DL/RAL Joint Accelerator Workshop 21 st January 2009 Free Electron Laser Studies David Dunning MaRS ASTeC STFC Daresbury Laboratory

DL/RAL Joint Accelerator Workshop 21st January 2009

Free Electron Laser Studies

David Dunning

MaRS

ASTeC

STFC Daresbury Laboratory

David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009

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?

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

Accelerator-based photon source that operates through the transference of energy from a

relativistic electron beam to a radiation field.

Molecular & atomic ‘flash photography’


How does an FEL work?

Basic components

N S SNS

N S NSN

B field Electron path E field

B

E

z

v

x

y

vx


Coherent emission through bunching


What is a FEL?

NOT a quantum source!

En

En-1

e-

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

vz


r

e-

u

2nd Harmonic

3rd Harmonic

Harmonics of the fundamental are

also phase-matched.

Resonant wavelength, slippage and harmonics

2

RMSu u

u

e Ba

mc

2

20

1

2u

r u

a


Lose energy

Gain energy

Axial electron velocity

r

Electrons bunch at resonant radiation wavelength – coherent process

Resonant emission – electron bunching


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


Low Gain – needs cavity feedback


ALICE IR-FEL


Single pass high-gain amplifier

Self-amplified spontaneous emission (SASE)


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


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

HHG

Proceedings FEL 2006 New Journal of Physics 9, 82 (2007)

*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,


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.


SASESpike

FWHM ~ 10fs

Mode-LockedSpike

FWHM ~ 400 as

Mode-Coupled

Spike FWHM ~ 1

fs

Schematics and simulated output

Neil Thompson and Brian McNeil, PRL, 2007


1D Simulation: Mode locking mechanism

Mode-locked SASE - 1D simulation


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

HHGP

[W

]

1 . 5 × 1 09

1 . 0 × 1 09

5 . 0 × 1 08

0

s [ m ]

3 23 02 82 62 4

P(

) [a

.u.]

6 × 1 04

4 × 1 04

2 × 1 04

0

[n m ]

1 4 .01 3 .51 3 .01 2 .51 2 .01 1 .51 1 .0

spectrumP

()

[a.u

.]

1 . 5

1 . 0

0 . 5

0 . 0

[n m ]

1 51 41 31 21 1

P [

W]

3 × 1 06

2 × 1 06

1 × 1 06

0

s [ m ]

2 42 22 0

Drive λ=805.22nm, h =65, σt=10fs


1D Simulation: HHG amplification mechanism

Amplified HHG – 1D simulation


0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 20 40 60

Module number

Sca

led

inte

nsi

typMA = 0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 20 40 60

Module number

Sca

led

inte

nsi

ty

pMA = 0

pMA = 1

pMA = 2.4

pMA = 5

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60

Module number

Sca

led

pu

lse

wid

th

pMA = 0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60

Module number

Sca

led

pu

lse

wid

th

pMA = 0

pMA = 1

pMA = 2.4

pMA = 5

Amplification of an HHG seed

Comparison of simulations with varying energy modulation amplitude – including case with no modulation.


1D Simulation: HHG amplification mechanism with energy modulation period and slippage at multiple of pulse spacing

Amplified HHG – increasing pulse spacing


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%


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.


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.


Thanks for listening.

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

Documents

DL/RAL Joint Accelerator Workshop 21 st January 2009 Free Electron Laser Studies David Dunning MaRS ASTeC STFC Daresbury Laboratory