1. Eduard Prat / Sven Reiche :: Paul Scherrer Institute
Optimization of the soft X-ray beamline Athos at SwissFEL  with integrated chicanes
Designing Future X-ray FELs Workshop
2 September 2016, Daresbury Laboratory, UK

2. Contents Athos CHIC: Chicanes for High-power and Improved Coherence
Concepts:
Optical klystron
HB-SASE
High-power and short FEL pulses
Optimization of undulator module length
Conclusion

3. Athos: the soft X-ray beamline of SwissFEL
SwissFEL will serve two beamlines:
Aramis: nm. Commissioning starts now, first light in 2017.
Athos: nm. First FEL light expected for 2020
e- charge: 10 – 200 pC  RMS pulse length: 2 – 30 fs
Athos undulators:
APPLE devices, K = , λu= 38 mm (recent design change to cover shorter wavelengths)
Module length is 2 m, 16 modules available from beginning (20 in a future upgrade)
Installed chicanes between the modules, inter-undulator space is 0.8 m
Allow for special configurations, e.g. transverse-gradient undulator (TGU) and continuous taper
Typical simulation parameters
Parameter
Value
e- charge
200 pC
Current profile
Flat
Peak current
2-3 kA
Pulse duration fs
e- Energy
3-3.5 GeV
Emittance
300 nm
Energy spread
350 keV
Almost all simulations presented here are with λu= 40 mm (equivalent results)

4. CHIC design for FELs Chicanes for High power and Improved Coherence
We propose to include small chicanes made of dipoles between the undulator modules. Chicanes have two physical effects: longitudinal dispersion (R56) and delay. Benefits:
Reduction of saturation length (more space to taper) using the optical klystron effect (R56)
Increase of brightness using the HB-SASE concept (delay)
Generation of short and high-power FEL Pulses (TW-as pulses) based on superradiance with a multiple slotted foil or a transversely tilted beam (delay)
We have optimized the undulator module length for best performance:
Optimal use demands short undulator modules
The delay of the chicanes is up to ~1 µm
Present design with permanent magnets: maximum delay of 1.5 µm, total length is 0.2 m
[E. Prat, M. Calvi, R. Ganter, S. Reiche, T. Schietinger, and T. Schmidt, JSR 23, 861 (2016)]

5. Concept: optical klystron
[Y. Ding, P. Emma, Z. Huang, V. Kummar, Phys. Rev ST Accel. Beams 9, (2006)]
FEL: Simultaneous growth of field, bunching and energy modulation
q+f Dg
q+f Dg
q+f Dg
Optical Klystron: enhance bunching due to energy modulation with chicane and then emit coherently
q+f Dg
q+f Dg
R56 of Chicane
Reduction of saturation length
Less undulator length is required
More space for tapering

6. Concept: iSASE / HB-SASE
[J. Wu et al, Proceeding FEL Conference, 2012 p. 237].
[B. W. J. McNeil, N. R. Thompson, and D. J. Dunning Phys. Rev. Lett. 110, (2013)]
Distributed Optical Klystron  Induced bunching due to R56 of the chicane
iSASE/HB -SASE  Increase coherence length due to delay of the chicanes
Delay
FEL interaction
Original Idea (delay doubled with each step, using isochronous chicanes)
Field
etc
Bunching
Inverted Sequence (delay halved with each step, dispersive chicanes)
Field
Bunching
etc
Original idea with dispersive chicanes (simpler and more compact)  overbunching
With the inverted sequence we avoid overbunching and there is optical klystron effect at the beginning 

7. Concept: short and high-power FEL pulses
There are some good ideas to achieve shorter pulses by reducing the electron pulse length (e.g. slotted foil, low charge) and using external lasers. Tanaka proposed a complicated scheme to get TW-as pulses [T. Tanaka, PRL 110, (2013)].
We propose two simpler methods using small chicanes between the modules:
With multiple slotted foil in BC [E. Prat and S. Reiche, PRL 114, (2015)]
With a tilted beam [E. Prat, F. Löhl and S. Reiche, PRSTAB 18, (2015)]. This method is more efficient and offers better tunability. Results in this talk are for this method.
The scheme has N undulator sections and (N-1) chicanes. The beam is split in N subpulses
In the first undulator section only the first subpulse (tail) produces XFEL radiation
Then the electron beam is delayed and aligned such that the 2nd subpulse overlaps with the XFEL pulse. Only this part is amplified.
This is repeated until all the electrons have contributed to amplify the short pulse

