Fast Electron Production by Irradiation of
Ultrashort Laser Pulses on Copper
Yuji Oishi*, TakuyaNayuki*, Takashi Fujii*, Koshichi Nemoto*,
Tsutomu Kayoijif ,Yasuaki Okano ^, Yoichiro Hironaka ^ ,
Kazutaka G. Nakamura ^ , and Ken-ichi Kondo ^
* Central Research Institute of Electric Power Industry, 2-11-1, Iwado kita, Komae-shi,
f
Tokyo 201-8511, Japan
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259,
Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan
^Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho,
Midori-ku, Yokohama 226-8502, Japan
Abstract. Fast electrons were produced by laser irradiation of 43-fs, 2.7X 1018 W/cm2 on a 30
li m thick copper target. The energy spectra of the electrons were directly measured using a
magnetic spectrometer with an imaging plate. The typical temperature was 350 keV for 15°
irradiation and was found to be close to the ponderomotive potential at the intensity 2.7X 1018
W/cm2. The energy spectra of high-energy photons, which are expected to be produced from the
electrons, were also calculated.
INTRODUCTION
The chirped pulse amplification technique opened new fields of laser interaction
physics including high energy particle generation such as electrons, x-ray photons,
ions, neutrons, and positrons[l]. Among them, the electron generation is the most
basic phenomena because other particles are generated by the interaction of electrons
with matters. Although relativistic electrons are obtained using a subpicosecond pulse
laser, when shorter pulses are used, a smaller energy laser can obtain the intensity
which is necessary for production of relativistic electrons. The result is that laser
systems become compact and high repetition rate irradiation is possible.
Recently, several groups reported the generation of fast electrons using pulse
durations of several ten femtoseconds. Gahn et al.[2] estimated 0.9 MeV as the
temperature of electrons which were produced by a 130-fs pulse of intensity higher
than 1018 W/cm2 when an artificial prepulse of an order of 1016 W/cm2 was combined.
Without an artificial prepulse, Schworer et a/.[3] measured the angular variation of the
hard x-ray spectrum at 5 X 1018 W/cm2 using a 60-fs laser, and estimated an electron
temperature of 0.7 MeV in the specular direction and 0.3 MeV in the laser forward
direction. However, the electron temperatures of these experiments using shorter
pulses were not directly estimated, but estimated from photon spectra.
CP634, Science of Superstrong Field Interactions, edited by K. Nakajima and M. Deguchi
© 2002 American Institute of Physics 0-7354-0089-X/02/$ 19.00
311
Here, we report on results of direct measurements of fast electrons penetrating
through copper film targets using 43 fs, 90 mJ pulse without an artificial prepulse [4].
The energy spectra of electrons were measured using a magnetic spectrometer with an
imaging plate (IP), which is used for electron microscopy and x-ray imaging. In
addition, spectra of high-energy photons originating from the electrons were simulated
byGEANT4[5].
EXPERIMENTAL SETUP
The diagram of the experimental setup is shown in Fig. 1. The experiments were
performed using a 4 TW Ti:Sapphire laser at the Materials and Structures Lab. in
Tokyo Institute of Technology [6], which is based on Chirped Pulse Amplification
technique and able to deliver up to 200 mJ, 43 fs pulse duration at the wavelength of
780nm with a repetition rate of 10 Hz. The contrast ratio of the main pulse to the
undesirable small prepulse that precedes it by 8 ns is grater than 106. In this study, the
delivered energy to the vacuum chamber was between 85 and 100 mJ. The p-polarized
laser beam was incident at 15 and 45 ° to the normal of the target and focused down
to a spot size of 5 \i m X 20 ^ m in FWHM, with an f/3 (feffective=152.4 mm) off-axis
parabolic mirror. Therefore, the maximum intensity was estimated to be ^2.7X
1018W/cm2. Tape-like targets (30 /zm thick) made of copper were irradiated and its
surface was translated for every shot. The surface displacements of the target caused
by tape movement was measured to be less than ±10 y, m which was small enough
compared with the collimated range of 90 jz m. The copper target was chosen because
this was the well-known material in the previous experiments for X-ray generation.
The energy of electron with the projectile range of 30 jzm is 120 keV, therefore,
electrons higher than 120 keV was expected to be able to penetrate through the copper
target from laser irradiation side to backside and to be captured by an electron detector.
Imaging
plate
Pb block
ThSapphire1
laser
FIGURE 1. Experimental setup
312
The electron energy spectrum was measured with a magnetic electron spectrometer
located 13 cm from the backside surface of the target. The energy of electron deflected
by 180° was derived from the two dimensional calculation of electrons trajectories
using a map of the measured magnetic field, which was about 1.3 kG. Because of
mechanical structure of a tape drive equipment, only electrons emitted to the target
normal direction were detected by the spectrometer. The slit width of the spectrometer
entrance was 5 mm, so an energy resolution is calculated to be about ±50 keV. The
IP [Fuji Film FDL-UR-V] were used to monitor the electron flux and were covered
with 15 ^ m thick aluminum film as a light shield. A lead block was used to shield the
imaging plates from X-ray generated by laser produced plasma. The electron flux was
calibrated using the sensitivity data provided by the manufacturer in the energy range
between 50 keV and 1 MeV. For the range higher than 1 MeV, the electron flux was
corrected by a linear extrapolation of the sensitivity data. The IP sensitivity has a
maximum value at 150 keV and gradually decreases to about 0.18 of the maximum
value at 1 MeV.
