Nonthermal particle spectra in ultra-intense laser-plasma interaction

Relativistic laser-plasma interaction can produce nonthermal electron spectra in power-law scaling. The power-law index for near-critical density plasmas changes between 1.8 and 2.0 as the laser intensity varies from the relativistic threshold to the radiation-dominated regime. While effective temperature of electrons is strongly suppressed by radiation-radiation effect, it is found that the nonthermal power-laws are consistent between the radiation OFF and ON cases. It decreases to similar to 1.4 as plasma density becomes over-critical. Similar to electrons, proton energy spectra also exhibit power-law distribution characteristics. As the laser intensity increases, the power-law index stabilizes around 1.6. We find that most nonthermal electrons constituting the power-law spectrum are distributed at the edge of the plasma channel as well as outside the channel, which is less sensitive to the radiation-reaction. This work provides a new physical mechanism relating to cosmic ray acceleration mechanisms in astrophysical environments with a strong electromagnetic field generated by neutron stars, relativistic collisionless shocks, and so on.