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Electron Acceleration beyond 200 MeV in
Underdense Plasmas Using Table Top Laser
Systems
S. Fritzler*, V. Malka*, E. Lefebvre1, M.-M. Aleonard**, F. Burgy*,
J.-R Chambaret*, J.-F. Chemin**, K. Krushelnick*, G. Malka**,
S.P.D. Mangles*, Z. Najmudin*, J.-P. Rousseau*, J.-N. Scheurer**,
B. Walton* and A.E. Danger*
*Laboratoire d'Optique Appliquee - ENSTA, UMR 7639, CNRS, Ecole Poly technique, 91761
Palaiseau, France
^Departement de Physique Theorique et Appliquee, CEA/DAM Ile-de-France, BP 12,91680
Bruyeres-le-Chdtel, France
**Centre d'Etudes Nucleaires Bordeaux Gradignan, IN2P3-Universite de Bordeaux 1,33175
Gradignan, France
^Blackett Laboratory, Imperial College of Science, Technology, and Medicine, London SW7 2BZ,
United Kingdom
Abstract. In this article we present electron acceleration by the interaction of a multi-TW, high
repetition rate, table top laser at moderate intensity with an underdense plasma. We optimized this
unique electron source in terms of the energy as well as the quality of the generated electron beam.
The plasma period was chosen to be of the order of the laser pulse duration by adapting the plasma
electron density. The waist of the laser focal spot was chosen to be larger than typical for comparable
experiments. As a result, a very energetic, well-collimated, and bright electron beam was generated,
which has the highest energy ever produced by laser-plasma interactions.
1. INTRODUCTION
Some of the tasks of modern large scale accelerator facilities include the generation of
particles as well as research on sub-nuclear structures. For this, to resolve a dimension
d, the required corresponding wavelength - either of particles or light - has to be equal
or smaller than 2nd. For a particle with momentum p the limit d > h/p is given, which
means that for smaller structures the particle's momentum has to increase. Up to now,
no limit has been found for this evolution in High-Energy Physics (HEP), whilst this
condition is met by continuously increasing the size of the accelerators used. The newest
and most remarkable goal is the TeSLA project planned at DES Y in Hamburg, Germany
[1]. This is a projected 33 km long linear superconducting £+£~-collider which can reach
center-of-mass energies beyond 1 TeV. This enormous size is a direct consequence of the
limited accelerating field that can be achieved in superconducting cavities, which cannot
extend 55 MV/m due to material breakdown considerations. Hence, for continuing
progress in HEP, new and more efficient acceleration techniques are required.
One alternative has been demonstrated in a number of proof-of-principle experiments
CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli
© 2002 American Institute of Physics 0-7354-0102-0/02/$19.00
54
over the past few years. In these experiments, high amplitude relativistic plasma waves
(RPW's) were excited during laser-plasma interactions and have been used to accelerate
electrons in fields which exceed several 100 GV/m [2, 3, 4, 5]. The self-modulated laser
wakefield (SMLWF) scheme [6, 7, 8] has aroused much attention in proof-of-principle
experiments, since no external electron injection is needed. The energetic electrons
observed were generated in the plasma itself, initially being trapped in the plasma wave
and then accelerated to high energy. In the SMLWF regime the laser pulse length ci has
to be longer [9] than the plasma wavelength Kp. Since even for small amplitude plasma
waves, the index of refraction is no longer constant but oscillates periodically, the laser
pulse envelope becomes modulated at Kp. This modulated beam in turn resonantly drives
the amplification of the plasma wave. As a signature of this interaction the transmitted
laser spectra exhibit Raman forward satellites at frequencies of (coo ± nwp), where coo
and (Op are the laser and the plasma frequencies, and n an integer. It has been possible to
accelerate electrons up to 100 MeV using VULCAN, a 100 J laser that delivers energetic
laser pulses of 1 ps every 20 min [10]. This encouraging experiment boosted research
in this field raising the question how this mechanism can be optimized regarding the
energy as well as the collimation of the generated electron beam.
