1. External Seeding Approaches for Next Generation Free Electron Lasers
Erik Hemsing
SLAC
Greg Penn
LBNL

2. Improving the temporal coherence in the free electron laser (FEL)
The Need to Seed
FEL output power
FEL power
FEL spectrum
SASE FEL
Improving the temporal coherence in the free electron laser (FEL)
e-beam energy vs time space (many slices)

3. The effect of coherent seeding
FEL output power
FEL power
FEL spectrum
SASE FEL
Dream FEL
Seeding would make an FEL an extraordinarily good laser
e-beam energy vs time space (many slices)

4. Seeding Methods
Ultimate goal: Seeding to generate transform limited x-ray pulses
Several seeding approaches:
High Harmonic Generation (HHG)
High Gain Harmonic Generation (HGHG)
Various Self-seeding techniques (HXRSS and SXRSS)
Echo-Enabled Harmonic Generation (EEHG)
Echo is a new approach where laser challenges are traded for beam manipulation challenges
Has advantage in that bunching is weak function of harmonic number and only small relative energy modulations required
Echo (EEHG) demonstration to benchmark critical accelerator and laser physics issues
Find optimal combination of high-harmonics and short wavelength seeds
Seeding methods potentially relevant to future SLAC programs…

5. Classical external seeding with HGHGλ
Energy modulation in a modulator
Energy modulation converted to density modulation with a chicane
Coherent radiation at amplified to saturation in a radiator

6. Limitations on single stage HGHG
Low up-frequency conversion efficiency:
Modulator exit
Chicane exit
Current distribution
Outcome: Bunching (large ∆E ) OR Gain (small ∆E)
But seeded FEL wants: Bunching AND Gain

7. Echo-Enabled Harmonic Generation (EEHG)
G. Stupakov, PRL, 2009;
D. Xiang, G. Stupakov, PRST-AB, 2009
First laser generates energy modulation in electron beam
First strong chicane stratifies the longitudinal phase space
Second laser imprints energy modulation
Second chicane converts energy modulation into harmonic density modulation

8. EEHG FEL: Advantages and Challenges
Excellent frequency up-conversion efficiency from small energy modulation
UV laser up-converted to soft x-rays in a single stage
Tunable through dispersion
(Relatively) insensitive to e-beam phase space distortions
Challenges
Preservation of fine-grained phase-space correlations
Sensitive to instabilities, CSR, quantum diffusion, intrabeam scattering, etc.
Depends on laser quality, stability, and spectral phase errors

9. Echo Demonstration at SLAC (starting in 2010)
Used existing NLC Test Accelerator for demonstration
Rf gun (S), high gradient rf (X) and multiple laser systems pre-installed
Added 60 MeV acceleration, 3 chicanes (C0-C2), 3 undulators, and lots of diagnostics in 2010
Subsequently installed two TCAVs, VUV spectrometer and upgraded energy spectrometer
~50 m

10. Echo Experiment at SLAC’s NLCTA
Existing
Main echo beam line constructed after 10/2009
C-1
TCAV1 X2
TCAV2 C1 U1 U2
spectrometer

11. First unambiguous Echo signal with chirped beam
1590 nm laser on
1590 nm laser on
(a)
1590 nm laser on H3
(a)
(a) H4
795 nm laser on
795 nm laser on H2 H2
(b)
795 nm laser on H2
(b)
(b)
Both lasers on H4 H2 H3
(c)
Echo
350
400
450
500
550
600
Radiation wavelength (nm)
D. Xiang et al., PRL 105, (2010)
EEHG and HGHG have different dependence on structures in the e-beam phase space

12. Pushing EEHG to realistic scenarios
The advantage of EEHG lies in efficient upconversion even for
Typically a ‘laser heater’ is used to increase beam slice energy spread
We use RF TCAV used to increase slice energy spread

13. 7th harmonic Echo (2012) 1590 nm only, V=0
Retuned U3 radiator for resonance at 227 nm
4th to 7th harmonics from HGHG are suppressed with increased beam slice energy spread
7th harmonic reappears with the first laser on
Energy modulation is about 2~3 times the beam slice energy spread
1590 nm only, V=85 kV
1590 nm only, V=170 kV
1590 nm only, V=255 kV
795 nm nm, V=255 kV

