Ion-Cyclotron Wave Excitation by Double-Resonance Coupling

A modulated high frequency wave is used to remotely excite low frequency oscillations in a linear, strongly magnetized plasma column. An electromagnetic wave is launched as an extraordinary mode across the plasma by an external waveguide in the upper-hybrid frequency regime f almost-equal-to f(UH) almost-equal-to f(ce) almost-equal-to 8 GHz, with P less-than-or-equal-to 2 W. By frequency modulating (at f(FM) almost-equal-to 1-60 kHz, with f(ci) congruent-to 30 kHz) the pump wave, the resonant layer is swept radially across the profile and perpendicularly to the field lines at f = f(FM). The resulting radial oscillation of the electron linear and nonlinear pressure can be considered to act as a source term for the ion wave. A localized virtual antenna is thereby created inside the plasma. Measurements of the ion dielectric response (interferograms and perturbed distribution functions) via laser-induced fluorescence identify the two branches (forward, or ion-acoustic-like, and backward, or Bernstein, modes) of the electrostatic dispersion relation in the ion cyclotron frequency range. By changing the modulation bandwidth, and thus the spatial excursion of the oscillating resonant layer, a control on the perpendicular wavelength of the excited mode can be exerted. In particular, the possibility of selective excitation of the ion Bernstein wave is demonstrated experimentally.