Properties of a plasma-based laser-triggered micro-lens

This paper investigates the characteristic properties of a laser triggered micro-lens for focusing and energy selection of laser generated MeV proton and ion beams. Both experimental and computational studies that have been carried out leading to the understanding of the physical processes driving the micro-lens. After a one side irradiation of a hollow metallic cylinder a radial electric field develops inside the cylinder. Hot electrons generated by the interaction between laser pulse and cylinder wall spread inside the cylinder generating a plasma at the wall. This plasma expands into vacuum and sustains an electric field that acts as a collecting lens for proton or ion beams propagating axially through the cylinder. Various measurements including the reduction of the intrinsic beam divergence, the focusing quality, the energy selection and temporal response were carried out which contribute to the understanding of the lens properties. In addition, proton imaging was employed to study the electron transport inside the cylinder, revealing a transport along the wall surface. Each aspect studied experimentally is interpreted using 2D PIC and ray tracing simulations. A very good consistency between experimental and computational data was found.