The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

FAST PROTON GENERATIONBY LASER IRRADIATION OF THIN FILMS
Tetsuo Kubota, Yoshio Wada, Koji Matsukado 1, Kensuke Homma2,Fumitaka Nishiyama3, Akio Seiki, Yuji Tsuchimoto2 , Atsushi Ogata
Graduate School ofAdSM, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
1Advanced Photon Reserch Center,Kansai Reserch Establishment, JAERI, Kyoto 619-0215,Japan
2

Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
3 Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739-8526, Japan

parallel to the target surface. A Faraday-cup is also used
for the proton detection.

Abstract
Fast proton generation by laser irradiation of plastic
films such as polyvinylidene chloride, mylar etc. isinvestigated. We irradiated 1 TW, 50 fs, 800 nm laser
pulses onto thin film surfaces. Energy of protons
generated normal to the film surface is analyzed by CR39.
Protons with energy up to 1 MeV were observed
perpendicular to the thin film.

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laser

1. INTRODUCTION
Recently, high-energy ion generation up to several tensof MeV/nucleon by high-intensity laser irradiation ofsolid target is reported by Michigan University [1],Rutherford Appleton Laboratory (RAL) [2-4], Lawrence
Livermore National Laboratory (LLNL) [5-7]. There arealso many theoretical studies [8-10]. These experiments
use lasers whose powers exceed 10 TW and pulse lengths
exceed 100 fs. If this phenomenon were reproduced by acompact laser, it could be applied in construction of anion/proton source of an accelerator. This is the motivation
to start the present work.
The next section describes our laser system andexperimental setup. Section 3 describes the results of theexperiments and their discussion. Section 4 describes thepresent summary.

Fig. 1. Schematic of the experimental setup.
We have found that, if both the target and the CR39 are
put on the laser axis normal to it, we cannot distinguishthe tracks produced by laser and those by protons.
The target is bored by a single irradiation of the laser,
so it is moved after each shot so that the laser pulse hits
the virgin surface. A shutter extracted a fewer pulses fromthe 10 Hz pulse train lest the target should be exposed tothe laser during the move.
Sensitivityi II
I I i IIiIIII ~r{lI!

2. EXPERIMENTAL SETUP
Our experiments used a 1 TW, 50 fs, 800 nm laser with
10Hz pulse frequency. This laser system equips no pulse
cleaner, so there exists a pre-pulse at ns area with 100 fs
to 1 ps pulse duration and 5 J.1l to 50 J.1l pulse energy.
Figure 1 is the schematic of the experimental setup. Acylindrical vacuum vessel with 180 mm in radius and 170
mm in depth is used under typically pressure of 2 X 10"3Pa. An f = 300 mm lens outside of the vacuum vessel
focused the laser enabling the peak intensity of 5 x 10 16 10 17 W cm· 2 at the focal spot with diameter of - 20 f.lm.
We used as the target polyvinylidene chloride (C 2H2Clz)nfilms with thickness of -10 f.lm and density of 1.7 g cm-3 ,and mylar (polyethylene terephthalate, (C 10H80 4 ) 0 ) films
with various thickness. We put the target surface at anangle of 45° to the laser axis, and put CR39 detectors [11]100!Ig)

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proton energy

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0.15

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0.05
50(MeV)t(
100.150

2002SO300

REL(MeV cm2g" 1)

Fig. 2. Left is the relation between proton energy andrestricted energy loss (REL) at CR39. Right is the relation
between restricted energy loss (REL) and sensitivity at
CR39.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

We mainly used CR39 [11] for energy measurement.
It is a plastic nuclear track detector sensitive only to ionswith energy greater than 100 keY/nucleon. The irradiatedCR39 plates were etched in NaOH solution. Typicalconditions of the etching are use of 5N solution for 15hours at 70 °C.
Figure 2 was obtained by exposing proton beams from
Van de Graaff accelerator in Hiroshima University. It
gives the relation between proton energy and restrictedenergy loss (REL is linear energy transfer (LET)
including effects of recoil electrons), and that between
REL and sensitivity at CR39. Sensitivity is defined by
S = 2D 2 /( 4B 2 - D 2 ), where D is a pit diameter and B is
the etched depth ofunbombarded surface.
We measured D and B, and convert the data into
particle energies using Fig. 1. We have assumed that theall pits are caused by protons.

