Experimental study of short pulse laser generated electron collimation for EFI using resistive focusing

Electron fast ignition (EFI) is a promising approach to achieving high gain in inertial confinement fusion. It reduces driver and symmetry requirements compared to conventional central hot spot ignition by separating the compression and ignition phases. Short pulse (sub-picosecond) lasers incident on solid targets generate energetic electrons which can then ignite a fuel capsule pre-compressed by nanosecond lasers. The short pulse laser energy is coupled very efficiently into electrons; however, they experience a large angular divergence in the solid material, making it challenging to deposit their energy into the limited hotspot volume. Resistive focusing via self-generated magnetic fields has been demonstrated as a viable electron collimation technique wherein a resistivity gradient in the material drives strong azimuthal fields that collimate the electrons. In this work we present results from an experimental campaign conducted at the Apollon and Titan laser facilities studying resistive focusing of MeV electrons. We make simultaneous measurements of the electron spot size, angular-dependent energy distribution, and sub-picosecond time-resolved temperature gradients in coaxial wire targets with a radial resistivity gradient. Preliminary results from this campaign show strong evidence of electron collimation, which along with the electron temperature measurements can be used to benchmark resistivity models and improve the predictive capability of EFI design codes.