Excitation of the half-frequency of the input signal in the response of an antenna immersed in a plasma

The nonlinear response of a cylindrical monopole antenna immersed in a steady-state collisionless magnetic-field-free plasma and driven by a single-frequency RF source is investigated. The thickness of the ion sheath surrounding the antenna is controlled by a dc bias applied to the latter. It is found that a frequencies of the driving signal close to twice the frequency of a sheath wave resonance, the 1/2-subharmonic is excited, provided that the input power level is sufficiently high. The dependence of this process on the sheath thickness and on the electron plasma density is studied in the vicinity of several sheath wave resonances