We then recorded the proton tracks caused by ten
successive laser pulses onto five pieces of 20 mm x 10mm CR39 plates located 150 mm behind the target.
Figure 3 shows the distributions of the etched tracks
whose diameters are larger than 5 jlm. It contains threefigures corresponding to three proton energy regions; <0.2 MeV, 0.2- 0.5 MeV, and> 0.5 MeV. We found thatthe protons with higher energies concentrate around thecenter of the distribution.
Figure 4 shows the energy spectrum. Horizontal error
bars were estimated from Fig. 2. Decrease of the numberof pits at the low energy side is due to the lack ofsensitivity of the detector against low energy protons. We
can observe here protons with energies exceeding 1 MeV.
number of pits,.··,

3. RESULTS AND DISCUSSION
In the first experiment using polyvinylidene chloride
targets, we observed the Faraday-cup signal by an
oscilloscope, in which a first negative signal was takenover by a large positive signal. The maximum value ofpositive signal appeared 20 ns after the peak of thenegative signal. Assuming that negative signal is caused
by electrons with the light speed, and that the positive
signal is caused by protons, we calculate the mean energyof proton as - 300 ke V.
<0.2MeV166 (mrad)

0-166-3300(mrad)

330··f· ···+.

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energy (MeV)

Fig. 4. Energy spectrum of protons
polyvinylidene chloride targets.

fromtheFigure 5 shows the energy spectra obtained using mylar
targets with various thickness, recorded by a single laser
shot. The length between the targets and CR39 was 45mm. Figure 6 summarizes the data in Fig. 5. It gives thenumber of the traces and the slopes of the fits or the ion
temperature defined by lie, in which 1 mm2 correspond to10-4 str. The thinner foils give the more particles and thelower temperature. Tracks concentrate on the area 10 mmin length and 5 mm in width. The total number ofparticles is - 25,000, if the average particles density is 500mm- 2 .

0.2 MeV- 0.5 MeV
166 (mrad)

0-166-3300(mrad)

330>0.5MeV166 (mrad)

00(mrad)20-166-330

330

energy (MeV)

FIG. 3. Distributions of the etched tracks on CR39 causedby polyvinylidene chloride target in three proton energy
regions.

FIG. 5. Energy spectra of protons from mylar with variousthickness.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

the maximum proton energy exceeds 1 MeV.
It has been reported that plasma generated by apre-pulse lowers the energy of the protons [7], because
the electric field gradient decreases as the plasma expands.
Though our laser has prepulses, we do not observe any
harmful effect of it. In order to clarify the effect of thepre-pulses, we plan to equip a pulse cleaner in the laser
system which kills them.
We are going to introduce a Thomson parabola toobtain two-dimensional spectra of mass and energy of theions [14]. We expect that it will give better energy spectra.
Moreover, it will enable us to locate both the target andthe CR39 detector on the laser axis. Hitherto experiments
report that the normal incident laser produces ions withhigher energy. We will verify it in the nearest future.

ion temperature (ke V)
pit density (mm- 2)

600
500
400
300

100············-·--·+I
12film thickness (J.lm)

FIG 6. The number of particles (solid line) and ion
temperature (broken line) dependences on myler filmthickness.
In spite that our laser has smaller energy and shorter
pulse width than those of other experiments [ 1-7], Figures
4-5 show that the maximum proton energy exceeds 1MeV. Certainly, there have been some experiments [3]and simulations [8] reporting the production of MeV
protons at the laser intensity of 10 16- 17 W cm- 2, but theshortest laser pulse width among their works were around
1 ps.
According to the LLNL model [9], the proton energy
gained at rear side of the irradiation is proportional to theelectron temperature. No data of the electron temperature
are available. If we estimate it using the formula That- [(In 2I
2.8xl0 rs ) v2 - l] x 511 keY [12], That becomes+ 11\,
about 6 ke V. The proportion constant between the protonenergy and the electron temperature lies between 2 and 12[9]. Even if we take the maximum, the proton energy is
only 70 keY in this model.
It has been said that the energy identification using theCR39 detectors requires good skill. We however have anindirect evidence that support the maximum protonenergy. We placed AI foils (0.8 f.!m, 5.0 J.!m or 2 x 5.0 J.!m)in front of CR39. The 0.8 J.!m and 5 J.!m AI foil was drilled
to have holes, the numbers of which are almost equal to
the CR39 tracks. However, when we placed two 5.0 J.!m
The SRIM
foils, the second foil was not drilled.
code[ 13] tells that the range of 450 keV proton is about
5.0 J.!m in AI. The maximum proton energy should belarger than - 500 ke V.

4.SUMMARY
We have studied fast proton generation by 1 TW
laserirradiation of thin films. The CR39 detectors tell that

ACKNOWLEDGMENTS
The authors would like to thank Ryoko Katsube, for her
measurement of energy response of the CR39, Shuji
Sakabe for laser tuning, Nakahiko Yasuda for technicalsupport of CR39 etching and Hiromi Okamoto forvaluable discussions. The present work is supported bythe JAERrs Nuclear Research Promotion Program.

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