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? 2 August 7, 2014 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

3. Seeding schemes and layouts EEHG HGHG 3 August 7, 2014 UV seeds radiator mod1 mod2 UV seed fresh bunch delay mod1rad1 mod2 rad2 quadrupoles 15th harmonic (160 nm) demonstrated at NLCTA 65th harmonic (4 nm) demonstrated at FERMI@Elettra

4. 4 Common parameters for both schemes August 7, 2014 4 GeV beam energy ~ 1 kA peak current 260 nm external lasers final undulators -39 mm period, 3.4 m sections -  = 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

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 5 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 6 August 7, 2014 0.22 eV rms 0.12 eV rms 18  J 9 fs rms 25  J 16 fs rms

7. EEHG: 300 pC 7 August 7, 2014 power spectrum 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 2 extra undulator sections at end

8. longer pulse suppresses SASE only make first laser longer: same output pulse length also increase power of first laser? not worth the reduced power Suppressing SASE 8 August 7, 2014 1.5×10 9 do not rely on beam splitter for the 2 seed pulses

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(-t 4 ) 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 9 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 10 August 7, 2014

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

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

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 13 August 7, 2014

14. 100 pC beam properties 14 August 7, 2014 B mag =(        )/2 ≥ 1 measure of mismatch ~0.30 micron care about -50 fs to 30 fs current spikes can drive SASE in EEHG transverse offsets (not shown) of ~50 micron

15. 300 pC beam properties 15 August 7, 2014 B mag =(        )/2 ≥ 1 measure of mismatch ~0.43 micron care about -200 fs to 100 fs

16. Summary: Tradeoffs between EEHG and HGHG 16 August 7, 2014 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

17. 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 18 August 7, 2014 EEHG wavelengthEnergy spread at end of EEHG Energy spread at start of 1 nm 4 nm1.5 MeV6 MeV 2 nm1.8 MeV2.5 MeV 1 nm2.4 MeV

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, R 56 =11.0 mm, down from 14.4 mm 2nd chicane, R 56 =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 19 August 7, 2014 either choice yields ~100 microJ, pulse close to transform limit peak energy spread ~ 1.9 MeV

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

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

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 22 August 7, 2014

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

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

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

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