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An Office of Science User Facility

LCLS will offer self-seeded mode in Run 7

    General LCLS

Thursday, June 07, 2012

LCLS will offer self-seeded mode in Run 7
Since January 2012, LCLS has been testing the use of a diamond crystal monochromator located part way down the FEL undulator to produce self-seeded pulses of hard X-rays. The tests have successfully demonstrated seeding of very short FEL pulses, though R&D is continuing. It is anticipated that self-seeded operation may be advantageous to some user experiments, and that by the beginning of Run 7 LCLS will be in a position to reliably offer self-seeded operation to users in an exploritory mode.

What does self-seeding offer?
Self-seeding will be useful for some experiments, but not for all. In general, experiments that use the full bandwidth of the SASE FEL pulse can expect better machine reliability and significantly greater energy per pulse by continuing to operate in SASE (unseeded) mode. Also, seeded operation is not yet available for soft X-ray experiments. However, hard X-ray experiments (7.1-9.5 keV) that require the use of a crystal monochromator can expect increased energy per pulse in self-seeded mode.

Other restrictions
Currently, the self-seeded mode cannot be used with focused X-ray beams, due to concerns about beam containment. LCLS is working to address this issue – please contact the appropriate instrument scientist for updates about the use of focused seeded beams in a given experimental station.

Typical parameters for the self-seeded pulses are:

Photon energy 7.1-9.5 keV
Fractional bandwidth 1 x 10-4 (30 times smaller than the SASE bandwidth)
Maximum pulse energy about 0.5 mJ (0.2 mJ typical)
Average pulse energy 0.1 mJ
Shot-shot energy fluctuations 100%
Pulse duration < 50 fs
Expected tune-up time 2-4 hrs



SLAC SLAC National Accelerator Laboratory, Menlo Park, CA
Operated by Stanford University for the U.S. Dept. of Energy