24F05
Experimental study on the origins of cosmic rays
宇宙線の起源に関する実験的研究
Y. Kuramitsu1 , Y. Sakawa1 , N. Nakanii2 , K. Kondo3 et al.
蔵満康浩 , 坂和洋一 1 , 中新信彦 2 , 近藤公伯 3 , 森芳孝 4 , 三浦永祐 5 , 谷本壮 3 , 中村浩隆 2 ,反保元信 3 , 兒玉了祐 2 , 北川米喜 4 , 三間圀興 1 , 田中和夫 2 , 星野真弘 6 , 高部英明 1
11Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan
大阪大学レーザーエネルギー学研究センター 565-0871 大阪府吹田市山田丘 2-62Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan
大阪大学大学院工学研究科 565-0871 大阪府吹田市山田丘 2-1
3Kansai Photon Science Institute, Japan Atomic Energy Agency, 8-1-7 Umemidai, Kizugawa, Kyoto
619-0215, Japan
日本原子力研究開発機構関西光科学研究所 619-0215 京都府木津川市梅美台 8-1-7
4
The Graduate School for the Creation of New Photonics Industries, 1955-1 Kurematsu-cho, Nishiku,Hamamatsu, Shizuoka 432-1202, Japan
光産業創成大学院大学 431-1202 静岡県浜松市西区呉松町 1955-1
5National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki305-8568, Japan
産業技術総合研究所 305-8563 茨城県つくば市梅園 1-1-16
Department of Earth and Planetary Science, the University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033,
Japan
東京大学大学院理学研究科 113-0033 東京都文京区本郷 7-3-1
Experimental study on the origins of cosmic rays is presented. One possible mechanism for thenonthermal acceleration of the extragalactic cosmic rays is wakeﬁeld acceleration. In the upstreamof a relativistic collisionless shock, large amplitude electromagnetic (light) waves are considered to
be excited and particles can be accelerated by turbulent wakeﬁelds. Numerical simulations show
universal production of power-law spectra of the accelerated particles with an index of ∼ 2. By
substituting an intense laser pulse for the light waves in the upstreams of the astrophysical shocks,
a model experiment was performed to prove the preceding numerical model. The result is equivalent
to the numerical study and the ﬁrst experimental proof of a model for cosmic ray acceleration.

1Introduction

It is considered that the diﬀusive shock acceleration (DSA) is a standard acceleration mechanism
for energetic particles or cosmic rays within ourgalaxy. In order to understand the dissipation andtransport processes in the shock environments,
collisionless shock formations have been investigated in laboratories [1, 2]. On the other hand,the origin of extragalactic cosmic rays has been along standing unsolved issue. A possible candidateof the cosmic ray acceleration is the wakeﬁeld acceleration in astrophysical environments. For instance, eﬃcient particle acceleration by wakeﬁelds
induced by large amplitude precursor electromagnetic (light) waves in the upstream of relativistic
perpendicular shock waves was reported[3]. In astrophysical plasmas, however, it is impossible toobserve the ﬁeld and particle quantities directly,
which are crucial in the acceleration and transportof cosmic rays.

By performing two-dimensional particle-in-cell(PIC) simulations, a model experiment of cosmic
ray acceleration due to an incoherent wakeﬁeld induced by an intense laser pulse was proposed [4],where the laser pulse is substituted for the large
amplitude light waves in the astrophysical shocks.
When the spatial scales of the laser pulse are larger
than the plasma wavelength, relatively incoherent or quasi-turbulent waleﬁeld is excited and theelectrons are nonthermally accelerated, resulting
in power-law energy spectra with an index of ∼ 2
independent of the laser intensity, the plasma density, and the pulse size. Although it used to be
diﬃcult to prove a model for cosmic ray acceleration so far, current laser technology allows us toprove the above model by laboratory experiments.
Using an intense laser pulse for the astrophysicalprecursor light waves, the universality of powerlaw acceleration of relativistic electrons with respect to the plasma density has been experimentally proved [5, 6].

(a)-110101

γ-2100-2

a = 0.33
a = 3.3
a = 1010-1

10-31010-410) (f-3~gf(γ)-210-510-4

Eimp = 2.0 kJ-61010-5-6-2

gEimp = 1.9 kJ

10Eimp = 2.3 kJ-71010-81234 5 6 7810234 5 6 78

100

234 5 6 78

1000

γ(b)

8 9102345678 9234567100

8 9231000

g101

γ-2100

10-110ωp/ωL = 0.095
ωp/ωL = 0.19ωp/ωL = 0.38
ωp/ωL = 0.6

Figure 2: Energy distribution functions of theelectrons in logarithmic scales.

f(γ)

10-2
10-3

functions were diﬀerent for diﬀerent implosion energies; the higher implosion energy was the lower
10maximum energy. The distribution functions con10
sist of two parts; one is the thermal part and the10other is the nonthermal part, which is represented
1101001000
γ
as a power law. The nonthermal parts show aFigure 1: Energy distribution functions of elec- common feature, a power law with an index oftrons are shown with various (a) laser intensity ∼ 2 independent of the plasma density.
and (b) plasma density. The spectra are well represented by a power law with an index of ∼ 2.
4 Summary
10-4-5-6-7

2234 5 6 78

234 5 6 78

23

4 5 6 78

Numerical simulations

We have presented results from numerical and experimental simulations of cosmic ray acceleration
due to wakeﬁeld induced by an intense light pulse.
In our numerical study, the electrons were nonthermally accelerated by the incoherent wakeﬁeld,
resulting in a power-law spectrum with an index of∼ 2 independent of the plasma density [4]. Scaled
laboratory experiment shows that energy distribution functions of electrons have a power-law component with an index of ∼ 2 independent of theimplosion energy or the plasma density [5, 6]. Thisis the ﬁrst experimental proof of an analytic modelof the cosmic ray acceleration.

We perform particle simulations in two spatial andthree velocity dimensions. The details of the numerical conditions are found in [4]. Figure 1 showsthe energy distribution functions of the electrons
calculated by varying (a) the laser intensity and(b) the plasma density. In Fig. 1 (a) the larger
intensity results in the larger acceleration. Whenthe normalized laser intensity a = 0.33, the electrons are simply thermalized rather than the nonthermal acceleration. Except a = 0.33 the spectra
are well represented by the power law with an index of ∼ 2. The lower density results in the larger
acceleration, however, the spectra are commonly Referencescharacterized by the power law with an index of[1] T. Morita et al, PoP, 17, 122702, 2010.
∼ 2.
3Experiment[2] Y. Kuramitsu et al., PRL, 106, 175002, 2011.
[3] M. Hoshino, ApJ, 672, 940, 2008.

The experiment was performed using the PW laser
system at the Institute of Laser Engineering at
Osaka University. The details of the experimental setup and the diagnostics are found in [5, 6, 7].
Figure 2 shows three energy distribution functionsof relativistic electrons generated for three different implosion energies with logarithmic scales.
The maximum energies in the energy distribution[4] Y. Kuramitsu et al., ApJ, 682, L113, 2008.
[5] Y. Kuramitsu et al., PoP, 18, 010701, 2011.
[6] Y. Kuramitsu et al., PRE, 83, 026401, 2011.
[7] N. Nakanii et al., APL, 93, 081501, 2008.