Laser-driven proton beams: Acceleration mechanism, beam optimization, and radiographic applications

This paper reviews recent experimental activity in the area of optimization, control, and application of laser-accelerated proton beams, carried out at the Rutherford Appleton Laboratory and the Laboratoire pour l'Utilisation des Lasers Intenses 100 TW facility in France. In particular, experiments have investigated the role of the scale length at the rear of the plasma in reducing target-normal-sheath-acceleration acceleration efficiency. Results match with recent theoretical predictions and provide information in view of the feasibility of proton fast-ignition applications. Experiments aiming to control the divergence of the proton beams have investigated the use of a laser-triggered microlens, which employs laser-driven transient electric fields in cylindrical geometry, enabling to focus the emitted protons and select monochromatic beamlets out of the broad-spectrum beam. This approach could be advantageous in view of a variety of applications. The use of laser-driven protons as a particle probe for transient field detection has been developed and applied to a number of experimental conditions. Recent work in this area has focused on the detection of large-scale self-generated magnetic fields in laser-produced plasmas and the investigation of fields associated to the propagation of relativistic electron both on the surface and in the bulk of targets irradiated by high-power laser pulses.