Phase-locked heterodyne receiver and its application to lower hybrid wave detection by microwave scattering

The frequency stabilized, low-noise heterodyne receiver is developed for microwave scattering measurements in a plasma. Its application to the observation of a lower hybrid wave (LHW) externally excited in a linear test plasma is demonstrated. The frequency fluctuation of 70 GHz klystrons employed for scattering measurements is stabilized by the phase-locked loop. The frequency resolution of the receiver is about 10 kHz and its noise equivalent power is about 7×10−19 W/Hz, corresponding to the minimum detectable density fluctuation ñe/n̄e=6×10−5 under present experimental conditions. By the scattering of 70 GHz millimeter waves using this heterodyne receiver, LHW externally excited by the helical antenna can be detected. The spatial intensity distribution and the wave number spectrum of the scattered power are obtained. These observations make clear not only the magnitude, but also the direction of the wave number of the excited wave. The observed wave numbers in the several scattering positions over the plasma cross section are almost in the radial direction but not always consistent with the theoretical expectations. Their magnitudes are close to those determined from the auxiliary measurements using an electrostatic probe. The density fluctuation obtained from the absolute value of the scattered power is about ñe/n̄e=5×10−4. The theoretical estimation gives ñe/n̄e=2×10−3.