Since the maximum energy Wmax an electron can gain in a plasma wave is equal to
the product of an electrostatic field Ez with an optimum length, the dephasing length
Ldeph* which corresponds to exactly half a wavelength in the wave frame, Wmax is given
by 4jj)(Ez/Eo)moc2, where Ez is normalized to EQ = m§c(&pje, and oo^ the plasma
frequency. The plasma wave Lorentz factor jp is in turn equal to 1/(1 — P^)1/2, where
Pp is given by vp/c, which is the phase velocity of the plasma wave normalized to the
speed of light. In a plasma, Pp is equal to the index of refraction n, which is given by
(1 — rie/ric}1/2, where ne and nc are the electron plasma density and the critical density
respectively. Hence, jp is equal to the square root of nc/ne, indicating that for lower
electron densities, the plasma wave has a higher phase velocity, which in turn increases
the electron energy. Concerning the collimation of the generated and accelerated electron
beam, it is desired that the transverse electric fields are reduced. This can be achieved
if the focal spot size is much larger than the plasma wavelength, or, more precisely,
WQ • kp > 1, where WQ and kp are the waist of the focal spot and the plasma wave vector.
The goal of this paper is to show the efficient acceleration of electrons using table-top
laser systems. In Section 2 we will first give a detailed description of the experimental
set-up and present in Section 3 the main results obtained in this experiment. Since
the parameters for classical SMLWF acceleration are not entirely met, we present in
Section 4 the identification of a novel acceleration regime, which we term "Forced Laser
Wakefield."
2. EXPERIMENTAL LAY-OUT
In the following we present in detail the experimental set-up, starting with a description
of the laser facility, followed by the diagnostics used such as the electron spectrometer,
integrating current transformer (ICT), radiochromic film, emittance measurement set-up
and nuclear activation.
55
FIGURE1.1. View
Viewofofthe
thethree
threeamplification
amplification stages
stages of the “salle
"salle jaune"
FIGURE
jaune” laser
laser at
at LOA.
LOA.Details
Detailsare
aregiven
given
thetext.
text.
ininthe
2.1. “Salle
"Salle jaune"
2.1.
jaune” laser
laser
The experiment
experiment was
was performed
performed on
on the
the "salle
The
“salle jaune"
jaune” laser
laser [11]
[11] atat Laboratoire
Laboratoire
d'Optique
Appliquee
(LOA),
which
is
shown
in
Fig.
1.
This
short
pulse,
d’Optique Appliquée (LOA), which is shown in Fig. 1. This short pulse, high
high intensity
intensity
lasersystem
systemproduces
produces aa near
near diffraction-limited
diffraction-limited laser
laser
laser beam
beam based
based on
on the
the Chirp
ChirpPulse
Pulse
Amplification (CPA)
(CPA) technique
technique [12].
[12]. The
The laser
laser chain
sapphire self
Amplification
chain starts
starts from
from aa Ti:
Ti:sapphire
self
mode-lockedoscillator
oscillator pumped
pumped with
with an
an argon-ion
argon-ion laser.
mode-locked
laser. The
The oscillator
oscillator produces
producesaa88
88
MHz,300
300mW
mWtrain
trainof
of pulses
pulses of
of 15
15 fs
fs duration
duration at
at aa wavelength
MHz,
wavelength of
of 820
820 nm.
nm.Each
Eachpulse
pulse
firststretched
stretchedup
upto
to 400
400 ps
ps in
in an
an aberration
aberration free
free stretcher
isisfirst
stretcher and
and passes
passesthen
thenthrough
throughthe
the
actively
controlled
acousto-optic
programmable
dispersive
filter
(AOPDF),
actively controlled acousto-optic programmable dispersive filter (AOPDF), ininorder
ordertoto
control shape and phase of the laser spectrum. Thereafter, a pulse picker selects pulses
control shape and phase of the laser spectrum. Thereafter, a pulse picker selects pulses
at a repetition rate of 10 Hz. These 1 nJ pulses are sent into a 8-pass preamplifier in
at a repetition rate of 10 Hz. These 1 nJ pulses are sent into a 8-pass preamplifier in
order to reach 2 ml and then pass through a second Pockels cell, which acts both as a
order to reach 2 mJ and then pass through a second Pockels cell, which acts both as a
back-reflection isolator and a temporal gate to limit the amplified spontaneous emission
back-reflection isolator and a temporal gate to limit the amplified spontaneous emission
(ASE) energy. Subsequently, the pulses are amplified through a 5-pass power amplifier
(ASE)
energy.