14. Going to harmonics >10
In 2012 the entire Echo line (modulators, chicanes) moved upstream by 3m to accommodate new structures
Replaced X1 linacs with single RDDS (better alignment, no SLED)
Installed chicane bypass (cleaner phase space)
Upgraded laser systems and PLCs
U2 retuned to be resonant with 2400 nm laser
New OPA purchased and commissioned
RF undulator installed
Spherical mirror installed to enhance spectrometer signal

15. Going to harmonics >10
The 120 MeV e-beam energy limits the shortest wavelengths that can be detected with Echo 7 infrastructure
A ‘gift’ from RF structure testing program: RF undulator
lu=1.39 cm, 77 periods, variable tuning with input RF power (K=0-0.7)
~1 Tesla, 30 J of stored energy(!)
S. Tantawi, et al PRL 112, (2014)

16. 15th harmonic Echo (2014)
15th harmonic 2 orders of magnitude higher than incoherent signal
HGHG signals also visible, shows double peaked spectrum unless DE2 reduced by 20%
(n=-1, m=18)
EEHG
HGHG
(n=0, m=15)
160 nm from 2400 nm
DE1=80 keV
DE2=65 keV
R56(1)=4.8 mm
R56(2)=1.0 mm
EEHG has 60% higher spectral brightness and narrower bandwidth comparing optimized cases
E. H, et al PRST-AB 17, (2014)

17. EEHG signal has narrower bandwidth
Bandwidth comparison
EEHG signal has narrower bandwidth
(Dl/l=0.23% vs 0.62 %)
0.38 nm
1 nm
Two effects:
different dependence of EEHG and HGHG on local phase space distribution and
finite length laser pulse
Non-linear curvature adds more bandwidth to HGHG by shifting wavelengths across the beam
front is compressed, back is decompressed
EEHG less sensitive because strong initial R56 removes this smooth variation
HGHG
EEHG

18. Central wavelength stability
Reduced sensitivity of EEHG to phase space distortions stabilizes central wavelength
RF timing drift or jitter in e-beam can change chirp –> shift in central wavelength
OR, timing jitter between laser and e-beam (ie, energy jitter) changes laser overlap and selects differently chirped region
EEHG
HGHG
E-beam
laser

19. 15th harmonic efficiency at DE/sE~6
Because we are limited by chicane strength (R56<10mm), we use the TCAV to increase the slice energy spread to test harmonic efficiency in realistic regime
TCAV increased to 450 kV
sE =10 keV
EEHG signal persists, HGHG destroyed

20. Echo experiment status
Measured 15th harmonic with modulation ~6 times energy spread
EEHG less sensitive to phase space distortions
Narrower bandwidth
Higher spectral brightness
More stable central wavelength
Relevant for future seeded FELs in the presence of MBI and wakefields etc that can produce considerable higher order correlations
Numerous facility upgrades have been performed to improve beam quality and signal
Developing next phase of Echo program…

21. Toward higher harmonics at shorter wavelengths
Boost beam energy to 160 MeV
Beginning experimental studies of laser phase errors and stabilization (critical for EEHG and HGHG)
Implement a modified zero phasing technique to directly measure laser microbunching on beam
compare with spectral measurements
Installing 2m VISA undulator to access harmonic undulator frequencies (160 nm, 75 nm and 35 nm) and more photons
VISA
undulator
T105
(+40MeV)

22. VISA undulator at NLCTA
Undulator emission
Echo 75 possible at 5th harmonic of undulator
1st harmonic
3rd harmonic
5th harmonic
l (Echo harmonic)
164 nm (15)
55 nm (44)
33 nm (73)
VISA-II 2m undulator:
E=160 MeV
lu=1.8 cm
K=1.26
Nu=110

23. Spectral phase measurements with booster
Zero-phase crossing
Zero phasing technique plus R56 allows measurement of fine scale temporal structures in energy domain
Dispersion followed by linear energy chirp rotates phase space so that energy maps directly to time when 1+h R56=0:

24. Femtosecond visualization of microbunching
D. X, E. H et al, submitted to PRL (2014)

25. Upcoming Plans
Move progressively to Echo 75 at ~30 nm by end of FY 2015
Need to continue to study beam parameter space and feasibility of EEHG at ultra high-harmonics
Sensitivities to emittance, horizontal dispersion, IBS, etc
Install and commission SDL VISA undulators
Characterize undulator harmonic emission spectrum
Build and commission new VUV spectrometer down to 30 nm
Embark on complimentary spectral phase measurements using zero-phasing technique
fs 800 nm setup
2.4 um control station