Transcript Document

Acceleration of particles with lasers at RAL
Peter A Norreys
Physics Group Leader
Central Laser Facility
CCLRC Rutherford Appleton Laboratory
visiting Professor of Physics,
Imperial College London.
World's First Observation of Mono-energetic
Electrons from a Laser Plasma Accelerator
• Laser Plasma Accelerators have 1,000 higher electric field than conventional accelerators
• Implies kilometer’s to centimeters reduction in size for same electron energy - attractive
• To date have always produced broad range of energies which severely limited application
• Quasi Mono-energetic electrons up to 100 MeV produced for the first time at RAL
• Astra “Gemini” may increase this to the GeV level
IC / RAL / Strathclyde /
UCLA collaboration
Mangales et al
Nature, 431 , 535 (2004)
• Experiments performed on
the Astra ultra-high power
laser system in the CLF
• Major success for the RCUK
Basic Technology Programme
Electron Energy
Astra Target Area 2
Outline
• Introduction to the ASTRA laser facility
• Basic concepts for electron acceleration
• Laser wakefield acceleration
• Beam pointing
• Photon acceleration in laser wakefield accelerators
• Scaling to multi-GeV energies - Astra Gemini laser at RAL
• Physics group modelling team in the CLF
Laser characteristics
15 TW = 1.5 1013 W
Power =
Energy
=
pulse duration
0.6 J
= 1.51013 W
40 fs
To maximise the intensity on target, the beam must be focused to a small spot.
The focal spot diameter is 20 mm and is focused with an f/17 off-axis parabolic
mirror
Intensity =
Power
=
Focused area
1.5 x 1018 W cm-2
Astra
diffraction gratings
Astra laser
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•
•
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Single Beam Titanium Sapphire laser system
10 TW optical pulse at 10Hz / 25TW at 1Hz
Operates to 2 target areas
Experiments in Laser-Plasma Physics
Electron motion in an intense laser field
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A single electron in an intense infinite plane
polarised laser field exhibits a figure of eight
motion due to the vxB term in the Lorentz force
F = -e(E+vxB)
•
At relativistic intensities, electrons are
accelerated in the direction of the propagation
direction k twice every laser cycle.
•
E
B
k
The kinetic energy the electron acquires is
roughly proportional to the ponderomotive
potential energy Up


kT~E  ( osc  1 )mc  511 1  I1018 Wcm2 / 137
.