Subsequently,
pulses
amplified
through
5-pass
power amplifier
in order
to reach
an energy the
of 200
ml.areAfter
each of
these atwo
amplification
steps
intheorder
to
reach
an
energy
of
200
mJ.
After
each
of
these
two
amplification
steps
beam is spatially filtered and up-collimated through a 4-times magnification beam
the
beam
is
spatially
filtered
and
up-collimated
through
a
4-times
magnification
beam
expander in order to increase the spatial quality of the IR beam. The laser pulses are
expander
in order
to increase
the spatialcooled
quality
of theamplifier
IR beam.
pulses
are
then amplified
through
a cryogenically
4-pass
up The
to anlaser
energy
of 3.5
then
amplified
through
a cryogenically
4-pass
toparallel
an energy
of 3.5
J. Finally,
the beam
is re-compressed
to cooled
30 fs after
fouramplifier
passes onup
two
gratings.
J.The
Finally,
theenergy
beam is
30 fs afterwas
fourabout
passes
onwhich
two parallel
output
forre-compressed
the described to
experiment
1 J,
lead togratings.
a peak
The
output
energy
for
the
described
experiment
was
about
1
J,
which
peak
power of 30 TW. The contrast, i.e., the intensity ratio between the ASElead
and to
theamain
7
power
of
30
TW.
The
contrast,
i.e.,
the
intensity
ratio
between
the
ASE
and
the
main
impulsion was close to 10~ .
impulsion was close to 10−7 .
2.2. Experimental set-up
2.2. Experimental set-up
The scheme of the experimental set-up is given in Fig. 2. The laser beam was focused
Thean
scheme
of the experimental
set-up
is given
in Fig.
2. The
was focused
with
f/18 off-axis
parabolic mirror
onto
the sharp
edge
of a laser
2 mmbeam
diameter
superwith an f/18 off-axis parabolic mirror onto the sharp edge of a 2 mm diameter super-
56
Optical
Spectrometer
CCD
Off-axis
Parabola
SBD's
Lead Wall
FIGURE2.2. Sketch
Sketchofofthe
theexperimental
experimental set-up.
set-up. The
The laser
FIGURE
laser is
is focused
focused with
withaaf/18
f/18off-axis
off-axisparabola
parabolaonto
onto
sharpedge
edgeofofa aflat
flattop
tophelium
heliumgas
gasjet.
jet. The
The entire
entire charge
charge of
thethesharp
of the
the accelerated
acceleratedelectron
electronbeam
beamwas
wasmeasured
measured
withananICT.
ICT.ItsItsyield
yieldasasaafunction
function of
of energy
energy was
was determined
determined with
with
with aatunable
tunableelectron
electronspectrometer
spectrometerand
and
SBD's.The
Thetransmitted
transmittedlaser
laserspectrum
spectrum was
was obtained
obtained with
SBD’s.
with an
an optical
optical spectrometer
spectrometerand
andmeasured
measuredwith
witha a
18-bitCCD
CCDcamera.
camera.
18-bit
sonichelium
heliumgas
gasjet
jetto
toavoid
avoid refraction
refraction induced
induced by
sonic
by ionization
ionizationprocesses
processes[13].
[13].ItsItsneutral
neutral
density
profile
was
characterized
by
interferometry
and
found
to
be
uniform
density profile was characterized by interferometry and found to be uniform[14].
