Experimental studies of toroidal momentum transport in ASDEX Upgrade

Characteristics of toroidal plasma rotation have been experimentally investigated using charge exchange recombination spectroscopy in the ASDEX Upgrade tokamak. Ion cyclotron resonance frequency (ICRF) heating is found to cause a reduction of the toroidal rotation velocity, V, driven by neutral beam injection (NBI) in the co- and counter-current directions. The reduction of plasma rotation is attributed to an increasing momentum diffusivity connected with the confinement degradation by the additional ICRF power flux, and not to an ICRF induced toroidal force related to radial non-ambipolar transport of resonant particles. Toroidal momentum transport is found to be anomalous in various plasma regimes including standard and improved H-modes and ion-internal transport barrier (ITB) plasmas. In the inner half of those plasmas, except for high density H-modes with on-axis NBI only, the momentum diffusivity, ?, is found to be similar to the ion and electron heat diffusivities, ?i and ?e. In the outer half region, ? becomes smaller than ?i, while ? is still comparable with ?e except for ITB plasmas. It is found that the normalized gradient length of the toroidal rotation velocity, is smaller than that of the ion temperature, , in H-modes and ITB plasmas. The magnitude of in an ITB region exceeds that in H-modes, as seen for the Ti profile. In H-modes, the Ti profile is stiff ( ), while the V profile is not stiff, with ranging from 0 to 7. The V profile tends to become flat at high densities with on-axis NBI only. Additional ICRF heating can lead to a small decrease in both and , while it sometimes causes a flattening of the V profile in the inner region. It is shown that the neoclassical correction of V does not affect strongly the results obtained with the measured V.