1 / 28

Free Electron Laser Studies

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

argus
Download Presentation

Free Electron Laser Studies

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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.

More Related