[14].The
The
laser distribution at full energy in the focal plane was a Gaussian with a waist WQ of
laser distribution at full energy in the focal plane was a Gaussian with a waist w 0 of
18 /on, containing 50 % of the total laser energy. This resulted in on-target intensities
18 µm, containing 5018% of the
total laser energy. This resulted in on-target intensities
on the order of 3 • 1018 W/cm22, for which the corresponding normalized vector potential
onaothe
order
of
2 3 · 10 W/cm , for which the corresponding normalized vector potential
= eA/rriQC2 is 1.2. By changing the backing pressure the plasma period was chosen
a0to=vary
eA/m
1.2.and
By14
changing
the backing
plasma
period2was
chosen
0 c is 25
between
fs, by selecting
initialpressure
electron the
densities
between
• 1019
19and
3
to6-10
vary19between
25
and
14
fs,
by
selecting
initial
electron
densities
between
2
·
10
and
cm.
19
−3
6 The
· 10 energy
cm .of the generated electron beam was measured using a spectrometer, whose
The
energyfield
of the
generated
wastomeasured
using a spectrometer,
whose
magnetic
strength
couldelectron
be variedbeam
in order
measure energies
from 0 to 217 MeV.
magnetic
field
strength
could
be
varied
in
order
to
measure
energies
from
0
to
217
Here, the electron beam was first collimated by a 1 cm internal diameter aperture inMeV.
a4
Here,
the electron
collimated
1 cm
internal diameter
aperture
in a 4
cm thick
stainlessbeam
steel was
piecefirst
at the
entranceby
of athe
spectrometer.
The number
of eleccm
thick
steel
at the entrance
of the spectrometer.
The(SBD)
number
of electrons
wasstainless
measured
withpiece
five biased
silicon Surfaced
Barrier Detectors
placed
in
trons
was
measured
with
five
biased
silicon
Surfaced
Barrier
Detectors
(SBD)
placed
in
the focusing plane of the spectrometer.
the
focusing
plane
of
the
spectrometer.
It is well known that laser-plasma interactions suffer from large background signals due
Itto
is multiple
well known
that laser-plasma
interactions
large
signals
due
processes.
To ensure that
the signalsuffer
of thefrom
SBD's
arebackground
in fact caused
by entoergetic
multiple
processes.
Tonull
ensure
signal
of the SBD’s
fact caused
by enelectrons
several
test that
havethe
been
performed.
Firstly, are
anyin
source
of electronic
noise electrons
in the dataseveral
acquisition
washave
suppressed.
Secondly,Firstly,
the magnetic
field of
waselectronic
slowly
ergetic
null test
been performed.
any source
varied
its entire
range from
to 1.5 T, which
changed
dispersion
of was
the signal
noise
in over
the data
acquisition
was 0suppressed.
Secondly,
thethe
magnetic
field
slowly
correspondingly.
Since
SBD's
sensitive
to Bremsstrahlung,
which isofgenerated
varied
over its entire
range
fromare0 also
to 1.5
T, which
changed the dispersion
the signal
when electrons pass
through
thick lead
walls next to the which
collimator
as well
correspondingly.
Since
SBD’sany
arematerial,
also sensitive
to Bremsstrahlung,
is generated
as around
the detectors
were any
set up
to suppress
Furthermore,
since the as
specwhen
electrons
pass through
material,
thickthese
leady-rays.
walls next
to the collimator
well
has detectors
the specialwere
feature
the electron
butFurthermore,
obviously hassince
no influence
astrometer
around the
set to
upfocus
to suppress
thesebeam
γ-rays.
the specon the propagation
of Bremsstrahlung,
a clear
distinction
the signal
in influence
and out
trometer
has the special
feature to focus the
electron
beam between
but obviously
has no
of
the
focusing
plane
can
be
ascertained.
Only
signals
having
a
signal-to-noise-ratio
on the propagation of Bremsstrahlung, a clear distinction between the signal in and of
out
than 25:1
werecan
considered.
The SBD
signals
were
read-out
on an oscilloscope,of
ofbetter
the focusing
plane
be ascertained.
