Youichi Sakawa (Institute of Laser Engineering, Osaka University)
Collisionless shock, in which the coulomb mean-free-path is much longer than the shock-front thickness, is ubiquitous in space and astrophysical plasmas. It is believed that collisionless shocks are the sources of cosmic rays. In such collisionless plasmas, wave-particle interactions and collective effects of electromagnetic fields play an essential role in the shock formation. In addition to local observations of space plasmas by spacecraft and global emission measurements of astrophysical plasmas, laboratory experiments can be an alternative approach to study the formation of collisionless shocks.
We investigated Weibel-instability mediated collisionless shock (Weibel shock) in a self-generated turbulent magnetic field using large-scale laser systems.
On Omega (LLE, USA) laser experiments with CH/CH double-plane target, plasma parameters of counter-streaming flows were measured by collective Thomson scattering [1], and temporal evolution of Weibel filaments were observed by D-3He fusion produced proton radiography [2].
On the National Ignition Facility (NIF, USA) experiments, CD/CD, CD/CH, and CH/CH double-plane target was used. The transition from collisional to collisionless flows was investigated as the foil separation gets larger. Excess neutrons, when comparing the CD/CD and CD/CH interactions, indicated a strong thermalization [3]. We used D-3He proton radiography for the first time on the NIF and investigated temporal evolution of the Weibel-instability and shock formation. Finally, by using Thomson scattering measurements, we showed the formation of collisionless Weibel shock and non-thermal electron acceleration [4].
Reference
[1] J. S. Ross et al, Phys. Plasmas 19, 056501 (2012).
[2] C. M. Huntington et al, Nature Physics 11, 173 (2015).
[3] J. S. Ross et al, Phys. Rev. Lett 118, 185003 (2017).
[4] F. Fiuza et al, Nature Physics Published online 8 June (2020). https://doi.org/10.1038/ s41567-020-0919-4.