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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?

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free electron laser studies

Free Electron Laser Studies

David Dunning



STFC Daresbury Laboratory

free electron laser fel studies
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
what is a free electron laser and why are we interested

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.

how does an fel work


















B field

E field

How does an FEL work?
  • Basic components

Electron path

what is a fel


What is a FEL?

A classical source of tuneable, coherent electromagnetic radiation due to accelerated charge (electrons)


NOT a quantum source!



resonant wavelength slippage and harmonics

3rd Harmonic


2nd Harmonic

Resonant wavelength, slippage and harmonics


Harmonics of the fundamental are also phase-matched.



Lose energy

Gain energy

Resonant emission – electron bunching

Electrons bunch at resonant radiation wavelength – coherent process

Axial electron velocity


types of fel low gain and high gain
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

single pass high gain amplifier
Single pass high-gain amplifier

Self-amplified spontaneous emission (SASE)

some exciting fels
Some Exciting FELs
  • LCLS ( to 1.5Å !)

  • XFEL ( ~6nm to 1Å !)

  • JLAB (10kW average in IR)

  • SCSS (down to ~1Å )

fel studies
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)
seeding a high gain amplifier with hhg
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 Sheehyand P H Williams,

Proceedings FEL 2006

New Journal of Physics 9, 82 (2007)

modelocking a single pass fel
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.
schematics and simulated output
Schematics and simulated output

SASESpike FWHM ~ 10fs

Mode-CoupledSpike FWHM ~ 1 fs

Mode-LockedSpike FWHM ~ 400 as

Neil Thompson and Brian McNeil, PRL, 2007

mode locked sase 1d simulation
Mode-locked SASE - 1D simulation

1D Simulation:

Mode locking mechanism

amplification of an hhg seed in mode locked fel
Amplification of an HHG seed in mode-locked FEL

Brian McNeil, David Dunning, Neil Thompson and Brian Sheehy, Proceedings of FEL08

amplified hhg retaining structure



Drive λ=805.22nm, h =65, σt=10fs

Amplified HHG – retaining structure

Amplified HHG – 1D simulation

1D Simulation:

HHG amplification mechanism

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

Amplified HHG – increasing pulse spacing

1D Simulation:

HHG amplification mechanism with energy modulation period and slippage at multiple of pulse spacing

high gain oscillator fels
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.
vuv fel main features

Five 2.2m undulator modules. Gain 10,000%

2mm outcoupling hole: outcoupling fraction ~75%

VUV-FEL: Main features
high gain oscillators at short wavelengths
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.
  • 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.
Thanks for listening.
  • And thanks to Neil Thompson and Brian McNeil for the use of slides.