High Energy Gain, High Quality Laser Particle
Accelerator Development at JAERI-APR
Masaki Kando*, Hideyuki Kotaki*, Shin'ichi Masuda*, Shuji Kondo*,
Shuhei Kanazawa*, Takayuki Homma* and Kazuhisa Nakajima*1^
*Advanced Photon Research Center, Japan Atomic Energy Research Institute
8-1 Umemidai, Kizu, Souraku, Kyoto, 619-0215, Japan
^High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
Abstract. Recent activities related to laser-plasma acceleration at Advanced Photon Research Center, Japan Atomic Energy Research Institute are presented. Gas density measurements of a pulsed
gas valve and wakefield measurements have been performed with two types of interferometers. A
wakefield of 20 GV/m excited in the gas-jet plasma has been obtained at a plasma density of 7 x 1017
cm~3. We have shown by 1D-3V particle-in-cell simulation that a high quality electron beam with a
transverse emittance of 0.3 n mm-mrad and with an energy of 7.0±0.5 MeV can be produced using
a colliding optical injection. Our proposed experiments using a 100 TW laser beam and a 150 MeV
electron beam are also presented.
INTRODUCTION
Since Tajima and Dawson[l] have shown laser-plasma accelerator concepts in 1979,
the remarkable progress on short laser pulse technology realizes the concepts. A laserbased accelerator has high acceleration gradient such as ~GV/cm that is much greater
than those achieved in conventional accelerators. Both laser wakefield experiments
with and without external injection have been demonstrated, however, the obtained
electron energy spread were broad. To improve beam quality of accelerated electrons, we
are preparing second-phase laser acceleration experiments. A perfect matching will be
achieved when we can prepare a small emittance of comparable to laser beam and a short
pulse duration comparable to a plasma wavelength. One possible candidate that satisfies
such high qualities is colliding injection into the wakefield. It is shown theoretically that
an electron beam is obtained with a small energy spread and a small emittance[2, 3].
However, this method seems to be difficult to demonstrate because three short laser
pulses are required and two of them should have different frequencies. As an alternative
method, we decide to use an advanced accelerator system which is based on conventional
accelerator techniques. The accelerator system is a 150 MeV photocathode Microtron.
The accelerator can produce 10 n mm-mrad emittance, which is comparable to those of
the 800 nm wavelength laser beam. The temporal beam matching is difficult, however, an
improved energy spectrum is expected when a transverse beam matching is achieved[4].
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
701
EXPERIMENTAL APPARATUS
We describe the laser acceleration test facility at JAERI APR. We have four large
experimental halls. One is dedicated for high peak power laser development, and the
other one is for the laser acceleration research. In this section, we will give a brief
description of our experimental apparatus.
Laser System
Laser group of APR constructed a 100TW class Tiisapphire laser system based on
chirped pulse amplification (CPA) technique[5]. The detailed description is found in [6].
This system consists of a 10 fs TiiSapphire oscillator, a cylindrical-mirror based pulse
expander, a regenerative amplifier, a 4-pass preamplifier, a 4-pass power amplifier, and
a pulse compressor. The mode-lock Tiisapphire oscillator is pumped by a frequencydoubled Nd:YVO4 laser (Spectra-Physics, Millennia) and produces the pulse duration
of 10 fs at the repetition rate of 82.7 MHz. Synchronization between laser and electron pulses will be achieved by installing a timing stabilizer system (FemtoLaser, FEMTOLOCK) with the timing jitter of less than 100 fs. The final output laser pulse has
typically the pulse energy of 2 J and the pulse duration of 20 fs after the pulse compressor placed next to the laser room. The contrast with the time range of ±150 ps is the
order of 10~6 measured by cross correlation method. The main limitation of the contrast is amplified spontaneous emission (ASE) from the regenerative amplifier. After the
compressor chamber, laser pulse is transported in a vacuum (10~7 Torr) duct with the
length of 60 m and reaches to an experimental chamber. In the laser transportline, four
on-line position monitors are placed and motorized mirrors are installed in the appropriate section. These apparatus will reduce difficulties in alignment procedure. After the
transport we plan to measure characteristics of the laser pulse such as a pulse energy, a
pulse duration, and pointing stability at the chamber.
