The reflection of an ordinary wave from its cut-off at the electron plasma frequency can be exploited to measure the density profile of laboratory plasmas. Because of the large shear characteristic of reversed field pinches, however, coupling between the ordinary and the extraordinary modes in the plasma can be important. Mode mixing is investigated by solving the full wave equation for nearly equatorial waves, with a finite element code using cubic Hermite interpolating functions. The results show that indeed a nonnegligible amount of power can be transferred from the launched O-mode to the unwanted X-mode of (vice-versa). Although stronger in small devices with large shear, the effect can also be substantial when the shear length is much greater than the wavelength, if the optical path is sufficiently long. The mixing coefficient is found to be an oscillating function of frequency, due to the excitation of standing waves between the plasma edge and the cut-offs. Nevertheless, it is argued that the implementation of reflectometry should still be possible in sufficiently large RFP plasmas.
A study of quasi-mode parametric excitations in lower-hybrid heating of tokamak plasmas