7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Numerical modeling of GeV positron generationin relativistic laser-plasma interaction
K. Sugimoto1,2, N. Iwata2,3, T. Sano2, and Y. Sentoku2
Yukawa Institute for Theoretical Physics, Kyoto University
2
Institute of Laser Engineering, Osaka University
3
Institute of Advanced Co-Creation Studies, Osaka University
e-mail (speaker): pak9nacm4pfqhb05r24a@gmail.com
1

With the advent of ultra-intense laser system, such as
ELI-NP and LMJ facilities, one can access relativistic
laser plasma interaction in a laboratory. The strong
electromagnetic field and a resultant plasma’s collective
effect accelerate charged particles to relativistic regime.
Such energetic particles have been attractive due to themotivations in fundamental research and applications:detection of QED phenomena, laboratory astrophysics,
and bright γ-ray sources for radiation therapy andphoto-nuclear spectroscopy. In recent years, a lot efforts
have been devoted for detection of linear Breit-Wheeler(LBW) process using bright γ-ray sources driven by theintense laser light. The LBW process is one of QEDphenomena, where one electron-positron pair isgenerated via collision between two high-energy photons,
such as X-ray and γ-ray. LBW process supposedly
converts photons to pair-plasmas, so that it determines
opacities for energetic photons in the universe. However,nobody has verified LBW process of real photons inexperiments due to its small cross section(~10-25cm2). In
order to realize LBW process, one needs collisions
between high-energy photons with high-directivity andhigh-density, and signals of generated positrons iscrucially important. Although several verification
schemes of LBW using intense laser light have been
proposed with a help of a particle-in-cell (PIC)
simulations [1,2], the details of positrons’ dynamics inthe laser-plasma interaction has been unknown.
In this work [3], we have found that a simple regime
where a near-critical density plasma irradiated by theintense laser light induces positrons, and they form
collimated positron beam with relativistic energies.
We developed a numerical model of LBW process in a
PIC code, PICLS, which can compute the radiationtransport self-consistently within PIC calculation [4]. It
has been demonstrated that an ultra-short petawatt laser
light self-organizes a photon collider in a near criticalover-dense plasma and produces positrons via the LBWprocess. We have performed 2D-PIC simulation using
the PICLS code. In the simulations, an ultra-shortpetawatt laser light is focused on a carbon target
consisting of carbons. The laser peak intensity, pulse

duration, and wavelength are 3×1022 W/cm2, 30fs, and
0.8μm respectively. The initial electron density of thetarget is set to be 2.8nc, where nc is the non-relativistic
critical density of laser light. As radiation processes ofelectrons, we consider synchrotron radiation andBremsstrahlung. Such an ultra-intense laser pulse
propagates in the over critical plasma by its relativistictransparency with inducing a self-generated magnetic
channel structure inside the pulse. A positive electrostatic
field appears at the pulse front as the result of electron
accumulation due to the laser photon pressure. Due tothe self-generated magnetic field, the laser light candrive relativistic electrons, which emit collimatedgamma-ray photons via synchrotron radiation, via directlaser acceleration. At the pulse front, on the other hand,plasma electrons are accelerated backward withrelativistic energies by the strong positive electrostatic
field. The relativistic backward electrons emit photons
backward via radiative decay when they collide with thelaser pulse. The forward and backward photons thendecay and induce electron-positron pairs via LBWprocess. The generated positrons are accelerated by thesame positive electrostatic field to GeV energies with asmall divergence of ±10 degrees.
We also investigated the effects of target density
configuration on the positron generation and acceleration
processes. It was found that positrons are produced andaccelerated in the almost same way in the initial electron
density range of 0.5-5.6nc. We also observed thatpair-creation is triggered effectively when the laser pulse
reaches the near critical density region.
In the talk, we’ll introduce the simulation models inPICLS code including radiation transport and LBWprocess, and show details of the simulation results.
References[1] X. Ribeyre et al, Phys. Rev. E, 93. 013201 (2016)[2] Y. He et al, Comms. Phys., 4. 139 (2021)[3] K. Sugimoto et al, accepted in PRL (2023)[4] Y. Sentoku et al, Phys. Rev. E, 90, 051102 (2014)