Only
signals
having
a signal-to-noise-ratio
so that
only
electron
signals synchronized
the laser
pulse
were taken
account.
better
than
25:1
were considered.
The SBDtosignals
were
read-out
on aninto
oscilloscope,
so that only electron signals synchronized to the laser pulse were taken into account.
57
Finally, 1 cm thick copper pieces were installed directly in front of the SBD's. This
changed the signals accordingly with the corresponding energy. These measurements
gave the yield as well as the energy of electrons accelerated in a solid angle of 0.0785
msrad. The entire beam charge was determined by using an ICT, which had an inner
diameter of 10 cm and was installed 20 cm behind the gas jet. In order to prevent any
influence of the laser beam, which would also pass the ICT, it was entirely blocked with
a black pasteboard.
To obtain the collimation of the electron beam as a function of its energy, the FullWidth-at-Half-Maximum (FWHM) of the angular distribution of the electron beam was
measured with a stack of gafchromic film (RCF) to visualize and numerous 2 mm copper pieces to slow down the electron beam. This stack was placed on the beam axis and
shielded with aluminum wrapping to prevent illumination of the film by the laser. The
traces on the film show the beam size of the electrons as a function of the electron energy
required to pass the single copper and RCF pieces. The images have been corrected for
multiple scattering and superposition of the signals due to higher energies which pass
through the RCF.
To confirm this additional measurements of the nuclear activation of 63Cu and 12C
via (y, ri) reactions were done. In this case, the electron beam was first converted to
Bremsstrahlung by sending it through a 2 mm thick tantalum piece. The resulting yspectrum can be correlated with the initial electron spectrum by simulations with the
Monte-Carlo code GEANT. To trigger (y, ri) nuclear reactions in 63Cu and 12C, the incident photon energy must be above the g-values for these reactions, which are 10 and
18 MeV respectively. Consequently, this diagnostic is solely sensitive to the higher energy part of the spectrum [4]. The angular distribution of Bremsstrahlung was obtained
from measurements of the relative activity of a number of (4 x 10 x 10) mm3 targets,
which were placed in a circle 22.5 mm behind the converter. Their (3+-decay was measured using standard coincidence techniques in which the simultaneous measurement of
two counter-propagating 511 keV photons is taken to be due to the annihilation of the
positron inside the activation target.
To measure the emittance of this electron beam, i.e., its volume in the (x — xf) phase
space, a secondary set-up was used, where a 5 cm diameter magnet was installed behind
the gas jet. Stainless steel collimators of adequate apertures for the different opening
cones of the electron beam ensured that the beam halo was accounted for. To obtain the
angular divergence of electrons as a function of their position within the beam envelope,
the well known "pepper pot" technique was applied [15]. Here plates of different thicknesses are used to entirely stop the electron beam, except for a grid of holes in the plate,
which permits the electron beam at a certain position to pass. The propagation of the
electrons passing the (750 ± 100) ]um holes was then analyzed with RCF. Scanning the
entire beam envelope gave in turn the emittance.
The transmitted laser beam has been measured with an optical spectrometer after each
single shot using a high dynamic 18-bit CCD camera. This not only allowed measurements of the plasma electron density but also monitored any possible modification of its
spectrum due to the interaction.
58
10"
109
I io8
106
50
100
150
200
Electron Energy (MeV)
3
19cm~
FIGURE3.
3. Electron
Electron spectrum
spectrum for
for a plasma electron density
(squares).
FIGURE
density of
of 2.5
2.5 •· 1019
cm−3
(squares).An
Aneffective
effective
electrontemperature
temperature of
of (18
(18±
± 1)
1) MeV
MeV is
is obtained from
from a purely exponential
electron
exponential fit
fit for
forelectrons
electronsofofless
lessthan
than
130MeV
MeV(continuous
(continuous line).
line). The
The detection
detection threshold marks the
130
the signal
signal limit
limit for
forthe
thechosen
chosensignal-to-noisesignal-to-noiseratioof
of25:1.
