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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.