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Сер. 10. 2011. Вып. 1
ВЕСТНИК САНКТ-ПЕТЕРБУРГСКОГО УНИВЕРСИТЕТА
UDK 533.9
S. Kawata, K. Takahashi, D. Sato, D. Barada, A. A. Andreev, O. Klimo, J. Limpouch, Y. Y. Ma, Z. M. Sheng, W. M. Wang, Y. T. Li, Q. Kong, P. X. Wang
EFFICIENT ION BEAM GENERATION IN LASER FOIL INTERACTION -TOWARD A CONTROLLABLE LASER ION ACCELERATOR*)
Efficient Laser Ion Generation by a Sub Wavelength Structure. By chirped pulse amplification, higher laser intensity has been realized, and high intensity short pulse lasers
Kawata Shigeo — PhD, Professor of Utsunomiya University, Yohtoh 7-1-2, Utsunomiya 321-8585, Japan. Number of published papers: 130. Scientific directions: laser particle acceleration, ion beam inertial fusion, computer assisted simulation system and optical physics. E-mail: kwt@cc.utsunomiya-u.ac.jp.
Takahashi Kohki — B. Engineering, a master student of Utsunomiya University, Yohtoh 7-1-2, Utsunomiya 321-8585, Japan. Scientific adviser: Prof. Shigeo Kawata. Number of published papers: 1. Scientific direction: laser ion acceleration. E-mail: mt096629@cc.utsunomiya-u.ac.jp.
Sato Dai — B. Engineering, a master student of Utsunomiya University, Yohtoh 7-1-2, Utsunomiya 3218585, Japan. Scientific adviser: Prof. Shigeo Kawata. Number of published papers: 1. Scientific direction: laser ion acceleration. E-mail: mt106639@cc.utsunomiya-u.ac.jp.
Barada Daisuke — PhD, Assistant professor of Utsunomiya University, Yohtoh 7-1-2, Utsunomiya 321-8585, Japan. Number of published papers: 25. Scientific directions: optical sciences and laser particle acceleration. E-mail: barada@cc.utsunomiya-u.ac.jp.
Andreev Alexander A. — PhD, Professor of Vavilov State Institute, St. Petersburg, Russia. Number of published papers: about 100. Scientific directions: laser plasma interaction, laser particle acceleration. E-mail: alex2_andreev@yahoo.com.
Klimo Ondrej — PhD, Associate professor of Czech Technical University Prague, Prague, Czech Republic. Number of published papers: about 40. Scientific directions: laser plasma interaction, laser particle acceleration and numerical computation scheme for plasma simulations. E-mail: ondrej.klimo@fjfi.cvut.cz.
Limpouch Jiri — PhD, Professor of Czech Technical University Prague, Prague, Czech Republic. Number of published papers: about 90. Scientific directions: laser plasma interaction, laser particle acceleration and numerical computation scheme for plasma simulations. E-mail: jiri.limpouch@fjfi.cvut.cz.
Ma Yan Yun — PhD, Associate Professor of National University of Defense Technology, P.R. China. Number of published papers: about 50. Scientific directions: laser particle acceleration and laser plasma interaction. E-mail: plasim@163.com.
Sheng Zheng Ming — PhD, Professor of Shanghai Jiao Tong University, Shanghai, P.R. China. Number of published papers: about 250. Scientific directions: laser plasma interaction, laser particle acceleration, laser plasma radiation generation. E-mail: zmsheng@sjtu.edu.cn.
Wang Wei Ming — PhD, Researcher of Institute of Physics, CAS, Beijing, P.R. China. Number of published papers: 30. Scientific directions: laser plasma radiation generation, laser particle acceleration. E-mail: hbwwm1@126.com.
Li Yu Ton — PhD, Professor of Institute of Physics, CAS, Beijing, P.R. China. Number of published papers: about 250. Scientific direction: laser plasma interaction experiments. E-mail: ytli@aphy.iphy.ac.cn.
Kong Qin — PhD, Associate professor of Institute of Modern Physics, Fudan University, Shanghai, P.R. China. Number of published papers: 60. Scientific direction: laser particle acceleration. E-mail: qkong@fudan.edu.cn.
Wang Ping Xiao — PhD, Associate professor of Institute of Modern Physics, Fudan University, Shanghai, P.R. China. Number of published papers: 45. Scientific direction: laser particle acceleration. E-mail: wpx@fudan.edu.cn.