25:1.
ratio
3. SUMMARY
SUMMARY OF EXPERIMENTAL
EXPERIMENTAL RESULTS
3.
RESULTS
19
3
typical energy
energy spectrum
spectrum at
at a plasma density of 2.5
AAtypical
2.5 •· 1019cm~
cm−3 isis shown
shown in
inFig.3.
Fig.3.ItItisis
19
3
noted that
that similar
similar spectra
spectra have been obtained in the
noted
the range
range of
of 1.3
1.3 to
to 66 •· 10 19cm~
cm−3. .The
The
total charge
charge of
of this
this beam
beam was
was measured to be 5 nC.
total
nC. ItIt isis stressed,
stressed, that
that the
the maximum
maximum
energy gain
gain of
of 200
200 MeV
MeV was
was achieved
achieved on
on aa scale
energy
scale length
length of
of 22 mm
mm and
and an
an input
inputlaser
laser
energy,
which
was
50
times
less
than
on
VULCAN.
energy, which was 50 times less than on VULCAN.
Even though
though like
like all
all electron
electron spectrum
spectrum obtained
obtained from
Even
from laser-plasma
laser-plasma interactions
interactions up
up toto
now,
it
exhibits
a
broad
distribution
this
spectrum
is
particularly
noteworthy.
For
now, it exhibits a broad distribution this spectrum is particularly noteworthy. For low
low
energyelectrons
electrons of
of up
up to
to 130
130 MeV
MeV it
energy
it is
is possible
possible to
to fit
fit aaMaxwell-Boltzmann
Maxwell-Boltzmanndistribution
distribution
with aa longitudinal
longitudinal temperature
temperature of
of (18
(18 ±
± 1)
1) MeV.However,
MeV.However, this
with
this does
does not
not adequately
adequately
described the
the “hot
"hot tail”
tail" of
of the
the spectrum
spectrum -– this
described
this is
is the
the first
first time
time such
such aa non-Maxwellian
non-Maxwellian
profile has
has been
been measured
measured in
in any
any laser-plasma
laser-plasma acceleration
profile
acceleration experiment.
experiment.
However, itit isis not
not solely
solely the
the energy
energy gain
However,
gain of
of electrons
electrons in
in these
these RPW's
RPW’s that
that makes
makes
this
experiment
so
interesting.
It
is
also
the
quality
of
this
beam.
Starting
this experiment so interesting. It is also the quality of this beam. Starting with
with its
its
angular divergence
divergence measured
measured with
with the
the sandwich
angular
sandwich of
of RCF
RCF and
and copper
copper pieces
pieces one
one can
can
see in
in Fig.
Fig. 44 that
that the
the opening
opening cone
cone of
of the
the beam
see
beam decreases
decreases as
as the
the energy
energy of
of the
thebeam
beam
increases,
e.g.,
for
35
MeV
it
is
solely
(5
±
1)°.
This
has
been
verified
with
the
◦
increases, e.g., for 35 MeV it is solely (5 ± 1) . This has been verified with the above
above
mentionednuclear
nuclear activation.
activation. Assuming
Assuming aa Gaussian
mentioned
Gaussian angular
angular distribution
distributionfor
forthe
theelectrons
electrons
and
Bremsstrahlung
which
is
supported
by
the
data
obtained
with
the
and Bremsstrahlung – which is supported by the data obtained with the RCF
RCF -– the
the
FWHM of
of the
the angular
angular distribution
distribution was
was found
FWHM
found to
to be
be (16
(16 ±± 1)°
1)◦ and
and (10
(10±±1)°
1)◦ for
for 10
10and
and
18MeV
MeVelectrons
electrons respectively,
respectively, which
which corresponds
corresponds well
18
well to
to the
the data
data shown
shownin
inFig.
Fig.4.4.
This result promises a low emittance for the energetic electrons. The measurement gave
This result promises a low emittance for the energetic electrons. The measurement gave
a normalized vertical emittance e* of (2.7 ± 0.9);i mm mrad for (55 ± 2) MeV electrons.
a normalized vertical emittance εxn of (2.7 ± 0.9)π mm mrad for (55 ± 2) MeV electrons.
