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

Journal Article · · Physical Review Accelerators and Beams
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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.

Research Organization:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Organization:
USDOE
Grant/Contract Number:
AC02-76SF00515
OSTI ID:
1558730
Alternate ID(s):
OSTI ID: 1562493
Journal Information:
Physical Review Accelerators and Beams, Journal Name: Physical Review Accelerators and Beams Vol. 22 Journal Issue: 8; ISSN 2469-9888
Publisher:
American Physical SocietyCopyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 15 works
Citation information provided by
Web of Science

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Cited By (1)

What are the advantages of ghost imaging? Multiplexing for x-ray and electron imaging journal January 2020

Figures / Tables (20)

FIG. 1(p. 2)figure FIG. 1
FIG. 2(p. 2)figure FIG. 2
FIG. 3(p. 3)figure FIG. 3
FIG. 4(p. 3)figure FIG. 4
FIG. 5(p. 4)figure FIG. 5
FIG. 6(p. 4)figure FIG. 6
TABLE I(p. 4)table TABLE I
FIG. 7(p. 5)figure FIG. 7
FIG. 8(p. 5)figure FIG. 8
FIG. 10(p. 6)figure FIG. 10
FIG. 9(p. 6)figure FIG. 9
FIG. 11(p. 7)figure FIG. 11
FIG. 12(p. 8)figure FIG. 12
FIG. 13(p. 9)figure FIG. 13
FIG. 14(p. 10)figure FIG. 14
FIG. 15(p. 11)figure FIG. 15
FIG. 16(p. 13)figure FIG. 16
TABLE II(p. 13)table TABLE II
FIG. 17(p. 14)figure FIG. 17
FIG. 18(p. 14)figure FIG. 18

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