The work was partly supported by MEXT, JSPS, ILE/Osaka Univ. and CORE (Center for Optical Res. and Education, Utsunomiya University).
© S. Kawata, K. Takahashi, D. Sato, D. Barada, A. A. Andreev, O. Klimo, J. Limpouch, Y. Y. Ma, Z. M. Sheng, W. M. Wang, Y. T. Li, Q. Kong, P. X. Wang, 2011
are now available for experiments and applications. In recent researches, a laser intensity of I > 1020 W/cm2 has been available. On the other hand, ion beams are useful for basic particle physics, medical therapy, controlled nuclear fusion, high-energy sources, and so on. The energy of ions which are accelerated in an interaction between an intense laser pulse and a thin foil target has been over MeV. The ion acceleration in the laser-foil interaction is expected to be a new method of ion acceleration [1]. The issues in the laser ion acceleration include ion beam collimation [2], ion energy spectrum control [3], ion production efficiency [4], etc. In this paper we focus on the energy conversion efficiency from laser to protons in the laser-ion acceleration, and so far the energy conversion efficiency was still relatively low in actual experiments.
When an intense short pulse laser illuminates a thin foil target, electrons are first accelerated by the laser, and oscillate or move around the thin foil. The electrons form a high current and generate a magnetic field. In the laser-foil interaction, the ion dynamics is affected directly by the behavior of the electrons. The electrons form a strong electric field, and the ions are accelerated by the electric field. When a foil target has multiholes at the target rear side and the hole size is the order of the laser spot size, the holes help to produce a collimated ion beam [2]. In this case the transverse divergence electric field is shielded by the hole wall so that a collimated ion beam is created. On the other hand, the foil target surface reflects a significant part of the laser energy. The energy conversion efficiency from laser to ions tends to be low. The subwavelength-multiholes transpiercing the planar target help to enhance the laser-proton energy conversion efficiency [5]. The subwavelength microstructured targets, such as the multihole foil target, clusters, nanotubes, foam, subwavelength gratings, etc, are propitious to enhance the laser energy absorption. In this paper we employ the subwavelength-multihole microstructure to increase the laser-proton energy transfer efficiency at the target rear side.
In our study, we employ an intense short-pulse laser and a double-layer tailored hole target, which consists of hydrogen and aluminum. The reason why we employ aluminum is to increase the number of generated energetic electrons. Aluminum ions are heavier than hydrogen, and the aluminum supplies electrons more than hydrogen. Throughout the paper wider holes at the rear side are employed to ensure the collimated proton beam generation [2] (see fig. 1, a-c). In this paper, we perform 2.5-dimensional particle-in-cell simulations to investigate the improvement of the energy conversion efficiency from laser to protons in the laser-foil interaction. We clarify the role of the target hole thickness and depth at the laser side. The optimized multi-hole foil target provides a remarkable increase in the laser-proton energy conversion efficiency. The total laser-proton energy conversion efficiency becomes 16.7% in an optimized multi-hole target (see fig. 1, a), while a conventional planar foil target (see fig. 1, c) serves a rather low efficiency of a few percent.
Multi-Hole Target for Efficient Ion Generation. In this paper, we perform 2.5-dimensional (x, y, vx, vy, vz) particle-in-cell simulations. Figure 1, a shows a conceptual diagram of the multihole target. The laser intensity is I = 1.0 • 1020 W/cm2, the laser spot diameter is 4.0A, and the pulse duration is 20 fs. The laser transverse profile is in the Gaussian distribution. The laser wave length is A= 1.053 ^m. The simulation box is 80A in the longitudinal direction and 30A in the transverse direction. The mesh size is Ax = Ay = 0.02A. The free boundary condition is employed so that the boundaries do not reflect particles and waves. In this research, we employ a double-layer target, which consists of Al and H. The heavy material Al layer prevents the target deformation and supplies more electrons compared with the H layer. The ionization degree of the Al layer is 11. The initial Al target peak density is the solid one (n = i = 42nc) and the H layer density is flat and
a be
Multihole w/o left A1 holes Plain
A1 wing length
Fig. 1. Thin-foil targets a -a multihole target; b - a thin foil target without an Al hole layer; c - a plain target (a conventional target with a lain Al layer).
In fig. 1, c the conventional target has the Al plain layer with its thickness of 0.5A.
42nc. The Al layer has a linear density gradient of the Al thickness to include a laser prepulse effect. The initial particle temperatures are 1 keV. Here nc stands for the critical density.
