Abstract
Construction of the Linac Coherent Light Source II (LCLS-II) is underway for the world's first hard x-ray free-electron laser. A central part of the LCLS-II project is a 4 GeV superconducting radio frequency electron linac that will operate in the continuous wave (cw) mode. The linac is segmented into four sections named as $L0$, $L1$, $L2$, and $L3$. Two 3.9 GHz cryomodules, each housing of eight third-harmonic cavities similar to the cavities developed for the European X-ray Free Electron Laser (XFEL), will be used in section $L1$ of the linac for linearizing the longitudinal beam profile. In this paper, we present a study of trapped high order modes (HOMs) excited by a cw electron beam in the third-harmonic cavities of the LCLS-II linac. A detailed comparison of the original XFEL design and the LCLS-II design with a modified end group is performed in order to estimate the effect of a reduced beam pipe aperture on the efficiency of HOM damping. Furthermore, we apply a statistical analysis of the eigenmode spectrum for the estimation of the probability of resonant HOM losses and influence of HOMs on beam dynamics.
Highlights
A continuous wave operation regime of the 3.9 GHz Linac Coherent Light Source II (LCSL-II) accelerating structure at a maximum average beam current of 0.3 mA and ∼1 MHz bunch repetition rate might result in significant coherent rf losses in the form of trapped high order modes (HOMs) [1,2]
The initial rf design of the 3.9 GHz cavity was developed at Fermilab for the Deutsches Elektronen-Synchrotron FLASH facility, and later with minor changes it was adapted to the X-ray Free Electron Laser (XFEL) project [3,4,5]
Results of resonant HOM excitation in the ideal 3.9 GHz cavity are presented in Fig. 6 for the 0.3 mA average beam current
Summary
A continuous wave (cw) operation regime of the 3.9 GHz Linac Coherent Light Source II (LCSL-II) accelerating structure at a maximum average beam current of 0.3 mA and ∼1 MHz bunch repetition rate might result in significant coherent rf losses in the form of trapped high order modes (HOMs) [1,2]. Peak values of rf power radiated to the HOM couplers and estimate a cumulative probability of the transverse kick for a resonant HOM excitation in the LCLS-II thirdharmonic cavity.
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