Speaker
Yasuhiko Sentoku
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.2023.pdf
Formation of power law electron energy distribution in picosecond
interaction of relativistic laser and dense plasma
N. Iwata , Y. Sentoku , T. Sano and K. Mima
1 1 1 1,2
1
Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka, Japan
2
The Graduate School for the Creation of New Photon Industries, 1955-1 Kurematsu,
Nishiku, Hamamatsu, Shizuoka, Japan
High power lasers with relativistic intensities above 1018 W/cm2 and pulse lengths
exceeding picosecond (ps) have been developed in recent years. In over-ps laser-plasma
interactions, energy slope of high-energy electrons tends to be higher than the scaling laws
used in the sub-ps regime. One of the key mechanisms of such a superthermal electron
generation is stochastic heating in a laser-irradiated thin foil, where fast electrons
recirculate around and suffer multiple kicks from the laser field during the pulse duration
[1]. The blowout of hot plasma towards the laser, which takes place under the ps laser
heating [2], also enhances the multiple interactions of fast electrons with laser light.
Understanding characteristics of the energy distribution resulted from the new accelerations
arise in ps relativistic regime is essential for various applications for intense lasers.
Furthermore, the stochastic acceleration by superthermal fields is related to the acceleration
of cosmic rays which exhibit power law spectrum in high energy tail. For a non-relativistic
laser interaction with underdense plasma, the electron energy spectrum is found to be a
kappa distribution which becomes power law in high energy limit [3].
Here, we model the electron acceleration in the laser-thin foil interaction and study the
resulting electron energy distribution based on the relativistic Fokker-Plank equation in
momentum space. We introduce new diffusion and friction coefficients that represent the
stochastic heating at the front side and the energy dissipation by the sheath potential trap at
the rear side, respectively. We find that the steady solution of the Fokker-Plank equation
becomes a power law when the diffusion by the laser kick is in proportion to the
momentum p. This analysis can specify the origin of the power law formation, and provide
an insight for further development of theoretical models for complex laser interactions in
multi-ps time scale.
[1] N. Iwata et al, Phys. Plasmas 24 (2017) 073111
[2] N. Iwata et al., Nat. Commun. 9:623 doi: 10.1038/s41467-018-02829-5 (2018)
[3] A. Hasegawa, K. Mima and M. Duong-van, Phys. Rev. Lett. 54 (1985) 2608
