Transverse diagnostics based on dipole mode signal fitting method in TESLA-type accelerating cavities at the free-electron laser FLASH

Abstract

TESLA cavities are used at the free-electron laser in Hamburg (FLASH) to accelerate electron bunches for generating intense free-electron laser beams. Two specially designed couplers mounted at both ends of each cavity damp the higher order modes (HOMs) excited by the beam in these cavities. By using a specific dipole mode signal extracted from these couplers, one can determine the transverse beam position as well as the dipole mode polarizations and center in each cavity. We introduce a new method based on fitting the dipole mode signal to correlate the dipole mode amplitude and phase to the measured beam position. With this method, the TESLA cavities can be used as HOM-based beam position monitors (HOMBPMs), delivering consistent results over several months, with a resolution better than 10 μm RMS. We implemented this method for a whole accelerating module for beam orbit monitoring. The method also delivers the polarization axes and the center of the dipole mode for all eight cavities in this module. Our measurements show that the cavities did not move inside the module within almost one year.

Submacropulse electron-beam dynamics correlated with higher-order modes in Tesla-type superconducting rf cavities

“…Since there were different filter boxes being tested, a patch panel was used to quickly switch between cavities and boxes. Unlike the downconverter electronics design with a narrow band filter on the TE111 mode used by others [12,13], the HOM detectors' bandpass filters were optimized for two dipole passbands from 1.6 to 1.9 GHz, and the 1.3-GHz fundamental was reduced with a notch filter.…”