Here as well it was found that the emittance improves as the electron energy increases,
Here as well it was found that the emittance improves as the electron energy increases,
as is shown in Fig. 4. Since this value is well below the emittance of most modern
as is shown in Fig. 4. Since this value is well below the emittance of most modern
LINAC's, this emphasizes the quality of this energetic electron beam.
LINAC’s, this emphasizes the quality of this energetic electron beam.
59
§> 20
-S10
20
20
30
40
60
Electron Energy (MeV)
Electron Energy (MeV)
FIGURE4.4. Left
Left: :Angular
Angulardivergence
divergenceof
of the
the electron
electron beam
beam as
as aa function
: Results
FIGURE
function of
of its
its energy.
energy.Right
Right:
Results
themeasurements
measurementsofofthe
thenormalized
normalizedvertical
vertical emittance
emittance ej.
εxn .
ofofthe
NEW ACCELERATION
ACCELERATION REGIME
4.4. NEW
REGIME
Eventhough
thoughthe
thedescribed
described experiment
experiment appears
appears to
to be
be similar
Even
similar to
to previously
previously performed
performed
experimentson
onthe
theSMLWF
SMLWFscheme,
scheme, this
this result
result cannot
cannot be
be fully
experiments
fully explained
explained by
bythis
thisknown
known
theory.As
Asthe
thelaser
laserpulse
pulse length
length was
was only
only 30
30 fs,
fs, itit was
was only
theory.
only slightly
slightly longer
longer than
than the
the
plasmaperiod,
period,which
whichvaried
varied between
between 14
14 and
and 25
25 fs.
Hence, these
plasma
fs. Hence,
these conditions
conditions were
were not
not
sufficientfor
forclassical
classical SMLWF
SMLWF acceleration,
acceleration, since
since no
no cascading
cascading of
sufficient
of the
the laser
laser energy
energy
to
satellite
frequencies
can
occur.
But
efficient
electron
trapping
and
acceleration
to satellite frequencies can occur. But efficient electron trapping and acceleration was
was
obtained in this experiment, which was due to an impulsive generation of a plasma wave
obtained
in this experiment, which was due to an impulsive generation of a plasma wave
and its breaking in what we term the ”Forced Laser Wakefield“ regime (FLWF).
and
its breaking in what we term the "Forced Laser Wakefield" regime (FLWF).
Here, the laser pulse is compressed by group velocity dispersion [16, 17, 18], when the
Here, the laser pulse is compressed by group velocity dispersion [16, 17, 18], when the
front of the pulse pushes electrons forward, while its back propagates in the density
front
of the pulse pushes electrons forward, while its back propagates in the density
depression of the RPW. Consequently, the back of the pulse propagates faster than its
depression
of the RPW. Consequently, the back of the pulse propagates faster than its
front, compressing it to an optical shock. The resulting amplification of the ultrashort
front,
compressing
an opticalofshock.
The resulting
amplification
ofdrive
the ultrashort
pulse, in particular itthetoformation
an extremely
sharp leading
edge, can
an RPW
pulse,
in
particular
the
formation
of
an
extremely
sharp
leading
edge,
can
drive
RPW
beyond its wavebreaking limit. It is noted that in this case there can be no an
spectral
beyond
its
wavebreaking
limit.
It
is
noted
that
in
this
case
there
can
be
no
spectral
cascading of laser energy, and the only signature in the transmitted laser spectra will
cascading
of laserofenergy,
and the
signature
in the transmitted
laser investigated
spectra will
be a broadening
the driver
laseronly
frequency
bandwidth.
This has been
beduring
a broadening
of
the
driver
laser
frequency
bandwidth.
This
has
been
investigated
the experimental runs and Fig. 5 shows a typical spectrum of the transmitted laser
during
runs and
Fig. 33
5 shows
typical to
spectrum
of the
transmitted
laser
beam.the
Its experimental
FWHM increased
initially
nm inavacuum
48 nm for
shots
at full energy
19 cm
beam.
