30-TW laser–plasma interactions at ILE, Osaka

Surface acceleration of fast electrons with relativistic self-focusing in preformed plasma

We report an observation of surface acceleration of fast electrons in intense laser-plasma interactions. When a preformed plasma is presented in front of a solid target with a higher laser intensity, the emission direction of fast electrons is changed to the target surface direction from the laser and specular directions. This feature could be caused by the formation of a strong static magnetic field along the target surface which traps and holds fast electrons on the surface. In our experiment, the increase in the laser intensity due to relativistic self-focusing in plasma plays an important role for the formation. The strength of the magnetic field is calculated from the bent angle of the electrons, resulting in tens of percent of laser magnetic field, which agrees well with a two-dimensional particle-in-cell calculation. The strong surface current explains the high conversion efficiency on the cone-guided fast ignitor experiments.

Fast ion acceleration in ultraintense laser interactions with an overdense plasma

In order to study the ion acceleration processes in ultraintense laser-plasma interactions with solid targets, neutron spectra from deuteron-deuteron (D-D) nuclear reactions were measured. Spectra were obtained when (50-100 TW, 0.5-1 ps) laser light irradiated obliquely incident deuterated plastic targets as a function of laser polarization, intensity, and density scale length of the preformed plasma. The experimental data are compared with three-dimensional Monte Carlo simulations. The results indicate that the ion momentum distribution is collimated and directed into the bulk of the target to the target normal direction with an energy that is linearly proportional to the laser intensity. The distribution of the accelerated ions was observed to change from isotropic to anisotropic with laser prepulse intensity. All the results indicate that the ion acceleration is dominated by an electrostatic field generated from a charge displacement of the hot electrons at the target surface.