Electron Injector
We constructed a compact accelerator in order to deliver a high quality electron pulse
into a laser wakefield. A photocathode radio frequency (RF) gun is used as an electron
source. The gun is improved version of one developed by BNL/SLAC/UCLA and can
produce the charge of 2 nC, the emittance of less than 10 Trmm-mrad and the pulse
duration of 10 ps at the repetition rate of up to 60 Hz. A stable frequency quadrupled
NdiYLF laser is used as a driving laser for the gun. The root-mean-square energy
stability is 0.5 %. We use a timing stabilizer composed of a double balanced mixer
(DBM) and a piezoelectric transducer in order to synchronize the phase of the laser
oscillator to a reference sinusoidal RF (79.3 MHz) signal that is the 36th subharmonic
of the master oscillator (2856 MHz). The electron beam from the gun is deliver to a racetrack microtron (RTM) (Sumitomo Heavy Industries, Inc.). The RTM has a two dipole
magnet and a standing-wave accelerating structure in an straight section. The beam is
702
accelerated 25 times and each energy gain is around 6 MeV and finally beam energy
reaches 150 MeV. Extensive beam measurements were made. Beam charge at beam
dump and transmissivity from the injection point to the beam dump were measured
by the CM and the Faraday cup. The maximum charge obtained was 95 pC/pulse
(transmissivity was 77 %) and the maximum transmissivity was 92%. In the simulation,
a charge of 1 nC can be transported at a transmissivity of 26 %. The charge was limited
because the quantum efficiency of the copper cathode was low (2xlO~ 5 ) which was
expected to be 1 x 10~4. The high transmissivity was due to low charge that reduced
the space-charge effect at the injection beam line. The obtained charge is high enough
for the laser wakefield experiments. Emittance was calculated by measuring beam size
as a function of focal strength of a quadrupole magnet. Since the quadrupole magnet
is located at dispersive section, measured horizontal emittance included the dispersion
effects. The horizontal and vertical emittances were measured to be 14 and In mmmrad at a charge of 50 pC, respectively. Assuming that the energy dispersion was 0.1 %,
the horizontal emittance was estimated to be 7 n mm-mrad. Emittance is acceptable in
focusing electron beam to tens of microns. The pulse length of the electron beam and
timing jitter between the electron and laser beams were measured by femtosecond streak
camera. For pulse length measurement, the synchrotron radiation from the circulating
electron in the RTM were delivered to the streak camera. The measured pulse length at
21-25th laps were 10 ps (rms) and no apparent difference was observed at each lap.
A timing jitter was directly measured by observing the time difference between the
fundamental laser radiation (1052 nm) that irradiates the photocathode and synchrotron
radiation generated from the 25 th lap electron bunch in the RTM. The jitter for the long
and short term were 5.5 and 2.2 ps (rms), respectively.
Plasma Waveguide
Various types of plasma waveguided have been developed and tested to overcome
the acceleration length limitation of laser wakefield acceleration (LWFA). We have
developed a noble waveguide by a fast Z-pinch capillary discharge[7]. A high current
fast Z-pinch discharge generates strong azimuthal magnetic field, which contracts the
plasma radially inward down to 100 jum in diameter. An imploding current sheet drives
a converging shock wave ahead of it, producing a concave electron density profile in
radial direction. In the proof-of-principle experiment, we have succeeded in guiding a 2
TW laser pulse over 2 cm, 12.5 times larger than the diffraction length, in a gas-filled
capillary. This method is advantageous in working high repetition rate and stability.
