Abstract
The microbunching instability (MBI) is a well-known problem for high brightness electron beams and has been observed at accelerator facilities around the world. Free-electron lasers (FELs) are particularly susceptible to MBI, which can distort the longitudinal phase space and increase the beam’s slice energy spread (SES). Past studies of MBI at the Linac Coherent Light Source (LCLS) relied on optical transition radiation to infer the existence of microbunching. With the development of the x-band transverse deflecting cavity (XTCAV), we can for the first time directly image the longitudinal phase space at the end of the accelerator and complete a comprehensive study of MBI, revealing both detailed MBI behavior as well as insights into mitigation schemes. The fine time resolution of the XTCAV also provides the first LCLS measurements of the final SES, a critical parameter for many advanced FEL schemes. Detailed MBI and SES measurements can aid in understanding MBI mechanisms, benchmarking simulation codes, and designing future high-brightness accelerators.
Highlights
The linac microbunching instability (MBI) is a persistent challenge for high brightness electron beams, and freeelectron lasers (FELs) in particular
With the development of the x-band transverse deflecting cavity (XTCAV), we can for the first time directly image the longitudinal phase space at the end of the accelerator and complete a comprehensive study of MBI, revealing both detailed MBI behavior as well as insights into mitigation schemes
While OTR diagnostics are more sensitive to low levels of microbunching and can probe shorter wavelengths, the XTCAV allows more complete, direct measurements of MBI
Summary
The linac microbunching instability (MBI) is a persistent challenge for high brightness electron beams, and freeelectron lasers (FELs) in particular. The high brightness beams, long accelerating sections, and strong dispersive regions at the Linac Coherent Light Source (LCLS) result in strong microbunching; the amplified density modulations can disrupt FEL operation both by producing intense radiation that disables diagnostics and by deforming the electron beam phase space, which directly degrades the FEL performance. Though typically small enough not to affect LCLS lasing, SES limits more advanced schemes such as high gain harmonic generation [26], harmonic lasing [27,28], longitudinal space charge amplifiers [29,30], and dispersive noise suppression [31,32]. We report LCLS’s first measurements of SES of the accelerated beam, as well as SES dependence on the laser heater and MBI
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