DL/RAL Joint Accelerator Workshop 21 st January 2009 Free Electron Laser Studies David Dunning MaRS...
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Transcript of DL/RAL Joint Accelerator Workshop 21 st January 2009 Free Electron Laser Studies David Dunning MaRS...
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
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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’
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Coherent emission through bunching
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Lose energy
Gain energy
Axial electron velocity
r
Electrons bunch at resonant radiation wavelength – coherent process
Resonant emission – electron bunching
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Low Gain – needs cavity feedback
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
ALICE IR-FEL
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Single pass high-gain amplifier
Self-amplified spontaneous emission (SASE)
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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)
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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,
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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.
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
1D Simulation: Mode locking mechanism
Mode-locked SASE - 1D simulation
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Amplification of an HHG seed in mode-locked FEL
Brian McNeil, David Dunning, Neil Thompson and Brian Sheehy, Proceedings of FEL08
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
1D Simulation: HHG amplification mechanism
Amplified HHG – 1D simulation
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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.
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
1D Simulation: HHG amplification mechanism with energy modulation period and slippage at multiple of pulse spacing
Amplified HHG – increasing pulse spacing
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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.
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
VUV-FEL: Main features
Five 2.2m undulator modules. Gain 10,000%
2mm outcoupling hole: outcoupling fraction ~75%
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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.
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
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.
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Thanks for listening.
And thanks to Neil Thompson and Brian McNeil for the use of slides.