Speaker
Natsumi Iwata
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O4.204.pdf
Plasma density limits of laser hole boring and superthermal electron
generation by relativistic picosecond lasers
N. Iwata , Y. Sentoku , S. Kojima , M. Hata and K. Mima
1 1 2 1 1,3
1
Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka, Japan
2
Advanced Research Center for Beam Science, Institute for Chemical Research, Kyoto
University, Gokasho, Uji, Japan
3
The Graduate School for the Creation of New Photon Industries, 1955-1 Kurematsu,
Nishiku, Hamamatsu, Shizuoka, Japan
Intense lasers can penetrate into overdense plasmas by the laser hole boring (HB) where
giga-bar-level radiation pressure pushes the critical plasma surface forward. The HB
proceeds with making a sharp laser-plasma interface which plays an important role in
energy transfer from laser to electrons. Hence, the HB is a fundamental concern in
applications such as ion acceleration and fast ignition-based laser fusion. Conventionally,
the HB had been considered to proceed as long as the laser pulse continues [1]. However,
recent studies have found that during over-picosecond (ps) laser irradiation, surface plasma
starts to expand towards the laser resulting the superthermal electron production [2].
In this study, we find that under the continuous laser heating in ps time scale, the
pressure balance between plasma and laser light is established being assisted by the sheath
electric field, which acts as the surface tension, and the HB stops consequently. Based on
the pressure balance equation, we theoretically derive the limit density for the HB, i.e., the
maximum density laser light can reach, as 8a02nc where a0 is the normalized laser amplitude
and nc is the critical density [3]. The time scale for the laser front to reach the limit density
is found to be in the ps regime. After the laser front reaches the limit density, the hot
plasma starts to blowout back towards the laser. In the blowout plasma, electrons interact
with the intense laser multiple times and stochastic electron heating can be enhanced. This
results in generating copious superthermal electrons and affects the subsequent phenomena
in the laser-plasma interaction, such as ion acceleration [4].
[1] S. C. Wilks et al., Phys. Rev. Lett. 69, 1383 (1992) ; Y. Sentoku et al., Fusion Sci. Technol. 49, 278 (2006)
[2] S. Kojima et al., J. Phys. Conf. Ser. 717 (2016) 012102; A. J. Kemp and L. Divol, Phys. Rev. Lett. 109,
195005 (2012); N. Iwata et al, Phys. Plasmas 24 (2017) 073111; A. Yogo et al., Sci. Rep. 7 (2017) 42451
[3] N. Iwata et al., Nat. Commun. 9:623 doi: 10.1038/s41467-018-02829-5 (2018)
[4] N. Iwata et al, Phys. Plasmas 24 (2017) 073111
