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

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

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

4 Common parameters for both schemes August 7, 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 pC -from Lanfa Wang, April pC 300 pC note: longitudinal dynamics not fully modelled

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

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, eV rms 0.12 eV rms 18  J 9 fs rms 25  J 16 fs rms

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

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, ×10 9 do not rely on beam splitter for the 2 seed pulses

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

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

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

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

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

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

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

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 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 August 7, 2014

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

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

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

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

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

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

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

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

More beam comparisons 26 August 7, pC 300 pC