Identification of Ions Generated by Ultrashort
Laser Pulses using Thomson Spectrometer
Yuji Oishi*, TakuyaNayuki*, Takashi Fujii*, Koshichi Nemoto*,
Tsutomu Kayoiji f , Kazuhiko Horiokaf ,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,
v
Midori-ku, Yokohama 226-8502, Japan
Abstract. Fast ions generated by irradiation of laser pulses of width 55 fs, intensity 8.6X 1018
W/cm2 on 5 JJL m thickness copper film were measured by use of a Thomson mass spectrometer.
From the spectgram, ions ejected from the target surface which was opposite side of the laser
irradiation were determined to be protons. Copper ions were not observed. From the enegy
measurements using Mylar filter method, the maximum proton energy was estimated more than
650 keV.
INTRODUCTION
High-energy particle generation by T-cubed laser is an attractive current issue and
many studies are reported [1,2]. However, most of their pulse durations are
subpicoseconds, and few investigations have been conducted with pulse durations of
several ten femtoseconds. Using shorter pulses, we can generate high-energy particles
with a smaller energy, so that the laser system becomes more compact and irradiation
at higher repetition rate is possible. Thus, it is important to clear whether high-energy
particles are accelerated in several ten femtoseconds.
Previously, we have measured the energy spectra of fast electrons produced by
ultrashort, high-intensity laser pulses using a magnetic spectrometer with an imaging
plate [3]. Relativistic electrons with energies more than 1 MeV were observed for
irradiation on 30 y, m thick copper film by pulses of width 43 fs, intensity 2.7 X 1018
W/cm2, repetition rate 10 Hz without artificial prepulses. The typical temperature of
electrons was estimated to be 350 keV and was found to be close to the ponderomotive
potential.
Here, we report the measurement of energetic ions from copper film target by use
of the same laser system. Identification of generated ions was conducted by a
Thomson mass spectrometer. The maximum energy of ions was also estimated using
Mylar filter method.
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
317
EXPERIMENTAL SETUP
Figure l(a) shows a schematic drawing of the experimental setup. The experiments
were performed using a 4 TW Ti:Sapphire laser, which is based on the CPA technique
and is able to deliver up to 200 mJ, 43-fs pulses at the wavelength of 780 nm with a
repetition rate of 10 Hz [4]. The contrast ratio of the main pulse to the undesirable
small prepulse that precedes it by 8 ns is greater than 106. In this study, the delivered
energy to the vacuum chamber was 185 mJ. The p-polarized laser beam was incident
at the angle of 30° relative to the perpendicular to the target surface and was focused
down to a spot size of 5 / z m X 10 ^m in FWHM with an f/3 (f e ff=152.4 mm) offaxis parabolic mirror. The maximum intensity was estimated to be ^8.6 X 1018 W/cm2.
Thin copper film, of which thickness was 5 # m, was used as a target. The copper
film was translated for every shot so that new fresh surface was irradiated. Ions ejected
from the target surface which was opposite side of the laser irradiation were
collimated by two 500 /z m pinholes and passed through a Thomson mass spectrometer.
A Thomson mass spectrometer analyzes ions into different energy and ratio of mass to
charge. Ions with the same charge to mass ratio reach the same parabola on the
detector plane with higher energy ions closer to the center. The schematic diagram of
our Thomson mass spectrometer is shown in Fig. l(b). When a high voltage was
applied to the electrodes, parallel electric and magnetic fields existed between the
electrodes, deflecting ions according to their energies and ratios of mass to charge.
The electric and magnetic fields were 3.3X10 5 V/m and 5.1 k Gauss, respectively.
The deflection length by the fields was 4.5 cm. A nuclear track plate as known CR-39
located 10 cm from the center of the deflector was used as the ion detector. The CR-39
is sensitive to ions with energies greater than 100 keV/nucleon. The tracks on the CR39 were observed with an optical microscope and a CCD camera after etching for 6
hours in 6.25 N NaOH solution.
