Narrow bandwidth active noise control for microphonics rejection in superconducting cavities at LCLS-II

LCLS-II is an X-Ray Free Electron Laser (XFEL) commissioned in 2022, being the first Continuous Wave (CW) hard XFEL in the world to come into operation. To accelerate the electron beam to an energy of $\SI{4}{\giga \eV}$, 280 TESLA type superconducting RF (SRF) cavities are used. A loaded quality factor ($Q_L$) of $4 \times 10^7$ is used to drive the cavities at a power level of a few kilowatts. For this $Q_L$, the RF cavity bandwidth is 32 Hz. Therefore, keeping the cavity resonance frequency within such bandwidth is imperative to avoid a significant increase in the required drive power. In superconducting accelerators, resonance frequency variations are produced by mechanical microphonic vibrations of the cavities. One source of microphonic noise is rotary machinery such as vacuum pumps or HVAC equipment. A possible method to reject these disturbances is to use Narrowband Active Noise Control (NANC) techniques. These techniques were already tested at DESY/CMTB and Cornell/CBETA. This proceeding presents the implementation of a NANC controller adapted to the LCLS-II Low Level RF (LLRF) control system. Tests showing the rejection of LCLS-II microphonic disturbances are also presented.