Frequency-sweeping calibration source for the dual-frequency CTS radiometer at W7-X

The Collective Thomson Scattering (CTS) diagnostic at the Wendelstein 7-X stellarator must be calibrated. The current calibration procedure uses a hot-cold method, with one thermal source at room temperature and the other at the boiling point of nitrogen (77 K). The CTS receiver is dual-band: 138.5-141.5 GHz and 171-178 GHz, thus, the receiver is calibrated in both bands. Because the black body sources produce weak signals, they are averaged over hours to achieve an acceptable signal-to-noise ratio and discernible difference between the two signals. These time scales and the handling of liquid nitrogen make the calibration process cumbersome and difficult to make impromptu calibrations or changes to the receiver. In this contribution, we will discuss the design, characterization, and first results of a new sweeping calibration source that replaces the hot-cold sources. This calibrator consists of a tunable continuous wave GHz source and amplifier-multiplier chain with 70-140 GHz output, followed by a frequency doubler, attenuators, and waveguide switch. By comparing the calibrator-generated signal with the characteristic of the receiver, we are able to calibrate the receiver using one power level. We explore this calibrator's performance both as a relative and absolute calibrator and compare it with the hot-cold method. The calibrator is integrated with the CTS receiver and enables calibration on the order of minutes.