Laser-driven ion acceleration and nuclear activation

Ion acceleration driven by intense laser-plasma interactions has been investigated since the 1970s. In early experiments with long-pulse (nanosecond) CO₂ lasers, at intensities of the order of 10¹⁶W/cm², protons were typically accelerated to tens of keV energies. They were produced with poor beam characteristics, including high transverse temperatures. The source of the protons was found to be hydrocarbon or water contamination layers on the surfaces of the laser-irradiated targets. A review of this work is provided by Gitomer et al. [1]. The introduction of chirped pulse amplification (CPA) in the late 1980s made it possible to produce high-intensity laser pulses with picosecond duration. The relativistic threshold for laser-plasma interactions was crossed at 10¹⁸ W/cm², leading to collective effects in the plasma, and a renewed interest in ion acceleration. Recently, proton acceleration was observed by Clark et al. [2] and Snavely et al. [3] in short-pulse laser-plasma interactions. Protons with energies greater than 50MeV have been measured in low divergent beams of excellent quality. This novel source of laser-driven multi-MeV energy ions has also been used to induce nuclear reactions. Rapid progress in the development of this potentially compact ion source offers intriguing possibilities for applications in isotope production for medical imaging [4], ion radiotherapy [5], ion-based fast ignitor schemes for inertial fusion energy [6], and as injectors for the next generation of ion accelerators [7].