Free Electron Laser Studies

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. Molecular & atomic ‘flash photography’ What is a free electron laser? And why are we interested? Extremely useful output properties: • Extremely high brightness(>~1030ph/(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 Accelerator-based photon source that operates through the transference of energy from a relativistic electron beam to a radiation field.

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

5. Coherent emission through bunching

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

7. 3rd Harmonic r 2nd Harmonic Resonant wavelength, slippage and harmonics e- Harmonics of the fundamental are also phase-matched. u

8. Lose energy Gain energy Resonant emission – electron bunching Electrons bunch at resonant radiation wavelength – coherent process 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 !) http://www-ssrl.slac.stanford.edu/lcls/ • XFEL ( ~6nm to 1Å !) http://www-hasylab.desy.de/facility/fel/xray/ • JLAB (10kW average in IR) http://www.jlab.org/FEL/ • SCSS (down to ~1Å ) http://www-xfel.spring8.or.jp/ • 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 HHG *B W J McNeil, J A Clarke, D J Dunning, G J Hirst, H L Owen, N R Thompson, B Sheehyand 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 SASESpike FWHM ~ 10fs Mode-CoupledSpike FWHM ~ 1 fs Mode-LockedSpike FWHM ~ 400 as Neil Thompson and Brian McNeil, PRL, 2007

18. Mode-locked SASE - 1D simulation 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. HHG spectrum Drive λ=805.22nm, h =65, σt=10fs Amplified HHG – retaining structure

21. Amplified HHG – 1D simulation 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. Five 2.2m undulator modules. Gain 10,000% 2mm outcoupling hole: outcoupling fraction ~75% VUV-FEL: Main features

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.