Abstract
We present a design of a superconducting rf photonic band gap (SRF PBG) accelerator cell with specially shaped rods in order to reduce peak surface magnetic fields and improve the effectiveness of the PBG structure for suppression of higher order modes (HOMs). The ability of PBG structures to suppress long-range wakefields is especially beneficial for superconducting electron accelerators for high power free-electron lasers (FELs), which are designed to provide high current continuous duty electron beams. Using PBG structures to reduce the prominent beam-breakup phenomena due to HOMs will allow significantly increased beam-breakup thresholds. As a result, there will be possibilities for increasing the operation frequency of SRF accelerators and for the development of novel compact high-current accelerator modules for the FELs.
Highlights
Modern high-power free-electron lasers (FELs) continue to place serious demands on driver accelerators, which are required to provide high current, continuous duty, electron beams with minimal degradation to transverse and longitudinal emittances [1]
In order for superconducting rf photonic band gap (SRF photonic band gap (PBG)) cavities to be fully competitive with more traditional Superconducting radio-frequency (SRF) coupler configurations for high current accelerator applications, they need to be capable of withstanding the gradients higher than 15 MV=m without compromising their efficiency with respect to higher-order modes (HOMs) damping
Temperature distribution on the surface of a 2.1 GHz SRF PBG resonator with round rods immersed into superfluid liquid helium at 2 kelvin when operating continuous wave (CW) at 10 MV=m accelerating gradient
Summary
Modern high-power free-electron lasers (FELs) continue to place serious demands on driver accelerators, which are required to provide high current, continuous duty, electron beams with minimal degradation to transverse and longitudinal emittances [1]. The spacings of the array and the diameters of rods are adjusted so that the frequencies of HOMs fall outside of the rejection band In this manner, all parasitic wakefields are not confined at the center and may be extracted at the periphery of the structure [17]. In order for SRF PBG cavities to be fully competitive with more traditional SRF coupler configurations for high current accelerator applications, they need to be capable of withstanding the gradients higher than 15 MV=m without compromising their efficiency with respect to HOM damping. The research that is reported here presents a novel design of an SRF PBG resonator with elliptically shaped rods, which should be able to maintain gradients 40 percent higher than previously tested resonators with round rods
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