RF controls based on carrier suppression detection with attosecond resolution

In this paper, an RF control system with attosecond resolution based on carrier suppression interferometer operating a superconductive cavity at the Cryo Module Test Bench (CMTB) is presented. This novel application of the carrier suppression detector extends conventional heterodyne methods and improves the residual jitter of the regulated RF field in the cavity by more than one order of magnitude. The cavity was operated at 1.3 GHz frequency at a gradient of 8 MV/m and with a loaded quality factor of $\rm 10^7$. An out-of-loop phase noise detection floor of $\mathscr{L}=-180~dBc/Hz$ and outstanding time resolution of 189 as within an offset frequency range from 10 Hz to 1 MHz have been achieved. The phase noise budget of sub-components such as in-loop and out-of-loop detectors, high-power drive, microphonics and the reference source are reported. The facility RF reference and its phase noise in the offset frequency range from 1 kHz to 100 kHz is identified as the key noise contributor. Furthermore, above the closed-loop system bandwidth, due to filtering of the narrow band cavity the phase jitter contribution is further reduced to approximately 116 as only. The presented research using conventional receiver techniques with carrier suppression detectors in continuous-wave (CW) operation is an important milestone towards attosecond resolution pump-probe experiments at Free Electron Lasers.