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Title: Microbunching-instability-induced sidebands in a seeded free-electron laser

Measurements of the multishot-averaged, soft x-ray, self-seeding spectrum at the LCLS free-electron laser often have a pedestal-like distribution around the seeded wavelength, which limits the spectral purity and can negatively affect some user applications not employing a post-undulator monochromator. In this paper, we study the origins of such pedestals, focusing on longitudinal phase space modulations produced by the microbunching instability upstream of the free-electron laser (FEL) undulator. Furthermore, we show from theory and numerical simulation that both energy and density modulations can induce sidebands in a high-gain, seeded FEL whose fractional strength typically grows as the square of the undulator length. The results place a tight constraint on the longitudinal phase space uniformity of the electron beam for a seeded FEL, possibly requiring the amplitude of long-wavelength modulations to be much smaller than the typical incoherent energy spread if the output sideband power is to remain only a couple percent or less of the amplified seed power.
Authors:
 [1] ;  [2] ;  [3] ;  [3] ;  [3] ;  [3] ;  [3] ;  [3]
  1. SLAC National Accelerator Lab., Menlo Park, CA (United States); Tsinghua Univ., Beijing (China)
  2. Argonne National Lab. (ANL), Argonne, IL (United States)
  3. SLAC National Accelerator Lab., Menlo Park, CA (United States)
Publication Date:
Report Number(s):
SLAC-PUB-16487
Journal ID: ISSN 2469-9888; PRABFM
Grant/Contract Number:
AC02-76SF00515; AC02-06CH11357
Type:
Published Article
Journal Name:
Physical Review Accelerators and Beams (Online)
Additional Journal Information:
Journal Name: Physical Review Accelerators and Beams (Online); Journal Volume: 19; Journal Issue: 5; Journal ID: ISSN 2469-9888
Publisher:
American Physical Society (APS)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; ACCPHY, XFEL
OSTI Identifier:
1250549
Alternate Identifier(s):
OSTI ID: 1240361