In order to extend a waveguide length, a 10 cm waveguide is under development. The
system consists of a Marx generator which stores the energy up to 68 J and a water
capacitor. The maximum output from the Marx generator is 200 kV. A laser triggered
spark gap switch (LTSG) is used to minimize a jitter of the Marx generator. A 10 cm long
ceramic capillary is connected to the center of a water filled disk capacitor (4 nF). Four
LTSGs are symmetrically located on the disk capacitor to generate axially symmetric
current in the capillary. Four laser pulses, which is generated from a frequency doubled
NdiYAG laser pulse by beam splitters, are focused in each LTSG simultaneously. A
703
Gordon (RAL) Ips
Gahn (Max-Planck) 200 fs
X. Wang (U-Mich.) 29 fs
1
10 18
2
3 4 5 6 7
'
2
3 4 5 6 7
10 19
'
2
10 2°
2
Intensity (W/cm )
FIGURE 1. Recent experimental results of plasma cathode
solenoid valve is used to supply gas into the capillary. Pre-ionization by a YAG laser is
adopted to produce the stable plasma waveguide.
PLANNED EXPERIMENTS
In this section, we discuss on forthcoming experiments using a 100 TW laser beam and a
150 MeV electron beam at JAERI-APR. We are planning two types of laser acceleration
experiments.
Plasma Cathode Experiment
The term plasma cathode is used in this paper to represent an electron beam generation by focusing a laser pulse into a plasma without external injection of electrons, not
depending on whatever the generation mechanism is. There seems to be some mechanisms to explain high energy electron generation when we focus a single short laser
pulse into a plasma. Typical mechanisms are self-modulated laser wakefield acceleration
(SMLWFA)[8], direct laser acceleration[9] and stochastic acceleration[10]. To specify
the mechanisms actually happened in the plasma remains a still attractive and challenging problems.
Figure 1 shows the recent experimental results which represents the maximum energy
gain as a function of focused laser intensity [11] -[15]. We aim to give new experimental
data around an intensity of 1020 W/cm2. Shown in Fig. 2 is the experimental setup. A
peak power of up to 100 TW is focused into a 7 jum which corresponds to 1020 W/cm2
by an off-axis parabolic mirror (OAP) with a focal length of 177 mm. To reduce a strayrays from the initial laser beam, we put a collimating OAP with a 5 mm through hole
in the center axis. The tight focused laser is extracted from the vacuum chamber and
704
Electrondetector
detector
Electron
Plasticscintillator
scintillator
Plastic
32 ch,10 x 1mm t
32ch,10x1mmt
Spotsize
Spotsize
~7 m
~7|Aim
Off-axis
parabola
f/3.5
Electrons
high energy,
collimated part
Gas-jet
Wave-free
supersonic
<1020 cm-3
Laser
100TW, 20fs
60˚
Electronenergy
energyspectrometer
spectrometer
Electron
Collimator
low Z & high Z
B < 1 Tesla
uptoto600
600MeV
MeVwith
with10
10%%resolution
resolution
up
Vacuum
Vacuum
chamber
chamber
600x600x700(H)mm
600x600x700(H)mm
two
twoIMP
TMPpumps
pumps
FIGURE 2.
2. Experimental
Experimental setup
setup for
forthe
theplasma
plasmacathode
cathodeexperiment
experiment
is delivered to diagnostic
diagnostic apparatus.
apparatus. A
A helium
helium gas
gas isis served
served as
as aa plasma
plasma source
source by
by aa
pulsed gas valve , the
the nozzle
nozzle of
of which
which is
is specially
specially designed
designedto
toavoid
avoidshock-waves.
shock-waves.ItItcan
can
produce a spatially uniform
uniform gas.
gas. After
After the
the chamber,
chamber, apart
apart 1.5
1.5 m from
from the
the focal
focal point,
point, an
an
high energy electron spectrometer
spectrometer is
is placed.
placed. The
The spectrometer
spectrometer isis composed
composed of
of 11 Tesla
Tesla
dipole magnet and 32 channel
channel plastic
plastic scintillarors
scintillarors with
with photomultipliers.
photomultipliers. The
The energy
energy
range of the
detector
system
covers
up
to
300
MeV
with
a
resolution
of
10
%.
the detector system covers up to 300 MeV with a resolution of 10 %.The
Thefirst
first
experiment using a 100 TW experiment will be started
started this
this fall.
fall.
