, while the Er becomes more positive. The changes in magnetic topology calculated in the vacuum approximation for the radial perturbation field can be related to the experimental particle flux pattern at the DED target. This situation is characterized by field lines connecting the ergodized x-points of the 5/2 island chain to the target. With further increasing perturbation, the ergodic zone widens, the particle confinement increases, up to the point where the q=5/2 surface is completely ergodized and the particle confinement decreases again. Under conditions, where the q=5/2 surface is closer to the perturbation coils (smaller edge safety factor), the particle confinement is observed to decrease (particle Pump Out PO), local field line connections to the target are not established and enhanced transport within laminar flux tubes with short connection to the target directly leads to the loss in global confinement.Enhanced shear in Er is not observed for this case. Increased Er shear and increased particle confinement are correlated.They probably are connected to a reduction of density fluctuations as was observed in discharges under the influence of DED in m/n = 3/1 base mode configuration.