Nonlinear scattering dipole for electromagnetic field measurements in plasmas

Absolute electric field measurements of a 2 GHz electromagnetic wave in a nonuniform plasma are described. A remote measurement technique with minimum perturbations is used in which the information is obtained at the launching antenna from the backscattered signal of a small movable dipole in the radiation field. The weak scattering signal of the test dipole is discriminated against reflections from metallic walls and the plasma cutoff layer by introducing a small frequency shift (Δω/ω≃10−3) at the dipole. The dipole contains a passive nonlinear reactive element (varactor diode) which generates multiple sidebands when irradiated with two frequencies. The nonlinear behavior of the sidebands is used to obtain an absolute calibration for the electric field strength at the dipole. Mismatch effects due to the density dependent dipole radiation resistance are minimized by terminating the dipole with a high impedance.