Ultrashort Electron Beam Pulses and Diagnosis by
Advanced Linear Accelerators
M.Uesaka*, H.Iijima†, Y.Muroya*, T.Watanabe* and T.Hosokai*
*Nuclear Engineering Research Laboratory, University of Tokyo,
2-22 Shirakata-Shirane, Tokai, Naka, Ibaraki, 319-1188, Japan
†
National Institute of Radiological Sciences
9-1, Anagawa 4-chome, Inage-ku, Chiba-shi 263-8555, Japan
Abstract 240fs 18 MeV low emittance(6 pai mm.mrad) electron beam was generated and its pulse shape was diagnosed
by the S-band laser photocathode RF gun and linac. The maximum charge per bunch was 7 nC. This electron pulse was
synchronized with 100fs 0.3TW Ti:Sapphire laser with the timing jitter of 330fs(rms). Recently, the Cu cathode(QE10^4) was replaced by Mg cathode(QE10^-3). This system is utilized for radiation chemistry analysis for supercritical water.
We have adopted the four diagnostic methods(femtosecond streak camera, coherent transition radiation interferometer,
far-infrared polychromator, fluctuation method) and checked their time-resolution precisely. Further, we are doing the
experiment on laser plasma cathode by 12TW 50fs laser and He gas jet. Laser plasma wakefield acceleration and
electron injection via wavebreaking are planned. We have developed a new theory of self-injection scheme to generate
~10fs electron pulse. We have already succeeded in observing 40 MeV low emittance electron beam of 14 nC.
One of linacs, which is called the 35 MeV linac,
consists of the thermionic electron injector, the subharmonic buncher, the two accelerating tube and the
arc-type magnetic bunch compressor. The 35 MeV
linac generates electron bunches with the bunch
duration of 700 fs. The other is called the 18 MeV
linac where the laser photocathode RF injector (BNLGUN IV) is installed. Recently, the photocathode has
been changed from Cu to Mg [1,2]. Now we are
improving various stabilities. It was difficult to
provide a few nC high charge with sub-picosecond
bunch duration and the signal-to-noise ratio for
experiments using the 18 MeV linac was not enough
by using the Cu cathode. Therefore, we have
introduced the new RF injector with the Mg
photocathode. A quantum efficiency (QE) of the Mg,
which is in the order of 10-3, is 10 times larger than
one of Cu, so that more intense electron bunch is able
to be produced. The 18 MeV linac is operated in
combination with the 0.3 TW 100 fs laser. The 12 TW
50 fs laser is used to generate the ultrashort electron,
ion and X-ray beams via laser plasma. Details of the
12 TW laser and generation of the electron beam are
described.
TWO FEMTOSECOND LINACS AND
TWO FEMTOSECOND-TERAWATT
LASERS
At the linac facility of Nuclear Engineering
Research Laboratory (NERL), two femtosecond linacs
and two femtosecond-terawatt Ti:Sapphire lasers (0.3
TW 100 fs, 12 TW 50 fs) are operating to produce and
utilize ultrashort electron beams, laser lights, X-ray
and ion beams as shown in Fig. 1.
Laser Photocathode
rf-gun
Chicane-type
magnetic pulse compressor
Synchronizatio
n <1ps(rms)
Thermionic
gun(90kV)
Mutipass amplifier
Arc-type
magnetic
Pulse compressor
Multipass
amplifier
Regenerative
amplifier
Stretcher
Oscillator
100fs->200ps
Timing
stabilizer
Multipass
amplifier
Regenerative
amplifier
Stretcher
100fs->200ps
Mutipass
amplifier
Oscillator
Timing
stabilizer
FIGURE 1. Twin linac and terawatt laser system
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
968
The 18 MeV linac consists of the photocathode RF
injector as shown in Fig. 2, the accelerating tube and
the chicane-type magnetic bunch compressor.
and the vacuum pipe are deformed, which attribute to
the drift. We filled the pipe by N2 gas in order to avoid
its deformation of the pipe, especially the bellows
parts, by air pressure. Time-variation of the interval
between the electron beam and laser are shown in
Fig.4. Finally, the timing jitter was measured to be 1.6
ps (rms) for 2 hours and 1.4 ps (rms) for 1 hour.
laser (0.7ps, FWHM)
electron
(0.9ps, FWHM)
FIGURE 2. Mg photocathode RF injector
Maximum electron charge from the Mg cathode is
4 nC/bunch, corresponding to QE of 1.3 x 10-4, so far.
