24F13
Efficient Laser-Proton Acceleration from an Insulator Foil with an Attached
Metal Disk
局所的に金属蒸着した絶縁薄膜を用いたレーザー生成陽子線の高エネルギー化
Kazuto Otani2, Shunsuke Inoue1,2, Shigeki Tokita1,2, Masaki Hashida1,2 and Shuji Sakabe1,2
大谷一人, 井上峻介，時田茂樹，橋田昌樹，阪部周二
1Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
2
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
京都大学大学院理学研究科 〒606-8502 京都市左京区北白川追分町
京都大学化学研究所 〒611-0011 宇治市五ヶ庄
A new scheme of laser-proton acceleration under the interaction between a solid foil and an intense
femtosecond laser pulse is demonstrated. A polyethylene foil (88 mm in diameter and 10 m in thickness)coated with 0.2 m thick aluminum around laser spot is employed to the target. As reducing the coating
size from 88 mm to 150 m in diameter, the proton acceleration was enhanced. From the measurement offast electron distribution under some different aluminum disk size, the sheath field distortion was suggested.
We consider that the target conductivity affects to the sheath confinement and proton acceleration.

1. Introduction
Proton acceleration under the irradiation of an
intense femtosecond laser pulse is expected toseveral fields such as scientific, medical, andindustry [1]. Some schemes have been proposed
to increase the proton energy [2]. The mechanismof laser-proton acceleration is generally
understood by target normal sheath acceleration(TNSA) model under the intensity of ~10 19
W/cm2 [3]. Since it is considered that the protonis accelerated by not only fast electron but also
positively-charged foil, confinement of charge ofthe foil would be effective to enhance the proton
acceleration [4]. If the positive charge diffuses in
the conductive plasma and target foil, isolation of
the target in vacuum is a solution to keep thecharge much longer than a large foil. The effectof conductivity of the target foil to the proton
acceleration by using a metal-coated plastic target
has been reported [5]. The proton spectra andfast-electron distributions from a plastic target
coated with aluminum coating of several different
sizes are shown in this presentation.
2. Experiments
The experiment has conducted on T6, Ti:sapphirelaser system in Institute for Chemical Research,Kyoto-university. The central laser wavelength was
800 nm, and the pulse duration was 150 fs. Thelaser pulse was focused on the target with the spot
size of 3 × 5 m in FWHM by an F/3.2 off-axis
parabolic mirror. The laser intensity and energy at
the focal spot were calculated to 2 × 1018 W/cm2, 40mJ, respectively. The laser contrast was 10-7 of themain pulse intensity at 200 ps prior to the mainpulse. The laser pulse was irradiated with the angleof 10 degree from the normal incidence of the target
foil.
Three types of target foil were employed to theexperiment. The first is a polyethylene (PE) foilwith the thickness of 10 m as standard target. The
second target was PE foil coated whole irradiation
surface with 0.2-m-thick aluminum. The third
target was PE foil coated with aluminum as thick as
second target around the laser spot. The diametersof the aluminum coating were from 0.15 to 15 mmon third target. Note that these targets are called toPE, Al-coated, and Al-disk foil in this presentation.
The target foils were fixed on 8.8 cm metal ring,and corrected its position within 10 m in RMS.
The ion spectrum and fast electron distribution
were observed in experiment. The ion spectrum
emitted from opposite side of laser-irradiation
surface was measured by using a Time-of-Flight(TOF) technique. The scintillator and photo
multiplier tube was set 2.0 m away from the target
foil. The fast electron was deflected by a pair ofdipole magnet set at the midstream of the TOF tube.
The fast electron distribution was measured byimaging plates (IPs). The IPs were arranged at adistance of 80 mm behind the target over 180
degree horizontally on the laser incidence plane.
The IPs were stacked in two layers, and covered
with 11 m-thick aluminum filter. The second IP
was sensitive for electron with the energy of morethan 400 keV.

Al-coated foils are considered to be induced bydifference of the sheath electric field. The
relaxation of sheath field is affected by the disk size.
The positive charge would diffuse in the disk as
flowing of electron into the charged area, then thefinal peak intensity of the positive charge is reduced
and the area is broadened. The electric field near thetarget foil is distorted from radial to parallel
direction. As the result, much fast electron with thelateral momentum to the target can escape from thecharged target.
Fig. 1. Ion spectra from various target foils irradiatedby a laser pulse.

3. Results and Discussion
The ion spectra are shown in Fig. 1. In these
spectra, the upper half signal is composed only
proton. The maximum proton energy is 1.3 MeV
from PE foil. The proton energies from Al-disk foil
are increased more than twice of that from PE foil.
It is due to the fast electron yield is increased byemploying metal coating. With increasing the disk
diameter of aluminum coating, the maximum
energy is reduced. The proton energy fromAl-coated foil is lower than one from PE foil,
despite the proton yield with the energy of 0.7-1.0
MeV is significantly increased.
The fast electron distributions in horizontal planefrom rear surface of the target foil are shown in Fig.
2. The distribution from PE and Al-disk foil
(diameter: 200 m) are similar. The intensity of theelectron signal from Al-disk foil is larger than PE
foil because the yield of fast electron is considered
to be increased by aluminum coating. The electron
distribution from Al-coated foil is significantly
different from the others. Especially the electron
signal in lateral direction is much increased.
Since the experimental result of fast electron
distribution, differences between the Al-disk and180210150

24012090°

270050PE foilAl-coated foil
Al-disk foil (200 m in dia.)

100x 10150-11(a.u.)

Fig. 2. Fast electron (E > 400 keV) distribution fromrear side of target foil.

4. Summary
A new scheme to enhance the proton acceleration
under an intense femtosecond laser irradiation isdemonstrated. The proton energy was clearly
increased from a plastic foil restrict coated withaluminum. The proton energy is related to thecoating size. The sheath field distortion is suggestedby the measurement of the fast electron distribution.
In Al-disk target, the sheath field is possibly
confined by restricting of conductive area.
This work was supported by Global Center ofExcellence (GCOE) program of graduate school ofscience, Kyoto University, “the next generation ofphysics, spun from universality and emergence”
from the Ministry of Education, Culture, Sports,
Science and Tecnology (MEXT) of Japan.
References[1] S. Fritzler, V. Malka, G. Grillon, et al., Appl. Phys.
Lett. 83, 3039 (2003); J. Mackinnon, P. K. Patel, M.
Borghesi, et al., Phys. Rev. Lett. 97, 045001 (2006).
[2] R. A. Snavely, M. H. Key, S. P. Hatchett, et al., Phys.
Rev. Lett. 85, 2945 (2000); M. Nishiuchi, H. Daido,A. Yogo, et al., Phys. Plasmas 15, 053104 (2008); K.
Lee, S. H. Park, Y. –H Cha, et al., et al., Phys. Rev.
E 78, 056403 (2008); H. Kishimura, H. Morishita, Y.
H. Okano, et al., Appl. Phys. Lett. 85, 2736 (2004);
M. Kaluza, J. Shoreiber, M. I. K. Santala, et al., Phys.
Rev. Lett. 93, 045003 (2004).
[3] S. C. Wilks, A. B. Langdon, T. E. Cowan, et al.,Phys. Plasmas 8, 542 (2001).
[4] T. Toncian, M. Swantusch, M. Toncian et al., Phys.
Plasmas 18, 043105 (2001); S. Buffechoux, J. Piskal,
M. Nakatsutsumi, et al., Phys. Rev. Lett. 105,015005 (2010).
[5] K. Otani, S. Tokita, T. Nishoji, et al., Appl. Phys.
Lett. 99, 161501 (2011)