Transcript Free electron laser studies

Free Electron Laser Studies
David Dunning
MaRS
ASTeC
STFC Daresbury Laboratory
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?
Accelerator-based photon source that operates
through the transference of energy from a
relativistic electron beam to a radiation field.
Extremely useful output properties:

Extremely high brightness(>~1030 ph/(s mm2 mrad2 0.1%
B.W.)).
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High peak powers (>GW’s). High average powers – 10kW at
JLAB
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Very broad wavelength range accessible (THz through to xray) and easily tuneable by varying electron energy or
undulator parameters.
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High repetition rate.
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Short pulses(<100fs).
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Coherent
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Synchronisable
Molecular &
atomic ‘flash
photography’
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
How does an FEL work?
y

x
B
Basic components
E
vx
S
N
S
N
v
S
z
N
B field
S
N
S
Electron path
N
E field
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?
A classical source of tuneable, coherent electromagnetic
radiation due to accelerated charge (electrons)
e-
NOT a quantum source!
vz
En
En-1
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Resonant wavelength, slippage and harmonics
 1  au 2 
r  

2  u
 2 0 
eu BuRMS
au 
2 mc
3rd Harmonic
r
2nd Harmonic
e-
u
Harmonics of the
fundamental are
also phasematched.
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Resonant emission – electron bunching
Gain
energy
Electrons bunch at resonant radiation
wavelength – coherent process
Lose
energy
Axial electron velocity
r
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/
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XFEL ( ~6nm to 1Å !)
http://www-hasylab.desy.de/facility/fel/xray/
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JLAB (10kW average in IR)
http://www.jlab.org/FEL/
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SCSS (down to ~1Å )
http://www-xfel.spring8.or.jp/
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FLASH
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
FEL studies
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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:
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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
Journal ofWorkshop
Physics
82 (2007)
H L Owen, N R Thompson, B Sheehy
and
P H Williams,
st January
David
Dunning,
DL/RAL New
Joint Accelerator
219,
2009
*B W J McNeil, J A Clarke, D J Dunning, G J Hirst,
Modelocking a Single Pass FEL
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Borrow modelocking ideas from conventional lasers to
synthesise ultrashort pulses.
Modelocking in conventional lasers:
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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:
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Produce axial mode spectrum by repeatedly delaying electron
bunch by distance s between undulator modules.
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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
Schematics and simulated output
SASE
Spike
FWHM
~ 10fs
ModeCoupled
Spike
FWHM ~ 1
fs
ModeLocked
Spike
FWHM ~
400 as
Neil Thompson and Brian McNeil, PRL, 2007
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Mode-locked SASE - 1D simulation
1D Simulation:
Mode locking
mechanism
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
HHG
9
1.0 × 10
9
1.5
6
6
5.0 × 10
0
8
spectrum
1.0
P( ) [a.u.]
1× 10
1.5 × 10
P( ) [a.u.]
2× 10
6
P [W]
P [W]
3× 10
0.5
0.0
11
20
22
s [ m]
24
0
24
Drive λ=805.22nm,
26
h
28
s
[
=65,m]
12
30
13
[nm]
14
6× 10
4
4× 10
4
2× 10
4
15
0
32
11.0 11.5 12.0 12.5 13.0 13.5 14.0
[nm]
σt=10fs
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Amplified HHG – 1D simulation
1D Simulation:
HHG amplification
mechanism
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Comparison of simulations
with varying energy
modulation amplitude –
including case with no
modulation.
0.35
0.3
0.25
0.2
pMA = 0
pMA = 1
pMA = 0
pMA = 2.4
pMA = 5
0.15
0.1
0.05
0
0
20
40
60
Module number
Module
Scaled
Scaled intensity
intensity
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Scaled
Scaled pulse width
Amplification of an HHG seed
1.4
1.4
1.2
1.2
11
0.8
0.8
pMA = 0
pMA = 1
pMA = 0
pMA = 2.4
pMA = 5
0.6
0.6
0.4
0.4
0.2
0.2
00
00
20
20
40
40
60
60
Module number
number
Module
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
Amplified HHG – increasing pulse spacing
1D Simulation:
HHG amplification
mechanism with
energy modulation
period and slippage at
multiple of pulse
spacing
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009
High gain oscillator FELs
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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
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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
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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
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Thanks for listening.
And thanks to Neil Thompson and Brian McNeil for
the use of slides.
David Dunning, DL/RAL Joint Accelerator Workshop 21st January 2009