The electron bunch accelerated up to 22 MeV is
compressed by the chicane-type compressor. Typical
bunch duration is approximately 700 fs with the charge
of 2 nC/bunch. This ultrashort electron bunch is
usually used as a pump beam for a pulse radiolysis
method of chemical reaction of water. A probe laser
light and a driven laser for the RF injector are supplied
from the 0.3 TW laser. The 0.3 TW laser produces a
laser light with the wavelength of 795 nm, the energy
of 30 mJ/pulse, the pulse duration of 300 ps and the
repetition rate of 10 pps. The laser light is guided by
the optics in vacuum pipe and bellows, whose length is
approximately 50 m in order to aviod fluctuation by air
and realize precise synchronization between the pump
beam and the probe laser. One laser light is
compressed into the pulse width of 100 fs, and used as
the probe laser. The other is guided to the third
harmonic generator, which is provided the driven laser
with the wavelength of 265 nm, the energy of a few
hundred J/pulse and the pulse duration of a several
picosecond. Energy of the electron beam was
measured using the magnetic analyzer.
FIGURE 3. Streak image of compressed bunch
18.00
16.00
Syncronization
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
-200
20:00:00
20:30:00
21:00:00
21:30:00
22:00:00
22:30:00
Measurement time
FIGURE 4. The synchronization result
PULSE SHAPE DIAGNOSIS
The synchronization test was performed to set the
Xe chamber at the end of the chicane-type bunch
compressor. Cherenkov light emitted from the
chamber is guided to the femtosecond streak camera
with the probe laser as shown in Fig. 3.
Longitudinal pulse shape of the electron pulse was
measured by the femtosecond streak camera, the
Michelson interferometer, a 10-channel polychromator
and the fluctuation method. The experimental setup is
illustrated in Fig.5. Coherent transition radiation
emitted from the Al foil is sent to the Michelson
interferometer and the 10-channel polychromator.
Incoherent Cherenkov radiation emitted in air is
transported to the femtosecond streak camera. The
bunch distributions were reconstructed from the
interferogram taken by the Michelson interferometer
and the spectrum by the polychromator. The
experimental results were shown in Fig.6, where the
bunch distribution taken by the femtosecond steak
Before we had achieved the synchronization results
of 330 fs for a few minutes and 1.9 ps for an hour [1].
However, the most important subject to be overcome
is to suppress the long term drift of the time difference
between the electron and laser. For the purpose, we
renewed the water cooler of the resolution of 0.01 oC
for the RF gun and tube, and the air conditioner of the
resolution of 1 oC in the linac room. Even in a muchcontrolled temperature environment, concrete walls
969
camera is also shown for the comparison. As a result,
good agreements within 20 % were obtained among
the three instruments. The good agreement can be
obtained only with careful thought of error sources [3].
The misalignment of the interferometer or the
polychromator yields the error of the spectrum. The
transverse emittance of the electron pulse should be
taken in to consideration for the estimation of the
bunch form factor and the divergence factor. The
current error effects the estimation of the bunch form
factor as well. The spectrum contributed by a single
electron is calculated under several assumptions, such
that the radiator is an ideal conductor without
dispersion and has a infinite dimension. One has to be
careful about these factors. It is worth noticing that all
the error factors do not have an effect on the
measurement limit [5].
Measurement schemes which utilize coherent
radiation observe the first-order correlation of
radiation, while the schemes for incoherent radiation
detect the second-order correlation function. In other
words, a relation between the bunch length and the
wavelength is important for coherent radiation and that
between the bunch length and the bandwidth is
important for incoherent radiation [4]
PLASMA CATHODE
We have been studying generation of relativistic
electrons in a plasma wake wave-breaking scheme [6]
aiming at a compact ultra-short pulse relativistic
electron accelerator / injector which we called laserplasma cathode [7]. In this experiments an ultraintense laser pulse is focused on a gas jet, which is
sufficiently strong to generate a plasma inside the jet
and to excite a high-amplitude plasma wakefield.
Subsequently, electrons in the plasma are trapped and
accelerated in this field.
The fluctuation method was also applied to the
measurement of electron pulses. The method was
proposed by M..S. Zolotorev [4]. In our experiment,
the time-integrated power fluctuation of Cherenkov
radiation was observed as a function of the bandwidth.
However, the pulse durations evaluated from the
fluctuation were much longer than the actual value.
The reason is that the transverse beam emittance was
not considered and the formation length of Cherenkov
radiation was quite short compared with the radiator
length.
Streak
Camera
Al foil
θ-stage
Typical experimental setup is shown in Fig.7. An
axially symmetric supersonic nozzle with a pulse valve
was fixed inside the vacuum chamber. The pulse valve
was driven for 5 ms a shot at a repetition rate of 0.2 Hz
to generate helium supersonic gas jet. The density at
the exit of the nozzle ranged from 7 x 1018 to 3 x
1019cm-3. The background pressure of the vacuum
chamber was kept lower than 1.0 x 10-4 Torr during the
gas jet operation.
x-stage
LINAC 35L
Ti:Sapphire
Laser (5TW,50fs)
Supersonic Gas Jet
Imaging Plate
Michelson
Interferometer
Polychromator
Turbo
Pump
Streak camera
(FWHM = 1.0ps)
6
5
CCD Camera
Interferometer
(FWHM = 1.2ps)
FIGURE 7. Experimental setup.
4
3
2
Polychromator
(FWHM = 1.0ps)
-2
0
Filters
Gas Inlet
7
-4
Turbo
Pump
e
FIGURE 5. Experimental setup for electron pulse
measurement.
