1. Free Electron Laser Studies
David Dunning
MaRS
ASTeC
STFC Daresbury Laboratory

2. Free Electron Laser (FEL) Studies
What is a free electron laser? And why are we interested?
How does a free electron laser work?
What is the current state of the art?
What are we working on?
ALICE oscillator FEL
Seeding an FEL with HHG + harmonic jumps
Mode-locked FELs including HHG amplification
High-gain oscillator FELs
New Light Source FELs

3. What is a free electron laser? And why are we interested?
Accelerator-based photon source that operates through the transference of energy from a relativistic electron beam to a radiation field.
Extremely useful output properties:
Extremely high brightness(>~1030 ph/(s mm2 mrad2 0.1% B.W.)).
High peak powers (>GW’s). High average powers – 10kW at JLAB
Very broad wavelength range accessible (THz through to x-ray) and easily tuneable by varying electron energy or undulator parameters.
High repetition rate.
Short pulses(<100fs).
Coherent
Synchronisable
Molecular & atomic ‘flash photography’

4. How does an FEL work? Basic components y x B E vx v N S z B field
E field
Electron path

5. Coherent emission through bunching

6. What is a FEL?
A classical source of tuneable, coherent electromagnetic radiation due to accelerated charge (electrons) e- vz
NOT a quantum source! En
En-1

7. Resonant wavelength, slippage and harmonics
3rd Harmonic r e-
Harmonics of the fundamental are also phase-matched. u

8. r Resonant emission – electron bunching
Electrons bunch at resonant radiation wavelength – coherent process
Gain energy
Lose energy
Axial electron velocity r

9. Types of FEL – low gain and high gain
Low-gain FELs use a short undulator and a high-reflectivity optical cavity to increase the radiation intensity over many undulator passes
High-gain FELs use a much longer undulator section to reach high intensity in a single pass

10. Low Gain – needs cavity feedback

11. ALICE IR-FEL

12. Single pass high-gain amplifier
Self-amplified spontaneous emission (SASE)

13. Some Exciting FELs
LCLS ( to 1.5Å !)
XFEL ( ~6nm to 1Å !)
JLAB (10kW average in IR)
SCSS (down to ~1Å )
FLASH

14. FEL studies
So we have low-gain oscillator FELs which have a restricted wavelength range and high-gain FELs which have no restriction on wavelength range but random temporal fluctuations in output.
Recent research with ASTeC, in collaboration with the University of Strathclyde has been directed towards:
Seeding an FEL with HHG (improving temporal coherence in high-gain FELs)
Seeding + harmonic jumps (reaching even shorter wavelengths)
Mode-locked FELs (trains of ultra-short pulses)
HHG amplification with mode-locked FELs (setting train lengths in mode-locked FELs)
High-gain oscillator FELs (improved temporal coherence with low-reflectivity mirrors)

15. Seeding a high gain amplifier with HHG
*B W J McNeil, J A Clarke, D J Dunning, G J Hirst,
H L Owen, N R Thompson, B Sheehy and P H Williams,
Proceedings FEL 2006
New Journal of Physics 9, 82 (2007)

16. Modelocking a Single Pass FEL
Borrow modelocking ideas from conventional lasers to synthesise ultrashort pulses.
Modelocking in conventional lasers:
Cavity produces axial mode spectrum
Apply modulation at frequency of axial mode spacing to lock axial modes
The mode phases lock and the output pulse consists of a signal with one dominant repeated short pulse
In single pass FEL we have no cavity:
Produce axial mode spectrum by repeatedly delaying electron bunch by distance s between undulator modules.
Radiation output consists of a series of similar time delayed radiation pulses.
Lock modes by modulating input electron beam energy at frequency corresponding to mode spacing.

17. Schematics and simulated output
SASE Spike FWHM ~ 10fs
Mode-Coupled Spike FWHM ~ 1 fs
Mode-Locked Spike FWHM ~ 400 as
Neil Thompson and Brian McNeil, PRL, 2007

18. Mode-locked SASE - 1D simulation
Mode locking mechanism

19. Amplification of an HHG seed in mode-locked FEL
Brian McNeil, David Dunning, Neil Thompson and Brian Sheehy, Proceedings of FEL08

20. Amplified HHG – retaining structure
Drive λ=805.22nm, h =65, σt=10fs
spectrum

21. Amplified HHG – 1D simulation
HHG amplification mechanism

22. Amplification of an HHG seed
Comparison of simulations with varying energy modulation amplitude – including case with no modulation.

23. Amplified HHG – increasing pulse spacing
1D Simulation:
HHG amplification mechanism with energy modulation period and slippage at multiple of pulse spacing

24. High gain oscillator FELs
Improving temporal coherence in high-gain FELs through the use of a low-reflectivity optical cavity
Could be applied for very short wavelength FELs – where suitable seeds are not available.
Builds on the 4GLS design of a high gain oscillator FEL operating in the VUV wavelength range.

25. VUV-FEL: Main features
Five 2.2m undulator modules. Gain 10,000%
2mm outcoupling hole: outcoupling fraction ~75%

26. High gain oscillators at short wavelengths
Very low feedback fractions are required to improve the temporal characteristics for very high gain FELs.
There is an optimum feedback fraction for temporal coherence, above and below this the system reverts to SASE-like behaviour.

27. Summary
Low gain oscillator FELs and high gain SASE FELs are currently in operation.
ALICE FEL soon to be commissioned.
Schemes for improving the temporal properties of high gain FELs operating at short wavelengths are being studied.
New Light Source will have three FELs in its baseline design – next stage is deciding on suitable FEL schemes and optimising designs.

28. Thanks for listening.
And thanks to Neil Thompson and Brian McNeil for the use of slides.