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 $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}\text{ }$ 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.
Highlights
Higher order modes (HOMs) are used for a variety of diagnostics for the electron beam and the accelerating cavities [1,2,3]
The HOM-based beam position monitors (HOMBPMs) measurement based on the fitting method was implemented in the accelerating module ACC5 at free-electron laser in Hamburg (FLASH)
The signal measured from HOM2 on cavity three and HOM1 on the rest cavities was used for HOMBPM measurements
Summary
Higher order modes (HOMs) are used for a variety of diagnostics for the electron beam and the accelerating cavities [1,2,3]. While the usefulness of such a method has been shown before, we introduce a new method to be able to use dipole modes stably for operation We applied this method in the TeV-energy superconducting linear accelerator (TESLA) cavities at FLASH during several experiments. There are magnets between the BPM and the cavities, which make it difficult to infer the beam position in the cavities directly This information can be obtained from the dipole mode signal, which has a linear dependence on the beam offset with respect to the cavity axis. We developed a new method based on fitting of the dipole mode signal, and implemented it for all eight cavities in one accelerating module at FLASH.
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