1
0
-1
Off Axis Parabolic
Mirror (f=177mm)
2
The 12 TW Ti:Sapphire laser system based on CPA
technique gives up to 600 mJ, 50 fs laser pulses at the
wavelength of 790 nm at the repetition rate of 10 Hz.
In this study, available laser power at the target in the
vacuum chamber was up to 5 TW. The p-polarized
laser pulse with diameter of 50mm was delivered into
the vacuum chamber and focused on the front edge of
4
Time (ps)
FIGURE 6. Bunch distributions by the Michelson
interferometer, the polychromator and the streak camera.
970
the helium gas jet column at the height of 1.3 mm from
nozzle exit with an f/3.5 off-axis parabolic mirror. The
maximum laser intensity on the target was estimated to
be approximately 1.0 x 1019W/cm2. In order to
investigate correlation between the electron generation
and laser pre-pulse, ns-order laser pre-pulse was
produced before the main pulse by detuning of the
pockels cell of the Ti:Sapphire laser system.
Fast Capillary
Z-pinch Waveguide
Plasma Cathode
Ti-Sapphire
Laser Pulses
Ti-Sapphire
Laser Pulses
~2cm
Gas inlet
Capillary
Gas-jet
Accelerated
electrons
OAP
The electron emission from the gas jet was directly
measured by the imaging plates (I.P.). They were
laminated with an aluminum foil in 12µm thickness on
the surface to avoid exposure to X-rays or laser pulses.
The electron signals were accumulated for 300 shots.
The energy spectra of the generated electrons were
obtained by a compact magnetic electron spectrometer
set on the laser axis behind the jet.
OAP
Self-injection Scheme
Wall
D= ~1mm
DC
Channel
D<30µm
E > 40MeV, ~100pC
(Experiment)
axis
(Theory)
Relativistic Intensity
~1.5x1019Wcm-2
a0
~3.0
Laser Spot D~5µm
Ne
~5nF ~30kV
Typical e-density profile
in the plasma column
produced by fast Z-pinch.
17
-3
~5 x10 cm
Bθ ~104T/m
P< 5TW
FIGURE 9. Experimental setup of the channel-guided laser
wakefield acceleration (Future plan).
Figure 8(a)-(c) shows typical images of beam spot
on the bottom plate. In case of a laser pulse with 1ns
duration of the pre-pulse just before the main pulse
there was no electron beam generation as shown in (a).
In the second case of a 2.5ns pre-pulse, the narrow
cone energetic beam was generated and an enhanced
spot was observed as shown in (b). In the third case of
a 5ns non-monotonic pre-pulse, the beam was
exploded to pieces and smaller spots were observed as
shown in (c). The experimental results clearly show
that the beam generation depended strongly on the
laser pre-pulse, and a proper laser pre-pulse condition
is essential for generation of the narrow-cone energetic
electrons. In the case of (b) the maximal energy of the
beam we observed was 40MeV(detection limit).
REFERENCES
1.
2.
3.
4.
As a future plan, we will try further acceleration of
the electron bunch from the laser-plasma cathode
using channel-guided laser wakefield acceleration [8].
Figure 9 shows a setup of the channel-guided laser
wakefield acceleration experiment. A capillary
discharge plasma will be used for optical guiding
channel [9]. The electron bunch from the gas jet is
accelerated to higher energy through the capillary.
5.
6.
7.
8.
9.
FIGURE 8. Typical images of the bottom plate of the cupshaped I.P. detector. A laser pulse with 1ns duration of the
pre-pulse just before the main pulse(a), 2.5ns (b) and 5ns (c).
971
M. Uesaka et al, Trans. Plasma Sci. 28 (2000)
pp.1133-1142
T. Kobayashi, M. Uesaka, Y. Katsumura, Y.
Muroya, T. Watanabe, T. Ueda, K. Yoshii, K.
Nakajima, X. Zhu, M. Kando, Journal Nucl. Sci.
Tech., 39 (2002) 6
T. Watanabe, M. Uesaka et al., Nucl. Instrum.
Meth. A 437 (1999) 1.
T. Watanabe, et al., Proceedings of Advanced
Accelerator Concepts Workshop 2002,Oxnard,
CA
M.S. Zolotorev and G.V. Stupakov, SLAC-PUB7132 (1996).
S. Bulanov, N. Naumova, F. Pegoraro, and J.Sakai,
Phys. Rev. E, 58, R5257 (1998)
N. Hafz, M.Uesaka, J.Koga, and K.Nakajima,
NIM. A, 455, 148 (2000)
A.J.Reitsma, W.P.Leemans, E.Esarey, C.B.
Schroeder, L.P.J. Kamp, and T.J.Schep. Phys. Rev.
ST-AB, 5, 05131 (2002)
T.Hosokai, M.Kando, H.Dewa, H.Kotaki,
S.Kondo, K.Horioka, and K.Nakajima, Opt.Lett.
25,10 (2000)