External Seeding Approaches: S2E studies for LCLS-II

1. External Seeding Approaches: S2E studies for LCLS-II Gregg Penn, LBNL CBP Erik Hemsing, SLAC August 7, 2014

2. Why seed with an external laser? • More timing control over x-ray pulse • timing defined by laser seed • easy to adjust pulse duration • Shot-to-shot stability • Possibly narrower spectrum, even transform-limited • Tailored x-ray pulses • such as frequency chirps or pulse shaping • Concerns: • limits repetition rate, reduced x-ray energy per pulse • especially compared to self-seeding • very large harmonic upshift from conventional lasers • commissioning may be a challenge at highest photon energies August 7, 2014

3. Seeding schemes and layouts • EEHG • HGHG radiator mod1 mod2 15th harmonic (160 nm) demonstrated at NLCTA UV seeds quadrupoles rad2 mod1 rad1 mod2 65th harmonic (4 nm) demonstrated at FERMI@Elettra fresh bunch delay UV seed August 7, 2014

4. Common parameters for both schemes • 4 GeV beam energy • ~ 1 kA peak current • 260 nm external lasers • final undulators • 39 mm period, 3.4 m sections • b= 15 m • output at 1 nm • most challenging part of tuning range Two S2E electron bunches • 100 pC • from Paul Emma, October 2013 • 300 pC • from Lanfa Wang, April 2014 100 pC 300 pC note: longitudinal dynamics not fully modelled August 7, 2014

5. EEHG configuration: 260 nm directly to 1 nm Compact beamline to reduce IBS Low magnetic fields to reduce ISR • first chicane ~9 m long, B < 0.5 T • second undulator has 0.4 m period, B < 0.4 T Need energy spread < 3 MeV when start to radiate at 1 nm • but large energy modulations reduce impact of IBS and ISR • pushing limits at ~2.3 MeV induced energy spread • SASE starts to compete with seeded pulse • unless blow up energy spread everywhere All these constraints are less severe for longer wavelengths August 7, 2014

6. EEHG seeding results from 260 nm to 1 nm • ~ 700 MW peak power at 1nm • from ~ 1 GW laser power at 260 nm • allows long, coherent pulses • highly sensitive to laser quality, less so to electron bunch • 300 pC bunch uses 2 extra undulator sections • Examples: better than 2 × transform limit 0.12 eV rms 18 mJ 9 fs rms 25 mJ 16 fs rms 0.22 eV rms August 7, 2014

7. EEHG: 300 pC 2 extra undulator sections at end spectrum power note SASE from tail 21 microJ • two seed lasers: • 100 fs FWHM • 50 MW and 900 MW peak power • 1.5 MeV and 3 MeV modulation August 7, 2014

8. Suppressing SASE do not rely on beam splitter for the 2 seed pulses • longer pulse suppresses SASE • only make first laser longer: • same output pulse length • also increase power of first laser? • not worth the reduced power 1.5×109 August 7, 2014

9. HGHG configuration: 260 nm to 13 nm to 1 nm • Real estate within the bunch is at a premium • need short pulse, short delay • Laser seed • 20 fs to 40 fs FWHM • short enough to require extra laser power • consider using a super-Gaussian profile ~ exp(-t4) • Fresh-bunch delay • 25 fs to 100 fs shift of radiation relative to e-beam • dispersion weak enough that bunching from first stage survives fresh-bunch delay August 7, 2014

10. HGHG seeding from 260 nm to 13 nm to 1 nm • two stage fresh-bunch, pushed to high harmonics • ~ 500 MW peak power at 1 nm • from ~ 800 MW at 260 nm • highly sensitive to electron bunch quality • Examples: consistently poor spectrum • performance is much better at 2 nm August 7, 2014

11. HGHG: 100 pC spectrum power used super-Gaussian profile flatter, still 20 fs FWHM messy spectrum August 7, 2014

12. HGHG: 300 pC spectrum power regular Gaussian 40 fs FWHM x-ray pulse is short could make longer, but spectrum will be worse August 7, 2014

13. Some of the challenges for HGHG • Sensitive to incoherent energy spread • smaller energy spread would make HGHG easier • even if peak current has to be reduced • Fresh bunch delay • different regions of the electron beam have to co-operate • beamline sensitive to longitudinal variations in bunch • Twiss parameters and transverse offsets • CSR has a big impact • limits duration of x-ray pulse, little room for timing jitter • super-Gaussian profile for input laser helps August 7, 2014

14. 100 pC beam properties care about -50 fs to 30 fs Bmag=(b0g-2a0a+g0b)/2 ≥ 1 measure of mismatch current spikes can drive SASE in EEHG transverse offsets (not shown) of ~50 micron ~0.30 micron August 7, 2014

15. 300 pC beam properties care about -200 fs to 100 fs Bmag=(b0g-2a0a+g0b)/2 ≥ 1 measure of mismatch ~0.43 micron August 7, 2014

16. Summary: Tradeoffs between EEHG and HGHG EEHG • allows moderate energy modulation • in practice, set by energy scattering • good prospects for long, coherent pulses • challenging laser requirements (stability and phase control) • will be studied further at NLCTA • not yet tested at high harmonics, short wavelengths HGHG with fresh bunch delay • demonstrated good results down to ~10 nm (FERMI@Elettra) • best for short pulses • fresh-bunch delay limits pulse duration • hard to control spectrum • below ~ 2 nm seems to be pushing the limits Consider other seeding schemes as well August 7, 2014

17. August 7, 2014

18. Alternative: staged approach to 1 nm • Start with smaller harmonic jumps initially • At 2 nm or 3 nm could switch to 1 nm near saturation • “afterburner” configuration • only retuning of final undulators is required • peak power at 1 nm < saturation • blow-up of energy spread is a concern • see table for EEHG, similar behavior for 3-stage HGHG August 7, 2014

19. EEHG to 2 nm, with optional jump to 1 nm after • changes: • 2nd laser power reduced to 400 MW (2 MeV modulation) • first chicane, R56=11.0 mm, down from 14.4 mm • 2nd chicane, R56=82.0 micron, up from from 53 micron choose either 6 undulator sections tuned to 2 nm, or 3 sections tuned to 2 nm plus 11 tuned to 1 nm peak energy spread ~ 1.9 MeV either choice yields ~100 microJ, pulse close to transform limit August 7, 2014

20. EEHG to 2 nm results • power at 2 nm and 1 nm spectrum at 1 nm transform limited August 7, 2014

21. HGHG to 1.9 nm, possible 0.9 nm afterburner not bad at ~ 1 nm but low pulse energy August 7, 2014

22. HGHG ending at 1.9 nm • if continue to amplify 1.9 nm pulse • 23 microJ pulse energy • spectrum better than at 1 nm August 7, 2014

23. Better spectrum earlier, but only ~ 4 microJ August 7, 2014

24. EEHG: 300 pC spectrum power note SASE from tail 10 microJ • two seed lasers: • 50 MW and 900 MW peak power • 100 fs FWHM • 1.5 MeV and 3 MeV modulation August 7, 2014

25. Spectrum for longer HGHG pulse at 1 nm August 7, 2014

26. More beam comparisons 100 pC 300 pC August 7, 2014