Vacuum laser acceleration, relativistic plasmas, laser interaction with overdense plasmas

Compact particle accelerators driven by high-intensity laser pulses is of great interest for x-ray sources and related applications. Despite its simplicity, the VLA is very challenging in particle injection. The laser light travels at the light speed, yet electrons have to be pre-accelerated close to light speed in order to gain energy from the laser.

Recently, we identified a novel regime of VLA based on laser interaction with a thin high-density foil plasma. A dense plasma is usually opaque to an incident laser. Physically, abundant surface electrons respond timely such that the laser wave is completely backscattered (i.e. reflection). However, for a thin plasma, the back portion of the laser light may still penetrate the plasma via a transparency effect. This effect is partially caused by plasma expansion, with the plasma layer becoming less dense with time. Another major contribution is from the relativistic effect. That is, as the plasma electrons are heated up, they become heavier in (relativistic) mass and thus less prone to screen the light.

We show that this relativistic transparency process can be used to inject electrons for acceleration in a time-symmetric laser pulse. This kind of acceleration is normally very difficult to achieve because of the symmetry, i.e. particles gain and lose energy alternatively in time and they eventually gain zero net energy after exiting from the pulse. The transparency injection overcomes this problem by preacclerating electrons in the opaque phase and then during the transparency injecting the electrons around the middle (not from the beginning) of the pulse. This self-organizing process involves several phases where one is connected to the next seamlessly. Clear evidences of the injection-acceleration scenario are found both in experiments and extensive numerical work.

P. Singh#, F. Li#, C. Huang, A. Moreau, R. Hollinger, A. Junghans, A. Favalli, C. Calvi, S. Wang, Y. Wang, H. Song, J. Rocca, R. Reinovsky, and S. Palaniyappan, Vacuum laser acceleration of super-ponderomotive electrons using relativistic transparency injection, Nature Communications 13, 54 (2022). # Equal contribution

See also LANL STE Highlights