Laser
Laser Wakefield
Wakefield Acceleration
Acceleration
Laser wakefield
wakefield acceleration
acceleration experiments
experiments are
are planned
planned to
to demonstrate
demonstrate aa high
high energy
energy
gain of 1 GeV with
with improved
improved energy
energy dispersion
dispersion combining
combining the
the 100
100 TW
TW laser
laser system,
system,
the electron injector
injector and
and the
the plasma
plasma waveguide.
waveguide.Experimental
Experimentalparameters
parametersof
ofthe
theplanned
planned
LWFA experiments are
are listed
listed in
in Table
Table 1.
1. Since
Since our
our laser
laser system
system can
can produce
produce aahigh
high
peak
100 TW,
TW, rather
rather weak
weak focusing
peak power
power of
of 100
focusing is
is needed
needed to
to avoid
avoid wave-breaking.
wave-breaking. We
We
select aa normalized
select
normalized laser
laser intensity
intensity of
of aa00 =
= 1.3
1.3 which
which is
is accomplished
accomplished by
by using
using aa focal
focal
length
length of
of f/10.
f/10. For
For an
an ultrashort
ultrashort pulse
pulse duration
duration of
of 20
20 fs,
fs, itit isis not
not optimum
optimum toto operate
operate
at
the
k
<j
=
2,
in
which
case
the
wakefield
takes
its
maximum,
since
at the kpp σzz = 2, in which case the wakefield takes its maximum, sincedephasing
dephasinglimit
limit
is
The
higher
energy
gain
of
11 GeV
isis obtained
is dominant
dominant at
at higher
higher plasma
plasma densities.
densities.
The
higher
energy
gain
of
GeV
obtained
3
18 cm~
in
slightly lower
1018
in slightly
lower density
density of
of 1.5x
1.5×10
cm−3 (Case
(Case II)
II) instead
instead of
of operating
operating the
theoptimum
optimum
density
density (Case
(Case I).
I). When
When aa 10-cm-long
10-cm-long waveguide
waveguide isis used,
used, an
an energy
energy gain
gain of
of 4.5
4.5 GeV
GeV isis
expected.
expected. Proposed
Proposed experimental
experimental setup
setup is
is similar
similar to
to that
that of
of plasma
plasma cathode,
cathode,expect
expectfor
foraa
long
long focal
focal length
length (~
(∼ 1
1 m)
m) and
and the
the extra
extra spectrometer.
spectrometer.We
Weare
areconsidering
consideringaadouble-bend
double-bend
spectrometer to
spectrometer
to measure
measure 1
1 GeV
GeV electrons.
electrons.
705
TABLE 1.
Experimental parameters of LWFA
Mechanism
Energy gain
Injector
Laser peak power
Pulse duration
Spot radius
Laser intensity, aQ
Plasma density
Acceleration gradient
Diffraction length
Dephasing length
Pump depletion length
Optical guiding
Number of particles
GeV
TW
fs
jiim
cm-3
GeV/cm
mm
mm
mm
Case I
Case II
Case III
Standard LWFA
channel-guided LWFA
0.14
4.5
1.9
150 MeV Photocathode Microtron
100
100
100
20
20
20
40
40
40
1.3
1.3
1.3
8.8E+18 1.5E+18
6.3E+17
2.4
0.96
0.45
20
20
20
2.2
32
116
20
116
0.59
no
no
10cm capillary
4.3E+07 l.OE+08
1.1E+08
RECENT RESULTS AT JAERI-APR
Gas-jet Characterization
A pulsed gas valve makes a gas-jet in a vacuum, however, it is not easy to obtain its
spatial distribution at a time. In order to characterize the gas valve, we performed an
interferomeric measurement of the gas based on a Mach-Zehnder interferometer[16]. A
He-Ne laser is split into two arms and goes through different optical paths and merged
into a screen to generate an interferogram which was measured by a time-gated CCD
camera. Assuming that the gas density is axisymmetric, Abel inversion was used to
calculate a density distribution. From the extensive survey of the various conditions, we
find that a helium gas density of 3.6 x 1017 cm~3 is obtained at a backing pressure of 10
atm. An uniform, stable gas density distribution is formed at a point of 1.5 mm apart
from the nozzle and at a delay of 1.6 ms.