2
Intensity on target
Up
1016 Wcm-2
1 keV

1
2

 1 keV

1018 Wcm-2
1021 Wcm-2
0.4 MeV
14 MeV
Laser wakefield acceleration
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Perturbing ‘object’ passes through a
medium which is displaced from
equilibrium
The medium then returns creating
oscillations
Areas of high and low electron density
create extreme electric fields
High intensity
laser pulse
Gas Jet
Electron density
OSIRIS simulations
Electron density plot at 3.1ps
0
x2
100
200
300
0
200
400
x1
-0.125-0.100-0.075-0.050-0.025
Phasespace_x2x1
Electron acceleration
Plasma electrons are trapped and accelerated by the laser’s wakefield
Laser pulse
Wakefield
Electron
injection
Projection of
electron density
Collaboration between CCLRC, IST Lisbon, University of Strathclyde Glasgow,
UCLA, and Imperial College London
Mono-energetic spikes in the spectra observed
Measured electron
spectra
a) ne= 1.6 x 1019 cm-3
b) ne= 1.8 x 1019 cm-3
c) ne= 3 x 1019 cm-3
d) ne = 5 x 1019 cm-3
at E = 350 mJ, t = 40
fsec.
Analysis
Two independent measurements of the
plasma length
~600μm
~600μm
When the density is such that the
dephasing length,
Ld 
2c 02
 3pe
is shorter than the plasma, the
monoenergetic features are lost
Mono-energetic electron beams observed
Measured electron spectrum using
a 500 mJ laser pulse at a density of
2  1019 cm-3.
The energy spread is ± 3%.
Plasma density: 2.1 x 1019 cm-3
(1) Self-focusing of laser: electrons
first appear
(2) Wavebreaking first occurs
(3) More breaking occurs - multiple
bunches
(4) Dephasing causes smoothing of
the spectrum
Good beam properties measured at LOA
in France
• Fritzler et al., measured the emittance of an
electron beam from a laser wakefield accelerator
(with a ‘thermal’ distribution) using the pepperpot
technique and radiochromic film as the diagnostic.
• The normalised emittance is defined as the rms
correlation between the space (x) and reduced
momentum (x’) coordinates of all beam electrons in
the (x,x’) 2D phase space
 xn,rms  
x2 x'2  xx'
2
x’ = px/pz is the electron angle wrt laser axis.
• (x,x’) phase space plot is a measure of the
divergence of the electron beam as a function of
position across the beam.
• At 55 MeV electron energy, measurements of the
normalised emittance were 2.7 (0.9)  mm mrad.
S.Fritzler et al., Phys. Rev. Lett. 92 165006 (2004)
3D simulations of laser wakefield accelerators
Courtesy of Prof L.O.Silva, IST, Lisbon
According to Wei Lu and Warren
Mori (UCLA) the theoretical energy
gain E is
 k0 
E  Cm c a0  
k 
 p
2
2
provided that the laser focal width is
matched to the bubble size.
This indicates that the maximum
energy gain is simply proportional
to the laser power (within the
dephasing limit)
Should scale to multi-GeV energies
3D explicit PIC simulations using OSIRIS
typically take 1 week to run on a 256 node
cluster with 2GB memory per node.
2D simulations are needed for parameter
scans. They take 12 - 24 hours to run on 32node clusters
Proof of principle: towards a super compact
proton accelerator
Basic set-up (top view)
CPA
Laser
Target:
3-25 µm Al
Proton/ion beam
A.P.Fews, P.A.Norreys et al.,
Phys. Rev. Lett. 73,1801 (1994)
Proton/ion beam
E.L.Clark, K.Krushelnick et al.,
Phys. Rev. Lett. 84, 670 (2000)
• Large accelerating fields exist throughout the target
• Hydrocarbon surface contaminants provide the protons for acceleration on
both front and rear surface.
• Acceleration takes place over 10’s of µm (E~1012 V/m)
(Standard accelerators E~106 V/m, typical scale 10’s of metres)
The Physics Group, Central Laser Facility
Raoul Trines
Alex Robinson
Peter Hakel
James Green
Kate Lancaster
Christopher Murphy took the photo!
Dr Mark Sherlock has also joined us and Prof Roger Evans has arrived as a
consultant
Summary
• The first observations of mono-energetic electron beams from laser wakefield
accelerators has been made using the ASTRA laser facilities at RAL.
• These beams have excited wide interest because of the huge accelerating
electric fields generated (> GeV m-1).
• There is much to do - pointing stability of the beam, shot to shot energy
fluctuation, scaling with laser power.
• Theory indicates that the energy gain is proportional to the laser power multi-GeV energies may be possible on ASTRA-GEMINI. Scaling to 10 PW, it is
possible that energies of interest to HEP science can be generated - needs
experimental validation on the Vulcan 10 PW capability.
• There also have been a number of proposed applications for these beams such
as for injectors into subsequent conventional acceleration stages, new light
sources, probing of dense plasmas and for inertial fusion energy.
ACKNOWLEDGEMENTS
The mono energetic electron acceleration work
described here was performed as part of the RC UK
Basic Technology alpha-X grant
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S. P. D. Mangles, C.D.Murphy, Z. Najmudin,
A. G. R. Thomas, J. L. Collier, A. E. Dangor, E. J. Divall, P.S.
Foster, J.G. Gallacher, C. J. Hooker, D.A. Jaroszynski, A. J.
Langley, W. B. Mori, R. Viskup, B. R. Walton,
and K. Krushelnick
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CLF laser, target area and engineering staff.
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RCUK, EPSRC & CCLRC
ACKNOWLEDGEMENTS
Long-Wavelength Hosing Instability in a
Self-Injected Laser-Wakefield Accelerator
M. C. Kaluza, S. P. D. Mangles, A. G. R. Thomas, C. D. Murphy,
Z. Najmudin, A. E. Dangor, K. M. Krushelnick
Plasma Physics Group, Imperial College London
J. L. Collier , E. J. Divall, K. Ertel, P. S. Foster,
C. Hooker, A. J. Langley, D. Neely, J. Smith
CCLRC Rutherford Appleton Laboratory
Photon acceleration
The laser’s photons are accelerated by the laser’s own wakefield!
Scaled electron density
Wakefield
Photon frequency (rad/s)
Final photon
distribution
Initial photon
distribution
x – ct (m)
Image taken from simulations using a dedicated wave-kinetic code
Photon spectra
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Asymmetric redshift/blueshift qualitatively reproduced
Numerical shifts too big; caused by use of 1-D plasma model
Large blueshift because scaled wakefield amplitude exceeds 1
No blueshift of the spectrum as a whole
Rise and fall of blueshift with increasing density explained from wakefield
behaviour.
C.D.Murphy, R.Trines et al., Phys. Plasmas, March 2006