We obtained very close signal intensities per one shot with 100 times different shot
numbers, therefore, the response of an imaging plate was linear enough and the
deviation of imaging plates did not cause any significant errors to the temperature
measurements. The background noise obtained by reversing the magnetic field was
negligible.
RESULTS AND DISCUSSIONS
Figure 2 shows an example of an electron image recorded on the IP. Regions where
electrons reached are dark. From this figure, it is evident that relativistic electrons with
energies more than 1.0 MeV were generated. The energy spectra of electrons for laser
irradiation at 15° and 45° are shown in Fig. 3. These spectra were obtained with 20
shots in both cases.
$
10
Com }
FIGURE 2. Electron trajectories and electron image recorded on the imaging plate attached to
spectrometer.
313
——— Irradiation at 15°
- - - - Irradiation at 45°
_ — Ponderomotive
theory
•e<o
o
-S103
102
400
800
1200
Electrn kinetic energy (keV)
FIGURE 3. Spectra of fast electrons penetrate the 30 \i m thick copper target measured at normal
direction to the target surface for laser irradiation at 15° and 45° . The Broken line shows
ponderomotive scaling.
The electron temperatures were estimated from fitting to the Boltzmann distribution
in the range between 500 keV and 1 MeV because in the low energy region around
200 keV, the sensitivity of the IP changes significantly. The estimated temperatures
were 350 keV for the irradiation at 15° and 300 keV for the irradiation at 45° . The
broken
line
shows
ponderomotive
scaling
described
by
2
2
2
2
18
kBTh -m 0 c ( N /l + M [fF/cm -//m yi.38xl0 -l) [7]. Although the laser intensities of
these experiments were on the border of the relativistic region where the
ponderomotive force is dominant, the obtained electron temperatures were close to the
ponderomotive potential of 250 keV. We observed fast electrons for the irradiation at
15° , which was not expected to cause significant resonance absorption. Therefore,
the main acceleration mechanism should be due to the ponderomotive force. Figure 4
shows the relation between laser intensities and electron temperatures. Our result is
marked as an asterisk, which is almost on the curve of the poderomotive scaling.
These fast electrons can produce high-energy photons by bremsstrahlung, and the
estimation of photon energy and yield is important for photon applications. The
photon spectrum for the 30 JJL m thick copper target was calculated using a simulation
program GEANT4 and the results are shown in Fig. 5. In this calculation, x-rays were
generated by electrons which were assumed to be accelerated to a high temperature of
350 keV in the direction normal to the target surface and the cut-off electron energy
was assumed to be 3 MeV. The estimated temperature of the x-ray spectrum was 260
keV. In Schwoere's experiments,10 a laser pulse at an intensity of 5.0><1018
W/cm2 with 60 fs duration was irradiated on a 1 mm thick tantalum target and x-ray
temperature was estimated at 200 keV. We also calculated a photon spectrum for a 1
mm thick tantalum target, by which efficient x-ray production can be expected. The
calculated temperature was 250 keV, which was close to the results of Schwoerer et al.
314
[3] and the estimated intensity of photons was ten times larger than that for 30 n m
thick copper target, as indicated in Fig.5.
1000
100
10
a
CD
1. Ponderomotive Force
2. J.Yu. etal. Phys of Plasmas 6(4) 1318 (1999)
3. Q.Malka. etal.. PRL 77(1) 75(1996)
4. F.N.Bei. etal.. Phys of Plasmas 4. 447(1997)
5. C.Gahn, etal.. APL 73(25) 3662(1998)
6. T.E.Cowan, et.al, PRL 84(5) 903(2000)
7. PZhang, etal.. PRE 57(4) 57(1998)
8. S.Bastiani. etal.. PRE 56(6) 7179(1997)
9. C.Rouseeaux. etal.. Fhys. Fluids B4<8) 2589(1992)
10. W.Priedhorsky. etal.. PRL 47.1661(1981)
11. Our Results
8
0.1
0.01
0.001
10*
Laser intensity
IA 2 (w/cm2 x ^ m2)
FIGURE 4. Relation between laser intensities and electron temperatures. Our result is marked as an
asterisk.
c
Cu 30 II m target
104
Ta 1mm
>
<D
o
target
102
10°
Q.
•5
i_
<D
_Q
ID'2
10~4
1000
2000
3000
Photon energy (keV)
FIGURE 5. Calculated energy spectra of photons which are generated by the interaction of electrons
with 30 n m thick copper target and 1mm thick tantalum target.
SUMMARY
In summary, fast electrons were produced by laser irradiation of 43-fs, 2.7X 1018
W/cm2 on a 30 /z m thick copper target and the energy spectra were directly measured
315
using a magnetic spectrometer with an imaging plate. The typical temperature was 350
keV for 15° irradiation and was found to be close to the ponderomotive potential at
the intensity 2.7 X 1018 W/cm2. With ultrashort pulses, particles with energy
corresponding to the ponderomotive potential can be produced by a small laser.
Higher temperatures may be possible with an artificial prepulse, and a small and highrepetition ultrashort laser can be used for studies of high-energy particle generation.
ACKNOWLEDGMENTS
Authors would like to thank T. Fukuchi, S. Akita, S. Sasaki of Central Research
Institute of Electric Power Industry and T. Yamazaki of The Institute of Physical and
Chemical Research for their support and helpful discussions, and M. Hasegawa of
Tokyo Institute of Technology for experimental support. The sensitivity data of the
imaging plate was courtesy of Fuji film co. We also thank T. Suzuki of Univ. of
Tokyo for useful advice for simulation using GEANT.
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