Thomson
mass
spectrometer t=
(a)
318
.CR-39
Pin hole
Ions
(b)
FIGURE 1. (a) Experimental setup, (b) Thomson mass spectrometer
RESULTS AND DISCUSSIONS
Figure 2 shows the experimental results of the mass spectrograms. The magnetic
and the electric fields deflected ions toward horizontal and vertical directions,
respectively. A thin black curve is observed in the left hand side of Fig. 2(a). The
magnified image of this curve is shown in Fig. 2(b). From this figure, a black curve is
determined to be a Thomson parabola, because (i) the curve consists of ion tracks, (ii)
the width of the curve was about 500 jim, which corresponded with the diameters of
the pin holes, and (iii) the experimental curve was very close to the theoretical proton
parabola, which is shown as a broken line in Fig. 2(a). The origin of the protons was
derived from contaminants on the target surface. Parabolas for copper ions were not
observed, although the target was made of copper.
FIGURE 2. (a) Thomson spectgram for proton, (b) Magnified image of (a)
319
Figure 3 shows the energy spectrum of fast protons estimated by the counting of
track numbers in the parabola. The counting of the number was conducted at intervals
of 500 IJL m in the horizontal direction and the numbers of protons were translated into
the number per keV per sr. Proton intensity gradually decreases with increasing kinetic
energy and drops sharply around 210 keV. The maximum energy was estimated to 250
keV from the spectrum. The total number of protons was 1.1 X 109 /sr/shot.
109
108
I
Q.
•s
1_
0
10'
"E
106
100
150
200
250
300
Proton energy (keV)
FIGURES. Spectrum of fast protons.
In another experiment, we measured the maximum energy of protons using steps of
Mylar filter instead of a Thomson mass spectrometer. Figure 4(a) shows the
experimental setup. The CR-39 was covered with seven steps of Mylar filter of
thickness from 2.5 to 17.5 /zm at 2.5 /am intervals. The spatial distribution of the
thickness of Mylar filter looking from the ion beam axis direction is shown in Fig.
4(b). Figure 4(c) shows the image of protons penetrated the steps of Mylar filter.
Regions where protons reached are dark and where protons could not reach are light.
From the figure, it is evident that fast protons with energies more than 650 keV were
generated, while protons with energies more than 780 keV were not generated. The
cutoff energies by the filter were calculated using a program PSTAR [5].
Maximum energies of protons measured by the Thomson mass spectrometer and
measured using Mylar filter method are different, and both values are less than those
reported before [6-11]. Protons with energies of several MeV are reported for laser
intensities of order 1018 W/cm2 and 10 MeV are expected for our laser intensity of 8.6
X 1018 W/cm2, according to experimental equation E pmax - 1.2X 10" 2 XI ° 313 [keV]
suggested by F. N. Beg et al [6]. Thus, it seems that protons are not so accelerated
with pulse durations of several ten femtoseconds as to be with subpicosecond
durations, although laser intensities are same order. However, our values were the
preliminary results and higher energies can be obtained by optimizations. Further
320
experiments and optimizations are needed to discuss in more detail.
Vacuum
ILO
Mylar lllar
[/* rn]
(1)
(c)
FIGURE 4. (a) Experimental setup for measurement of proton energy with Mylar filter, (b) Spatial
distribution of thickness of Mylar filter looking from the ion beam axis direction, (c) Image of protons
penetrated the steps of Mylar filter. Protons reached in dark regions.
SUMMARY
In summary, fast ions were generated by irradiation of ultrashort (55fs) laser pulses
on copper film. From the Thomson mass spectgram, ions ejected from the copper
target surface which was opposite side of the laser irradiation were found to be protons.
Copper ions were not observed. From the energy measurements using Mylar filter
method, the maximum proton energy was estimated more than 650 keV. However, this
value was the preliminary result and higher energies can be obtained by optimizations.
321
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.
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