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Title: Experimental observations of seed growth and accompanying pedestal contamination in a self-seeded, soft x-ray free-electron laser

Abstract

Measurements of the soft x-ray, self-seeding spectrum at the Linac Coherent Light Source (LCLS) free-electron laser generally display a pedestal-like distribution around the central seeded wavelength. This pedestal limits the spectral purity and can negatively affect some user applications not employing a post-undulator monochromator. In this paper, we investigate the detailed experimental characteristics of both the amplified seed and its accompanying pedestal using data from a number of separate LCLS shifts over the 2015-2018 time period. We find that the amplified seed shows excellent wavelength stability and an exponential growth rate whose dependence upon energy detuning is consistent with theory. The pedestal’s spectral distribution and integrated strength vary strongly shot by shot, independent of electron beam energy jitter. Its shot-averaged strength relative to that of the seed grows at least linearly with z and can approach values of 15% or more. The pedestal is comprised of two separate components: (1) normal self-amplified spontaneous emission (SASE) whose total strength is nominally insensitive to energy detuning and laser heater (LH) strength; (2) sideband-like emission whose strength positively correlates with that of the amplified seed and negatively with energy detuning and LH strength. We believe this latter, non-SASE component arises from comparatively longmore » wavelength (i.e., λ~0.3–3 μm) amplitude and phase modulations of the main seeded radiation line. Its shot to shot variability and LH sensitivity suggests an origin connected to growth of the longitudinal microbunching instability on the electron beam.« less

Authors:
; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1558730
Alternate Identifier(s):
OSTI ID: 1562493
Grant/Contract Number:  
AC02-76SF00515
Resource Type:
Journal Article: Published Article
Journal Name:
Physical Review Accelerators and Beams
Additional Journal Information:
Journal Name: Physical Review Accelerators and Beams Journal Volume: 22 Journal Issue: 8; Journal ID: ISSN 2469-9888
Publisher:
American Physical Society
Country of Publication:
United States
Language:
English
Subject:
47 OTHER INSTRUMENTATION

Citation Formats

Marcus, Gabriel, Fawley, William M., Bohler, Dorian, Ding, Yuantao, Feng, Yiping, Hemsing, Erik, Huang, Zhirong, Krzywinski, Jacek, Lutman, Alberto, and Ratner, Daniel. Experimental observations of seed growth and accompanying pedestal contamination in a self-seeded, soft x-ray free-electron laser. United States: N. p., 2019. Web. doi:10.1103/PhysRevAccelBeams.22.080702.
Marcus, Gabriel, Fawley, William M., Bohler, Dorian, Ding, Yuantao, Feng, Yiping, Hemsing, Erik, Huang, Zhirong, Krzywinski, Jacek, Lutman, Alberto, & Ratner, Daniel. Experimental observations of seed growth and accompanying pedestal contamination in a self-seeded, soft x-ray free-electron laser. United States. doi:10.1103/PhysRevAccelBeams.22.080702.
Marcus, Gabriel, Fawley, William M., Bohler, Dorian, Ding, Yuantao, Feng, Yiping, Hemsing, Erik, Huang, Zhirong, Krzywinski, Jacek, Lutman, Alberto, and Ratner, Daniel. Fri . "Experimental observations of seed growth and accompanying pedestal contamination in a self-seeded, soft x-ray free-electron laser". United States. doi:10.1103/PhysRevAccelBeams.22.080702.
@article{osti_1558730,
title = {Experimental observations of seed growth and accompanying pedestal contamination in a self-seeded, soft x-ray free-electron laser},
author = {Marcus, Gabriel and Fawley, William M. and Bohler, Dorian and Ding, Yuantao and Feng, Yiping and Hemsing, Erik and Huang, Zhirong and Krzywinski, Jacek and Lutman, Alberto and Ratner, Daniel},
abstractNote = {Measurements of the soft x-ray, self-seeding spectrum at the Linac Coherent Light Source (LCLS) free-electron laser generally display a pedestal-like distribution around the central seeded wavelength. This pedestal limits the spectral purity and can negatively affect some user applications not employing a post-undulator monochromator. In this paper, we investigate the detailed experimental characteristics of both the amplified seed and its accompanying pedestal using data from a number of separate LCLS shifts over the 2015-2018 time period. We find that the amplified seed shows excellent wavelength stability and an exponential growth rate whose dependence upon energy detuning is consistent with theory. The pedestal’s spectral distribution and integrated strength vary strongly shot by shot, independent of electron beam energy jitter. Its shot-averaged strength relative to that of the seed grows at least linearly with z and can approach values of 15% or more. The pedestal is comprised of two separate components: (1) normal self-amplified spontaneous emission (SASE) whose total strength is nominally insensitive to energy detuning and laser heater (LH) strength; (2) sideband-like emission whose strength positively correlates with that of the amplified seed and negatively with energy detuning and LH strength. We believe this latter, non-SASE component arises from comparatively long wavelength (i.e., λ~0.3–3 μm) amplitude and phase modulations of the main seeded radiation line. Its shot to shot variability and LH sensitivity suggests an origin connected to growth of the longitudinal microbunching instability on the electron beam.},
doi = {10.1103/PhysRevAccelBeams.22.080702},
journal = {Physical Review Accelerators and Beams},
issn = {2469-9888},
number = 8,
volume = 22,
place = {United States},
year = {2019},
month = {8}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1103/PhysRevAccelBeams.22.080702

Citation Metrics:
Cited by: 3 works
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Figures / Tables:

FIG. 1 FIG. 1: Schematic of SXR self-seeding layout at LCLS (not to scale) adapted from Ref. [9]. The SXR self-seeding chicane and monochromator are at undulator segment position U9 while the hard x-ray self-seeding chicane and crystal-based monochromator (neither used in this experiment) are located at position U16.

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