8. Optimization of undulator module length
Better for
Focusing (β-function)
Short modules
Filling ratio / mode mismatch
Long modules
Optical klystron
iSASE/HB-SASE
Short and High-Power
Simulations done for the soft x-ray
beamline of SwissFEL (Athos)
Radiation wavelength: 1 nm
Electrons: 200 pC, 2-3 kA (flat current profile)
Energy: 3.0 GeV / Energy spread: 350 keV
Emittance: 300 nm
Fix space between undulator modules is 0.72 m, with a chicane and a quadrupole magnet.
Maximum delay of the chicane is ~1µm
Module lengths [m]: 0.5, 1, 1.5, 2, 3, 4, 6, 8
We have done 3 studies:
Minimization of saturation length considering optical klystron, beta-function and filling ratio
FEL bandwidth based on iSASE/HB-SASE
FEL performance for generation of short and high-power FEL pulses

9. Minimization of saturation length (w/o CHIC)
Beam sizes for emit = 300 nm
Only based on β-function and filling ratio
Studies for two emittances: 300 nm and 600 nm
Power along the undulator beamline
for emit= 300 nm
Gain Length
Best results for a module length between 3.m and 4 m

10. CHIC: Optical klystron
We scan the delay/R56 after each chicane, we take the value that maximizes the FEL power
We take minimum achievable β-function per each modulator length
FEL along the undulator beamline with and without optical klystron configuration (L = 4 m)
Power along the undulator beamline for the optimized cases
Saturation length reduced from ~30 m to ~20 m
For short modules we are penalized by the poor filling ratio
For long modules we are penalized by limited number of sections to apply optical klystron and β-function
Best results for a module length around 2 m

11. Simulation results for 1 nm (average over 10 seeds)
CHIC: iSASE/HB-SASE
Simulation results for 1 nm (average over 10 seeds)
We consider L=1, 1.5, 2, 3 and 4 m
For L<1m low filling ratio penalty is too large
For L> 4m HB-SASE does not work
Per each case we simulated different delay/R56 configurations (from higher to lower values to use optical klystron effect)
For L≤ 2 m the bandwidth gets reduced by about a factor of 10
Module length
FWHM Bandwidth x10-4
L=1m
1.6±1.5
L=1.5m
2.1±1.2
L=2m
2.4±1.5
L=3m
7.0±4.2
L=4m
16.0±6.9
L=2m (SASE)
19.4±10.3
Output spectra for L=1.5 m
Average over 10 shots
Single shot

12. CHIC: short and high-power FEL pulses
Simulations: I = 6 kA, λ= 2 nm, 8 undulator sections (7 chicanes)
Results for different module lengths and tilt amplitudes (average over 5 seeds)
L (m)
Offset along the bunch (mm)
Undulator modules
Undulator length (m)
Peak power (TW)
Pulse energy (mJ)
FWHM pulse duration (fs) 4
1.5
11 (4+7*1) 52
1.02±0.07
2.21±0.14
3.21±0.42 3
1.40±0.19
1.81±0.19
1.92±0.90 2
20 (6+7*2) 55
1.62±0.58
1.01±0.24
0.46±0.26
1.48±0.20
0.52±0.05
0.30±0.01 1
40 (12+7*4) 70
1.52±0.06
0.64±0.02
0.46±0.16
0.18±0.06
0.34±0.01
By tuning the tilt amplitude one can choose shorter pulses with less energy or longer but more energetic pulses
Shorter modules are generally better (stronger focusing, less tilt for same performance, delays can be applied more often…, but longer)
FEL radiation profile after each undulator section for L=2m and a tilt of 3 mm (1 seed)

13. Final module length is 2 m
Summary of FEL performance as a function of the undulator module length
Based on physics and costs
Final module length is 2 m
For 1.5 mm tilt

14. Wir schaffen Wissen – heute für morgen
We propose to implement a CHIC design to the undulator beamlines, i.e. to install chicanes between the undulator modules. Advantages:
Reduction of the saturation length (optical klystron effect)
Bandwidth reduction (iSASE/HB-SASE)
Generation of short and high-power FEL pulses
We have optimized the undulator module length. The best use of CHIC requires shorter undulator modules than the standard ones. For Athos, the best length is ~2 m.
Disadvantages: more hardware, setup becomes more difficult.
Additional possibilities of CHIC:
Phase-locked short pulse trains (CHIC+ laser) [N. R. Thomson and B. W. J. McNeil, PRL 100, (2008)]
Harmonic lasing [B. W. J. McNeil et al, PRL 96, (2006)]