CERN Accelerating science

 Wake field monitoring to improve FEL performance
 by Micha Dehler (PSI), Nicoleta-Ionela Baboi (DESY)

Down stream end of X band structure with wake field monitor pickup. Image credit: PSI

Detecting structure tilts in the spectral density of a WFM signal. Image credit: DESY

Beam degradation due to RF structure misalignments poses a challenge in the operation of ultra high brightness FELs. Wake field monitors offer a direct way to diagnose and correct these effects.

Misalignment between the beam and RF accelerating structures lead to transverse wake field excitation which degrades the beam quality. Wake field monitors (WFMs) are devices which couple these fields and their components, the Higher Order Modes (HOM), and therefore enable their reduction. This is of special interest in modern free electron lasers where the beam quality is of extreme concern.

Two teams from EuCARD2 WP12 are exploring this field, one developing a front end for the WFMs of a 12 GHz RF structure used for the SwissFEL and the other dealing with 1.3 and 3.9 GHz superconducting cavities of the European X-ray Free Electron Laser (E-XFEL).

The RF front end for the 12 GHz structure employs an innovative electro-optical down conversion scheme of the HOM fields, which is attractive in terms of band width, radiation hardness and the capability of transporting signals over kilometers. During first beam tests, by using a spectral analysis method, the system could identify the beam to structure offset and tilt. With the resolution improving to micron range, the device is expected to show even finer details such as structure bends or kinks.

Until recently, the HOM based system at FLASH was only used for beam alignment, but showed drifts over time as a beam position monitor. A way to stabilize the signals has been found based on a frequency domain analysis, such that a resolution of few microns was preserved over several months.  A new direct sampling approach for the E-XFEL 1.3 GHz cavities reduces the number of electronic components and therefore their  drifts with time. At the same time these are the first electronics to offer the possibility of monitoring the beam phase with respect to the RF pulse, important for optimization of the longitudinal beam properties.