Proceedings of International Conference on Physics in ... - KEK

In USA, LLNL has a user’s facility, Titan laser which
is 1PW system with 400fs--10ps, up to 530 J and
2shots/hour [12]. At LLE, University <strong>of</strong> Rochester, an
ultra-intense laser system OMEGA-EP is now in
operation. It is <strong>of</strong> 1 P multi-kJ, W , 1ps and intensity
higher than 10 20 W/cm 2 We demonstrated that the Bethe-Heitler process is main
process to produce the pairs, although it is the two step
process in the gold foil. We predicted the positron
spectrum and compared with Cowan’s experimental data.
What we found is that the experimental data looks like
[13]. In addition, the world- shifted by about 10 MeV to the higher energy side as you
biggest laser NIF will have four beams to be multi-P It W . can see in Fig. 2 [18]. The Monte-Carlo calculation
is called NIF-ARC [14]. NIF-ARC is originally motivated
for diagnostic purpose for NIF ignition campaign (NIC),
but it is also open to users in the world only for the
fundamental science.
PAIR PLASMAS
Einstein said “Imagination is more important than
knowledge”. We can spread the world <strong>of</strong> imagination with
the appearance <strong>of</strong> such ultra-intense lasers. The first
experiment with PW laser was done by T. Cowan et al.
[15] and he reported photo-nuclear fission <strong>of</strong> Uranium
and positron production in a gold foil. In order to analyze
his experimental data on positron production [16], we
pursued joint research starting from modeling the Trident
and Bethe-Heitler processes in Fokker-Planck equation to
relativistic electrons as main source for pair creation [17].
Figure 2: (a) The longitudinal momentum
distributions <strong>of</strong> positron (red) and electrons (black) at
back <strong>of</strong> the target. The target rear is at x=0. The solid
line indicates the sheath field normalized by 10 12 V/m.
(b) Positron spectrum calculated by PIC. The blue
dotted line is the electron spectrum used in PIC
calculation.
usually used in HEP field [17] predicted almost the same
as Fokker-Planck case.
In order to see the physics causing the difference, we
thought that since the laser-produced relativistic electron
density is much higher than the case <strong>of</strong> accelerator beams,
a strong electric field formation by charge separation
effect, namely; plasma effect, becomes important to
accelerate the created positron at the rear side <strong>of</strong> the target
foil [18]. We pointed out this is critically different point <strong>of</strong>
laser case compared to the accelerator case. We have
carried out two-dimensional PIC (Particle-in-Cell)
simulation [20] with the initial condition calculated with
the Fokker-Planck equation and successes to reproduce
the experimental positron spectrum as shown in line with
PIC. PIC simulation calculated Maxwell equations con-
sistently with charged particles and the electric field the
potential <strong>of</strong> about 20 MV is found to be produced. It
should be noted that since the electric field is the vertical
direction <strong>of</strong> the target rear surface and the positrons are
accelerated predominantly to this direction, we obtain a
jet like positrons with Lorentz factor about 10-20 the
number <strong>of</strong> which is the same as those <strong>of</strong> AGN-jets
(Cosmological jets) [21]. Recently, H. Chen et al. pub-
lished several papers and they also found the importance
<strong>of</strong> the electric field and jet-like emission <strong>of</strong> positrons [22].
We are now collaborating to demonstrate the charge
Fig. 3 Positron number scaling from 100’s Joule
region to Several KJ laser case.