In the simulations the Al thickness and its wing length in fig. 1, a are optimized. In the following sections we present simulation results and discussions on relating physics, and the results for the optimized multihole target demonstrate an extraordinary enhancement of the laser-proton energy conversion. In the section we study the role of the Al thickness and the Al wing length in the multihole target shown in fig. 1, a. At the target rear side the additional wider holes are installed to ensure the proton beam collimation based on our previous study [2] throughout the paper. Thin foil tailored targets, which have a subwavelength structure, enhance the laser energy absorption. In Ref. [5] we have proposed a multihole target for the laser proton generation, and we found that the laser-proton energy conversion efficiency became 9.3% from a few percent for a planar target which has no microstructures. In this paper we focus on and clarify the role of the target structure in the laser-ion production, and present a further significant increase in the laser-ion energy efficiency.
Figure 1, a shows the tailored multihole target. Firstly the Al thickness of the multiholes at the laser side is changed from 0.1 to 0.5A. For a comparison, we also simulate the target which has no multihole Al (see fig. 1, b). Figure 2 show that the maximum Ex acceleration electric field for the multihole target. Figure 2, a shows histories of the max Ex electric field, and fig. 2, b, c and d the space distributions of Ex at 60 fs for 0.5A, 0.3A and 0.1 A, respectively. At the subwavelength-structured Al surface, seen by the illuminating laser, the laser is partly reflected [6]. Therefore, when the Al is thinner in fig. 1, a, the laser enters into the target Al microstructure and generates hot electrons effectively. So the thinner Al-layer target produces the higher Ex electric field as shown in fig. 2. Figure 3 shows that the total
energy histories of the proton for each case. When the Al thickness is 0.1 A, the maximum proton kinetic energy is 12.2 MeV. For the other cases it is 9.51 MeV for 0.3A and 7.48 MeV for 0.5A. When the Al thickness becomes thin, the Al surface area seen by the laser is reduced and the laser reflection is also reduced. However, if the target has no Al microstructure at the laser side, it just becomes a plain target. As a comparison purpose, we also performed simulations for the target shown in fig. 1, b. The energy laser-proton conversion efficiency is 13.3% for 0.1A, 8.67% for 0.3A and 6.28% for 0.5A, though it is 5.74% for the target in fig. 1, b. The optimal thickness of the Al microstructure in fig. 1, a must be the order of the skin depth to reduce the laser reflection as low as possible. However, in actual applications and fabrications of the mictrostructured target, it may be so difficult to produce the skin depth thickness structure practically. In addition, particle-in-cell simulations have a limitation of computation space mesh size, which influences significantly for computation time required. So in the paper we performed the simulations ranging from 0.1 A to 0.5A for the Al thickness in fig. 1, a with the simulations for the target without the Al microstructure at the laser side shown in fig. 1, b. Figure 4 shows the histories of the proton total number and fig. 5 presents the spatial distributions of high energy protons for the multihole targets with the Al thicknesses of 0.1 A, 0.3A and 0.5A and for the target without the Al microstructure. As a reference we also performed simulations for the plain target in fig. 1, c, in which the Al thickness is 0.5A. In this conventional case the laser-proton energy conversion efficiency was 2.58%.
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Fig. 2. The maximal acceleration electric field Ex for the multihole target a - the time history of the maximal Ex; b, c - the Ex space distributions for the microstructured target with the Al thickness of 0.1A (b) and 0.5A (c); d - for target in fig. 1, b at 60 fs at the laser side.
The results, shown in figs. 2-4, present a significant effect of the Al microstructure thickness on the ion generation efficiency. As discussed above, the physics of the increase
t, fs
Fig. 3. Total energy histories of the protons accelerated for the structured target with the Al thickness in 0.5 A, 0.3 A and 0.1 A and for the target without the mictostructure in fig. 1, b
AN/N
Fig. 4. Histories of high-energy electrons over 1.0 MeV in the multihole target for the Al thickness of 0.1A, 0.3A, 0.5A and no Al layer at the laser side
in the proton generation efficiency comes from mainly the reduction of the laser reflection at the left surface area, seen by the laser [6]. The reduction of the Al reflection area, that is the reduction of the Al thickness in fig. 1, a, results in the significant increase in the laser energy conversion to the protons.
