S9-2

Possibility of Model Experiments onRelativistic Plasama Phenomena with Ultra-intense Lasers
Hideaki Takabe (Aki)Institute of Laser Energetics, Osaka University, Yamada-oka 2-6, Osaka 565-0871, Japan565-0871
2-6With use of ultra-intense lasers, we can efficiently convert the energy of lasers to that of relativisticelectrons with average energy of around 10 MeV. These relativistic electrons produce positron throughthe interactions with nuclei. We can create electron-positron plasmas with average Lorentz factor ofaround 10. This value is almost the same as seen in the jet from Active Galactic Nucleus (AGN). Inaddition, we can also study Weibel type instability which is thought to be important in the Gamma-raybursts. We describe the present status of laser produced pair plasma and its usage for model experiment
for astrophysical phenomena.

1. Introduction

It is clear that the high power laser has
grown as a new tool to investigate an advanced
physics in extreme conditions. This rapid
growth of the laser technology is partiallydriven by the need for laser fusion research andalso by the invention of CPA (Chirped PulseAmplification) technology [1]. I have strong
confidence that the lasers could be the second
accelerator to open a new field of thefundamental science, and of course they arealso used for many applications in science andindustry. So-called Laboratory Astrophysics
with intense lasers has been proposed and manycase studies has been done [2]. One of theinteresting case is the use of relativistic plasma
for studying high-energy astrophysics inlaboratory. For example, it is the creation ofanti-matter by use of ultra-intense lasers [3] andelectron-positron plasma formation. There arefull of electron-positron plasmas near Black
Hole and also in AGNs (Active GalacticNuclei). We plan to do an experiment where
the generated electron-positron plasma collides
with imposed magnetic field or matter. Suchexperiment may be a model experiment ofexpanding fire-ball believed to be important as
the energy source of the gamma-ray bursts [4].
At the same time, it is also interesting to trigger
nuclear reaction processes [5].
2. Laser Produced Relativistic Plasma

When ultra-intense and ultra-short laser isirradiated on a solid target, we can assume thations have no time to react and at rest.
Through a variety of interaction processes,
most of laser energy is converted into that ofrelativistic electrons. The injection of therelativistic electrons into the target induces thereturn currents by the background electrons,consequently a variety of instabilities beinginduced.
Such instabilities accompanying
with strong magnetic field are the origin of
anomalous transport of the relativisticelectrons.
In addition, the expansion of the substantial
amount of relativistic electrons into vacuum isprohibited by the generation of the am-bipolarelectrostatic field almost normal to the target
surface.
The relativistic electrons are
confined and interact with target ions manytimes. In the interaction with nuclei, thequantum-electro-dynamic (QED) process canbe seen, because the electron energy is far
beyond the rest mass energy of electron. For
example, high-energy g-rays generated throughBremstrahlung process induce photo-nuclear
reactions, and electron anti-matter positrons are
created through QED process.
In later time after the impinging of the laser,
the am-bipolar electrostatic field drives theacceleration of ions. Since the electrostatic
potential of the am-bipolar field is of order of
10 MeV, the ions are accelerated up to suchenergy range with very narrow angle spread [6].
Good and compact ion beam source can be
designed with the ultra-short pulse technique.

3. Positron Production

There are two processes which create
positrons in the present situation. Trident
process is a single process, while the BetheHeitler (BH) process is a two-stage process.
In the BH process, namely, the electron energyis once converted to that of g-ray and the g-rayinteracts with nucleus to create a pair. Which
one contributes most to generate positrons hasbeen studied [7].
The production of positron has alreadypointed out in early time [8], however, they justestimated the threshold laser intensity overwhich pair production occurs owing to theTrident process. They concluded that pairs
are produced when the laser intensity is higher
than 1019 W/cm2. Liang et al considered thecase when the high energy electrons areconfined in a relatively thin gold foil andconcluded that the Trident process is more
important than the Bethe-Heitler process for thecase of the foil with the thickness 1 mm [9].
On the other hand, Gryaznykh et al haveconsidered the case of relatively thick target
and concluded that the BH process is more
important [10].
4. Modeling High-Energy Astrophysics

The relativistic electron-positron plasma isan essential ingredient in high energy
astrophysics. In the high energy astrophysics,
for example, the high energy physics in thematter near Black Holes should be studied. Ingeneral, the temperature of the matter isextremely high because of the conversion of thegravitational energy to thermal energy in theaccretion disk surrounding the Black Holes.
The electron positron plasmas appear to beimportant in studying the plasmas over thesurface of pulsars, near black holes, at thecenter of our galaxy, at the active galactic
nuclei (AGN), and in the MeV epoch about one
second after the Big Bang.
We propose to do a model experiment of thepropagation of pair plasma jet in the gas
modeling inter-stellar matter in order to study
the propagation of AGN jets.
We alsoproposed to study the Weibel instability andmagnetic field structure formation in relativistic
plasmas relating to the physics of the gammaray bursts.
More details will be presented in theconference.

References[1] G. A. Mourou et al., Physics Today January issue,
22 (1998); C. Joshi and P. B. Corkun, Phys Today
48, No. 1 (1995), p. 36.
[2] H. Takabe, Progress of Theoretical Physics
Supplement, No. 143, 202-265 (2001).
[3] T. E. Cowan et al., Laser Partcl. Beams 17, 773(1999)[4] N. Gehrels et al., Scientific American, December,
287, 84 (2002).
[5] H. Takabe, “Laser Nuclear Physics”, Vol. 13, No.
1, AAPPS Bulletin, 2003 (Download at
http://www.aapps.org).
[6] S. Hatchett et al., Phys. Plasmas 7, 2076 (2000).
[7] K. Nakashima and H. Takabe, Phys. Plasmas 9,1505 (2002).
[8] J. W. Shearer et al., Phys. Rev. A8, 1582 (1973)[9] E. P. Liang et al., Phys. Rev. Latt. 81, 4887 (1998)[10] D. A. Gryaznkh et al., JETP Lett. 67, 257 (1998)