Page 4 - Combination Doppler backscattering/cross-polarization scattering diagnostic for the C-2W field-reversed configuration

10H116-4 Schmitz et al. Rev. Sci. Instrum. 89, 10H116 (2018) FIG. 4. (a) Layout of the optical board for DBS/CPS beam optics; scalar microwave horns with HDPE lenses, and polarizers are used to launch/receive Ka band (26-60 GHz, two channels) and U-band (40-60 GHz, two channels) for monostatic detection of DBS returns. Separate lensed scalar horns are used to detect the Ka-band and U-band CPS return; (b) diagnostic layout on C-2W. out the backscattered X-mode cross-polarization signals from the DBS O-mode backscattering return signals. One additional lensed scalar Ka-band horn antenna and one scalar U-band horn antenna are provided to detect the cross-polarization signals. Figure 5 shows a schematic of the heterodyne microwave transceivers and signal processing electronics for the U-band DBS/CPS system (a similar system will be implemented for the Ka band). The output of two tunable hyper-abrupt varactor-tuned 10-15 GHz oscillators (HTOs) is quadrupled via active frequency multipliers to generate the launched 40-60 GHz probing signals and the difference frequency used for heterodyning. Both channels are combined via a direc- tional coupler to generate the launch signal fed to a scalar U-band horn antenna (typically arranged for O-mode launch). Monostatic detection is used for DBS (the backscattered sig- nal is detected via the same scalar horn and separated out via a directional coupler). Two double-balanced U-band mix- ers are used to generate intermediate frequency (IF) signals for both channels that are amplified by ∼25 dB (amplifiers are not shown here for simplicity) and detected via hybrid in-phase/quadrature (IQ) detectors. The DBS IF section is highlighted in orange in Fig. 5. The local oscillator (LO) input for the IQ detectors is generated via a third double- balanced mixer from the two launched frequencies. The use- ful frequency separation range of the two channels (the IF frequency range) is determined by the properties of the IQ hybrid detectors (typically 1.5-10 GHz here). An additional U-band scalar horn, typically oriented to receive X-mode radi- ation, is used to detect the cross-polarization signal backscat- tered predominantly from the plasma cut-off locations. The signal paths for the backscattered CPS return signals are shown in green. The CPS signals for both launched fre- quencies are separated out via heterodyning, using two addi- tional balanced mixers, with the LO input derived from the microwave source signals. After amplification (not shown here), IQ detectors with the same IF frequency range (typically FIG. 5. Schematic of U-band microwave transceiver, including IF sections, for the combined DBS and CPS diagnostics. The U-band launch antenna (used also for monostatic DBS detection) and the horn used to detect the cross-polarized backscattered signal are also shown here. CPS signal paths are shown in green.