We also examine the role of the Al wing length on the laser-ion energy conversion. We change the Al wing length of the multihole target from 0.1 to 0.5A (see fig. 1, a). At 0.2A protons obtain the larger energy from the laser. Figure 6 shows the total energy histories
0.51 o.u w/o Al
Ml ftL
m W V
w mm w
0 25 0 25 0 25
Fig. 5. Spatial distributions of high energy protons in the multihole target for the Al thickness of 0.1 A, 0.3A, 0.5A and no Al layer at the laser side
t, fs
Fig. 6. Total proton energy histories for the multihole target and for the target without the Al microstructure The Al wing length was changed between 0.1 A and 0.5A. At 0.2A the protons obtain the larger energy from the laser.
of the protons. In fig. 6, at the Al wing length of 0.2A, the total energy of the protons reaches the maximum and the energy conversion efficiency from the laser to the protons becomes 16.7%. The maximum proton kinetic energy is 12.4 MeV for the case of 0.2A, 10.4 MeV for 0.1 A, 13.0 MeV for 0.3A and 12.2 MeV for 0.5A at 300 fs. Figure 7 shows the total energy histories of the Al ions, protons and electrons for the Al wing length 2A. Figure 8 shows the maximal acceleration electric field Ex for the multihole target (a), and the distributions of the high-energy electron density at 0.2A (b) and 0.5A (c). Depending on the Al wing length the acceleration field changes, and at 0.2A the acceleration field becomes high. The hot-electron high-density cloud is well located at the right space of the proton
s
a
ss
Ih
s
w
+J
o
H
fs
Fig. 7. Total energy histories of the Al ions, protons and electrons for the Al wing length 2A
layer in the case of 0.2A for the effective proton acceleration, so that the Ex becomes large compared with that for the other cases. For 0.5A, the hot-electron cloud center is located on the left part of the proton layer, and so the proton acceleration is not effective.
Fig. 8. The maximal acceleration electric field Ex for the multihole target (a), and distributions of the high-energy electron density at 0.2A (b) and 0.5A (c)
When the Al wing length is too short, the total electron number in the Al layer is reduced to create the proton acceleration field at the target rear side. The proton acceleration field, created by the hot electrons at the rear side, is produced by the hot electrons. When the total number of the electrons, which are mainly originated from the microstructured Al at the laser side, becomes small, the acceleration field at the rear side becomes weak. On the other hand, when the Al wing length is too long, the hot electron cloud cannot fully reach the target rear side (see fig. 8, c). In this case with the long Al wing the Al layer originally contains a sufficient number of electrons, and the sufficient number of hot electrons appears. However, the hot electron cloud experiences the interaction with the target plasma for a longer distance than that in the optimal case, and does not reach the correct position of the target rear side to create a strong acceleration electric field at the target rear side, as presented in fig. 8, c. Therefore, the acceleration field becomes small as shown in fig. 8, a, for example, for the case of 0.5A. However, for the optimal case, that is, the case with the Al optimal wing length of 0.2A, the hot-electron cloud location is appropriate to create the high acceleration field Ex for the effective proton acceleration as shown in fig. 8, b.
References
1. Nakamura M., Kawata S., Sonobe R. et al. Robustness of a tailored hole target in laser-produced collimated proton beam generation // J. Appl. Phys. 2007. Vol. 101. P. 113305.
2. Sonobe R., Kawata S., Miyazaki S. et al. Suppression of transverse proton beam divergence by controlled electron cloud in laser-plasma interactions // Phys. Plasmas. 2005. Vol. 12. P. 073104.
3. Ma Y. Y., Sheng Z. M., Zhang J. et al. Dense quasi-monoenergetic attosecond electron bunches from laser interaction with wire and slice targets // Phys. Plasmas. 2006. Vol. 13. P. 110702.
4. Sumeruk H. A., Kneip S., Symes D. R. et al. Hot electron and X-ray production from intense laser irradiation of wavelength-scale polystyrene spheres // Phys. Plasmas. 2007. Vol. 14. P. 062704.
5. Nodera Y., Kawata S. Improvement of energy-conversion efficiency from laser to proton beam in a laser-foil interaction // Phys. Revs. E. 2008. Vol. 78. P. 046401.
6. Wang W. M., Sheng Z. M., Zhang J. A model for the efficient coupling between intense lasers and subwavelength grating targets // Phys. Plasmas. 2008. Vol. 15. P. 030702.
Статья рекомендована к печати проф. Д. А. Овсянниковым.
Статья принята к печати 14 октября 2010 г.