Laser Wakefield Measurement
Previous measurements of wakefields were made in static gas-filled chambers [17][20] . In a gas filled chamber, nonlinear optical effects take place and makes it difficult
to understand the spot radius at a high intensity. In recent laser acceleration experiments,
a pulsed gas valve plays an important role to provide a stable high density plasma.
To confirm that plasma oscillation is excited in a gas-jet plasma, we have carried out
wakefield measurement in a gas-jet plasma by frequency domain interferometry[16]. A
gas density was set to 7x 1017 cm—3 which is close to the optimum plasma density for
wakefield generation. We have observed clear sinusoidal plasma waves and a decay of
the wake. A typical result is shown in Fig. 3. The maximum electric field of 20 GV/m
was observed in a 2 TW laser pulse is focused into a spot radius of 9 j
706
-300
0
50
100
150
200
250
300
Time [fs]
FIGURE 3. Measured wakefield in a plasma density of 7 x 1017 cm~3
PIC Simulation
We have developed a particle-in-cell (PIC) simulation code which is one dimensional
in space and three dimensional in velocity (1D-3V). The code is employed to analyze the
results of the wakefield measurement and a colliding injection scheme. Among various
optical injection schemes, the colliding scheme has a potential to obtain an ultrahigh
quality electron pulse, especially energy spread predicted by the theoretical investigation
and the beam tracking simulations [2] [3]. In order to investigate the feasibility of the this
method, we performed PIC simulation of the colliding scheme. Three pulses consist of
a pump pulse to generate a wakefield, and two counter-propagating pulses (colliding
pulses). The colliding pulses have a few difference in frequency, so as to form a beatwave in a plasma. The beat-wave injects plasma electrons into the wakefield generated
by the pump pulse, resulting in a high quality beam generation. We use moderate laser
intensities ofaQ = l,al=a2 = 0.3, Aco/co = 6 %, ne = 1 x 1017 cm~3, where a- denotes
normalized vector potential of pump and two injection pulses, Aco is the frequency
difference of injection pulses, and ne is the plasma density. Figure 4 shows the simulation
results. A high quality electron beam is clearly observed. The obtained beam has the
energy of 7 MeV, a low energy spread of 0.5 MeV, and the pulse duration of 14 fs at full
width at half maximum (FWHM). Assuming a beam radius of 15 jum, the normalized
transverse emittance and the charge per bunch are estimated to be 0.3 n mm-mrad and 25
pC, respectively. No small-energy-spread electron beam is observed when two colliding
pulses have the same frequency.
CONCLUSIONS
A wakefield of 20 GV/m in a helium plasma density of VxlO 1 7 cm~3 created by a
pulsed gas valve has been measured by frequency domain interferometer. We examined a
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FIGURE 4. Longitudinal phase-space plot of colliding pulse optical injection
colliding injection scheme by a 1 D particle-in-cell simulation, and found a generation of
high quality electron beam with a charge of ~ pC, an energy of 7 MeV, the energy spread
of 0.5 MeV, a normalized emittance of 0.3 n mm-mrad, and a pulse duration of 14 fs. We
also present two forthcoming experiments planned at Advanced Photon Research Center,
Japan Atomic Energy Research Institute. The plasma cathode experiments will give new
data around a focused laser intensity of 1020 W/cm2. A GeV electron demonstration will
be achieved by laser wakefield acceleration using a 100 TW laser and 10-cm plasma
waveguide with an injection of 150 MeV electrons.
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