Accretion theory of 'spontaneous' rotation in toroidal plasmas

The accretion theory of 'spontaneous' toroidal rotation connects this phenomenon to the energy and particle transport properties of the plasma column and to the relevant collective modes. The consequent prediction that an inversion of the velocity direction in the transition from the H to the L regime should occur has been verified by the experiments. The theory is consistent also with the observation that the velocity is depressed when a peaked density profile appears as a result of a transport barrier. The fact that the rotation velocity increases with the total energy content is explained by the fact that the inflow of angular momentum, whose source is at the edge of the plasma column, results from the excitation of modes driven by the plasma pressure gradient. A quasi-linear derivation of the angular momentum transport produced by these modes, whose novel feature is the inflow, is given. A model of the relevant (transport) equation that can be easily solved is discussed. Fluctuations at the edge of the plasma column are considered responsible for the scattering out of confinement of particles that transfer to the surrounding material wall angular momentum in the same direction as that of the phase velocity of the prevalent modes. Thus the fact that the plasma rotates in the direction of the ion diamagnetic velocity in the H regime, when the prevalent modes are expected to have phase velocity in the direction of the electron diamagnetic velocity, can be explained. The rate of rotation decrease observed when the plasma current is increased is also justified. (author)