The Mirnov oscillations observed in the ASDEX tokamak are analysed with regard to the generally accepted interpretation scheme according to which (1) Mirnov oscillations are caused by currents flowing parallel to the magnetic field on rational magnetic surfaces and (2) the field perturbation is frozen within the plasma. If the second statement holds, the frequency is obtained from the profiles of the electron density, the ion temperature and, if applicable, the toroidal or poloidal rotation velocity. It is shown that there are modes which are consistent with the above interpretation. On the other hand, mode coupling is observed. Mode coupling is also invoked to reconcile the experimental findings with the predictions of a theory based on statement (1); according to this theory, there is a poloidal variation of both the phase velocity and the amplitude of the Mirnov oscillations. While the observed phase velocity fits well into this picture, the poloidal variation of the amplitude cannot be ascribed to only one mode in the majority of cases. In addition, the possibility of coherent MHD activity due to currents in the scrape-off layer is discussed.

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Toroidal rotation velocity profiles during neutral beam injection on ASDEX have been investigated by means of charge exchange recombination spectroscopy. For both co- and counter-injection of neutral beams the ratio of the injected power and the line averaged electron density is found to be an essential experimental, parameter for the central rotation speed. With counter-injection, a continuous rise of rotation velocity and density is observed, indicating strong improvement of angular momentum confinement. This improvement is found to be strongly correlated with the peakedness of the density profile. Simple model calculations of momentum transport indicate that the increase of momentum confinement with density peaking can be partly attributed to the variation of torque deposition with the shape of the density profile.

A stationary discharge is produced bya current flowing parallel to the magnetic field ofa cylindrical coil. In the region where the field is homogeneous the pressure in the plasma column is much higher than that in the surrounding neutral gas. This is mainly caused by diamagnetic ring currents, as is shown by measuring the magnetic flux due to these currents. Two effects are primarily responsible for the ring currents in this region: The already known effect of the ambipolar diffusion across the magnetic field anda thermomagnetic effect, called NERNST effect, whose influence on the pressure build-up ofa plasma has not been investigated hitherto. Other phenomena causing ring currents occur in the plasma near the coil ends and outside the field coil.

In a cylindrical arc without external magnetic field the radius of the discharge current is identi-cal with that of the conductive plasma column. It is shown here that, in general, this is not the case in an arc with superposed magnetic field. Instead, the current is limited by the magnetic flux tube whose cross section is governed by that of the electrodes. This is still true if the radius of the conductive plasma column is much larger than that of the flux tube. In this case electromotive forces prevent radial expansion of the current carrying region and form a potential distribution in keeping with the current density distribution.