Optimal Frequency for Plasma Heating with a Single Electrostatic Wave
An experimental investigation of plasma heating by a single electrostatic wave (SEW) in a magnetized, rf-sustained plasma was carried out to establish the existence of an optimal wave frequency for maximized heating, which we recently predicted theoretically. A variable tuning network was constructed for electrostatic ion cyclotron wave launching, and laser-induced fluorescence measurements were made of ion heating and its dependence on SEW frequency over the tunable range of the second to the sixth harmonic of the ion cyclotron frequency. The measurements revealed a dependence of heating on frequency as predicted by the theory and the existence of an optimal frequency that is within one cyclotron harmonic from the theoretically predicted value. The findings establish a fundamental prescription for optimizing plasma heating by electrostatic waves, that is particularly useful for plasma propulsion where efficiency is of prime importance.