Nonlinear excitation of Alfvénic waves by helicon modulation

Nonlinear wave coupling is known to be an important mechanism in space plasma dynamics, but often a close correlation between different wave types, as observed by various satellites, is the only indication for the coupling. The underlying mechanism, however, often remain unclear. This motivates well defined laboratory experiments which can contribute with their excellent diagnostic capabilities and reproducible conditions to a better understanding. Recently, low frequency waves have been observed in the linear plasma device VINETA if the radio frequency (rf) helicon plasma source is amplitude modulated. Propagating waves are detected with electrostatic and magnetic fluctuation diagnostics. The frequency of the waves is given by the difference of rf source frequency and the modulation sideband and the phase velocity agrees well with the local Alfven velocity. Wave dispersion measurements reveal a strong dependence on the plasma density and the ambient magnetic field strength, whereas a striking change from normal to anormal dispersion is found if the magnetic field is increased. This suggests, that the waves are generated by coupling between the helicon wave and a third wave propagating at the modulation sideband frequency. Since the waves are of Alfvenic character the ion kinetic response is of special interest. Phase resolved ion velocity distribution functions are measured with laser induced fluorescense (LIF) during wave propagation. Preliminary results of LIF measurements indicate an increase of the ion temperature at the ion cyclotron frequency and higher/lower harmonics, which suggests, that wave energy is resonantly absorbed by the ions