FWHM
increased
initially
nm
in −3
vacuum
to 48 nmthere
for shots
atsignature
full energy
and aIts
plasma
electron
density
of 2.733
· 10
.
Additionally,
is
no
of
19
3
and
a plasma
electronasdensity
of 2.7 •in
10the
cm~
. Additionally,
theresuggests
is no signature
satellite
frequencies
are observed
SMLWF
scheme. This
the clearof
satellite
frequencies
are two
observed
in the regimes
SMLWFbut
scheme.
suggestsofthe
clear
distinction
between as
these
acceleration
also theThis
superiority
FLWF.
distinction
between
these
two
acceleration
regimes
but
also
the
superiority
of
FLWF.
Most significantly, since in SMLWF the growth of the RPW is amplified from an initial
Most
since in
SMLWF
growth
the RPW for
is amplified
from
initial
smallsignificantly,
level seed source,
which
itself the
depends
on of
instabilities
its creation,
thean
plasma
small
seed source,
itself depends
the FLWF.
plasma
wavelevel
characteristics
canwhich
vary greatly
from shotontoinstabilities
shot. This isfor
notits
thecreation,
case for the
wave
greatly
from shot
to shot.
Thistheis focusing
not the case
the FLWF.
Also,characteristics
in the FLWF can
the vary
electrons
interact
primarily
with
andfor
accelerating
Also,
electrons
interact
primarily
withimproved
the focusing
accelerating
fieldsinofthe
theFLWF
plasmathe
wave
and this
can result
in greatly
beamand
emittances,
as
fields
of the above.
plasma wave and this can result in greatly improved beam emittances, as
was shown
was shown above.
60
700
800
900
Laser Wavelength (nm)
FIGURE5.5. Spectrum
Spectrumofofthe
thetransmitted
transmitted laser
laser beam
beam in the FLWF regime
FIGURE
regimein
invacuum
vacuum(triangles
(trianglesand
andsolid
solid
line)and
anddistorted
distortedby
bythe
theplasma
plasma (circles
(circles and
and dashed
dashed line).
line). The
line)
The spectrum
spectrum isis broadened
broadenedfrom
froman
aninitial
initial3333
nmtoto48
48nm.
nm.Furthermore,
Furthermore,there
there isis no
no signature
signature of
nm
of satellite
satellite frequencies,
frequencies,as
asare
areseen
seenininthe
theSMLWF.
SMLWF.
5. CONCLUSION
CONCLUSION
5.
conclusion,itithas
hasbeen
been demonstrated
demonstrated experimentally,
experimentally, that
InInconclusion,
that itit isispossible
possibletotoaccelerate
accelerate
electrons
beyond
200
MeV
focusing
a
"table
top",
1
J,
30
fs
laser
electrons beyond 200 MeV focusing a “table top”, 1 J, 30 fs laser onto
ontoaa22mm
mmgas
gasjet.
jet.
This
means
this
"simple
scheme
of
small
steps"
between
laser
light,
underdense
plasma
This means this “simple scheme of small steps” between laser light, underdense plasma
andelectrons
electronsbeyond
beyond200
200MeV
MeV cuts-down
cuts-down dramatically
dramatically the
and
the actual
actualacceleration
accelerationlength
lengthofof
common
accelerators
from
20,000
to
2
mm,
whilst
the
challenging
beam
parameters
common accelerators from 20,000 to 2 mm, whilst the challenging beam parametersare
are
stillfulfilled.
fulfilled.
still
ACKNOWLEDGMENTS
ACKNOWLEDGMENTS
This work was partially supported by the EU large Facility Program under the Contract
This
was partially supported
byLIMANS3
the EU large
Facilityagreements.
Program under the Contract
No. work
HPRI-1999-CT-00086
under the
LIF-LOA
No. HPRI-1999-CT-00086 under the LIMANS3 LIF-LOA agreements.
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