Momentum transport studies on ASDEX

It has been reported from a number of tokamaks, that the global confinement times of energy and angular momentum show similar magnitudes and scalings with plasma conditions. Since the classical perpendicular viscosity is by far too low to explain the observed rotation damping, an anomalous transport mechanism is believed to cause the radial transport of momentum, which is a feature of the ions alone owing to the low mass of the electrons. The correlation of momentum and energy confinement can be taken as a hint for the thermal conduction and momentum diffusion sharing a common transport mechanism. There is also some evidence from theory supporting this assumption: E.g., the ion-temperature-gradient driven class of instabilities produces equal ion heat and momentum diffusivities (χi=χφ). Therefore, the investigation of momentum transport can be of special interest for the conditions of medium- or high-density discharges, where the ion- and electron heat transport channels often cannot be separated. We have analyzed rotation velocity profiles during steady-state conditions of about 50 neutral beam heated ASDEX discharges with different experimental parameters. To give an overview to the database, momentum confinement times τφ are plotted as a function of τE in Fig. 1. τφ and τE are equal within a factor of 2 through the whole parameter range with co-injection. There is a slight tendency for τφ being smaller than τE in the lower confinement branch. During ctr.-NI, the momentum confinement significantly improves over the energy confinement. This improvement has been found to correlate with the peaking of the density profile. For H-mode discharges, the improvement of the momentum confinement equals that of the energy confinement, resulting in data points to appear near the diagonal. In the following, we want to discuss the momentum transport for co-NI, L-mode discharges where a sufficient parameter variation for the derivation of scaling laws has been carried out. (author) 5 refs., 4 figs