Parametric excitation of ion quasi-mode by the pump near the ion cyclotron frequency

Parametric excitation of a nonresonant ion quasi-mode in a low density (i.e., ωpi≈Ωi) plasma is observed when the pump frequency is ω0≳2Ωi. From the interferometry measurement, the lower sideband is identified to be the cold lower hybrid wave and the low-frequency mode is shown to be a nonresonant ion quasi-mode. The dependence of the excitation process and the threshold upon density has been measured and was found to agree well with theory. The ion heating associated with this decay process is also observed. Such processes may take place near the plasma surface during ion cyclotron range of frequency heating experiments in tokamaks.