Masato Ota (Osaka University) et al.


　Collisionless shock is considered as one of the possible candidates of the sources of cosmic rays, or high-energy charged particles travel through space. It isformed in fast plasma flow created by gammaray bursts, active galactic nuclei, and supernova remnants. Theories and simulations predict that electromagnetic turbulence in front of the shock can result in stochastic charged particle acceleration and the high-energy and non-thermal energy spectrum of cosmic rays. Weibel-instability mediated shock (Weibel shock) is one of the collisionless shocks associated with current filamentation and turbulent magnetic field. Laser plasma experiments on the Weibel shock is conducted and the filamentary structure which indicate Weibal-instability [1] and non-thermal electron energy spectrum [2] are measured in plasma created by high-power laser with ns pulse width. On the other hand, Weibel filaments with typical spatial scale of 0.1 to 10 μm and shock can be produced by irradiating a high-intensity (HI) laser with sub-ps pulse width on sample, and their pico-second plasma dynamics can be observed by using X-ray Free Electron Laser (XFEL) at SACLA, in Japan. At the moment, the driving laser intensity available at SACLAis not yet high enough to produce Weibel shock. As a preparation step, we mimic the Weibel filaments by using a target consists of Si rod-assembly (diameter 0.5 μm, length 13 μm, 15×15 or 15×3 array, interval 1.0 μm) and observe the cold rod structure with Small Angle X-ray Scattering (SAXS) using the XFEL (spot size ~ 2.7×3.4 μm2 (FWHM), pulse duration < 10 fs (FWHM), photon energy 7.0 keV, photon number per pulse 4×1011) [3]. Observed SAXS diffraction pattern and peaks of theXFEL from the cold rod-assembly targetsshow a good agreement with our calculation. We then added the HI driving laser (central wavelength 800 nm, spot size ~100 μm (FWHM), pulse duration 50 fs (FWHM), intensity 7×1017W/cm2 ) to heat the targets. By varying the temporal delay between the driving laser and the XFELprobe, the time evolution of the expansion of the rod surface was successfully observed via SAXS. This successful proof-of-principle experiment can be extended to the future Weibel shock visualization at SACLA and other XFEL facilities.


References

[1] N. L. Kugland, et al., Nature Phys. 8, 809-812 (2012).

[2] F. Fiuza, et al., Nature Phys. 16, https://doi.org/10.1038/s41567-020-0919-4 (2020).

[3] T. Kluge, et al., Physics of Plasmas 24, 102709 (2017).