SOURCE OF MICROBUNCHING AT BNL NSLS SOURCE DEVELOPMENT LABORATORY
We report experimental studies of the origins of electron beam microbunching instability at BNL Source Development Laboratory (SDL). We eliminated laser-induced microbunching by utilizing an ultra-short photocathode laser. The measurements of the resulting electron beam led us to conclude that, at SDL, microbunching arising from shot noise is not amplified to any significant level. Our results demonstrated that the only source of microbunching instability at SDL is the longitudinal modulation of the photocathode laser pulse. Our work shows that assuring a longitudinally smoothed photocathode laser pulse allows mitigating microbunching instability at a typical FEL injector with a moderate microbunching gain. In this paper we investigated the source of microbunching instability at the SDL. To distinguish microbunching induced by shot noise from that arising from the longitudinal modulation of the photocathode laser, we studied the beam created by a very short laser pulse, thus eliminating the possibility of laser-induced microbunching. While the measured energy spectra of compressed beam did reveal severe longitudinal fragmentation, an analysis of the beam dynamics proved this to be due to self-fields acting on a beam with an initially smooth longitudinal profile, and not due to microbunching instability. Such fragmentation only was possible with the very short bunch chosen for these studies, and is absent in routine SDL operations. Our experiment shows that in the absence of the initial laser-induced beam modulation, microbunching instability at the SDL is not observed, and must be well below the levels that would limit the FEL performance. This result agrees with assumption of previous SDL studies that (when present under different machine conditions) microbunching instability at the SDL was laser-induced. Microbunching instability gain at the SDL is moderate. This is mainly because the SDL utilizes a single stage bunch compressor as well as due to the small compression ratio. Since the design of the SDL injector is typical of the majority of FEL injectors, our experiment proves that one possible way to control microbunching instability in such machines (that by design have a moderate microbunching gain) is to maintain a sufficiently smooth longitudinal profile of the photo-cathode laser. We note that the general principles for designing a machine with a moderate microbunching instability gain are presented in [12]. In conclusion, our experiment demonstrates that microbunching instability can be eliminated from a typical FEL injector with single stage bunch compressor (and operating without a laser heater) as long as the photocathode laser is longitudinally smooth. For machines with multi-stage bunch compressors, our results offer an important benchmark to establish a minimal laser heater power for instability-free operation.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- DOE - OFFICE OF SCIENCE
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 1016643
- Report Number(s):
- BNL-94944-2011-CP; 39KC02000; TRN: US1103087
- Resource Relation:
- Conference: Particle Accelerator Conference PAC11; New York, NY; 20110328 through 20110401
- Country of Publication:
- United States
- Language:
- English
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