Mode conversion of Alfven waves induced by quadrupole magnetic field modulation

A new type of the mode conversion of Alfven waves in the ion cyclotron range of frequency (ICRF) is presented, which is induced by the spatial modulation of a quadrupole magnetic filed. It is shown that the physical mechanism is analogous to a resonant parametric mode coupling; the coupling coefficient depends on the ellipticity of the cross section of flux surface and vanishes when the magnetic field is axisymmetric. Mode conversion of waves in a plasma is an important elementary process of plasma heating by waves. Recently a large number of theoretical studies on mode conversion have been done in the ion cyclotron range of frequency (ICRF) and in the electron cyclotron resonance heating (ECRH). Here, researchers present a new type of the mode conversion of Alfven waves in ICRF. They show here that one Alfven mode can convert to another Alfven mode due to the spatial modulation of a quadrupole magnetic field. They obtain coupled mode equations between two Alfven waves via the spatial quadrupole field modulation. If we suppose the quadrupole magnetic field modulation as a virtual mode, we can see that the conversion process is analogous to a resonant parametric mode coupling among two Alfven and the virtual modes. The solutions of the coupled mode equations show that the efficient mode conversion takes place when a resonant condition with respect to an axial wavenumber among these modes is satisfied. The authors study the mode conversion of Alfven waves propagating in a quadrupole magnetic field geometry. The basic equation to be analyzed, a Maxwell's wave equation, is given.