Transcript penn_external_seeding

External Seeding Approaches:
S2E studies for LCLS-II
Gregg Penn, LBNL CBP
Erik Hemsing, SLAC
August 7, 2014
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
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Seeding schemes and layouts
EEHG
mod1
radiator
mod2
15th harmonic (160 nm)
demonstrated at NLCTA
UV
seeds
quadrupoles
HGHG
mod1
UV
seed
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rad1
mod2
fresh
bunch
delay
rad2
65th harmonic (4 nm)
demonstrated at
FERMI@Elettra
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Common parameters for both schemes
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4 GeV beam energy
~ 1 kA peak current
260 nm external lasers
final undulators
100 pC
- 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
300 pC
- from Paul Emma, October 2013
• 300 pC
- from Lanfa Wang, April 2014
note: longitudinal dynamics not fully modelled
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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
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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
25 mJ
16 fs rms
18 mJ
9 fs rms
0.12 eV rms
0.22 eV rms
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2 extra undulator sections at end
EEHG: 300 pC
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
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Suppressing SASE
longer pulse suppresses SASE
do not rely on beam splitter
for the 2 seed pulses
1.5×109
only make first laser longer:
• same output pulse length
also increase power of first laser?
• not worth the reduced power
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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
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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
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HGHG: 100 pC
power
spectrum
used super-Gaussian profile
flatter, still 20 fs FWHM
messy spectrum
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HGHG: 300 pC
power
spectrum
regular Gaussian
40 fs FWHM
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x-ray pulse is short
could make longer,
but spectrum will be worse
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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
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100 pC beam properties
care about
-50 fs to 30 fs
Bmag=(b0g-2a0a+g0b)/2 ≥ 1
measure of mismatch
~0.30 micron
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current spikes can drive
SASE in EEHG
transverse offsets (not shown)
of ~50 micron
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300 pC beam properties
care about
-200 fs to 100 fs
Bmag=(b0g-2a0a+g0b)/2 ≥ 1
measure of mismatch
~0.43 micron
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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
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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
EEHG wavelength
Energy spread
at end of EEHG
Energy spread
at start of 1 nm
4 nm
1.5 MeV
6 MeV
2 nm
1.8 MeV
2.5 MeV
1 nm
2.4 MeV
2.4 MeV
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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
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either choice yields
~100 microJ,
pulse close to
transform limit
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EEHG to 2 nm results
power at 2 nm and 1 nm
spectrum at 1 nm
transform
limited
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HGHG to 1.9 nm, possible 0.9 nm afterburner
not bad at ~ 1 nm
but low pulse energy
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HGHG ending at 1.9 nm
if continue to amplify 1.9 nm pulse
23 microJ pulse energy
spectrum better than at 1 nm
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Better spectrum earlier, but only ~ 4 microJ
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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
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Spectrum for longer HGHG pulse at 1 nm
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More beam comparisons
100 pC
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300 pC
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