Space-charge effects on laser-accelerated proton beams captured by a solenoidal magnetic field

In context of the LIGHT project (Laser-Ion Generation, Handling and Transport) [1], studies on the capability of a solenoidal magnetic field to collimate and transport laser-accelerated proton beams were carried out. Results of preliminary particle-in-cell simulations [2] and experiments at the PHELIX laser facility [1, 3] demand a detailed analysis of the space-charge influence. Therefore, the WarpRZ simulation code [4] is used. The drawbacks of laser-accelerated proton beams such as large envelope divergence and broad exponential energy distribution are experimentally corrected by using a solenoidal magnetic field of 7.5 T to collimate and focus protons. The simulation setup exactly fits the experiment. An increase of almost 6% (10 particles) in proton transmission through the solenoid could be observed in comparison to simulations neglecting the co-moving electrons and the Poisson self-field solver (fig. 1). Due to the solenoidal magnetic field, the electrons are forced down on axis (fig. 2a) and attract protons (fig. 2b). As soon as the electron density decreases, the attracted proton distribution starts to expand, meanwhile the main distribution starts to focus (fig. 2c). A beam of 8.4×10 focussed protons of (6.7±0.1) MeV energy is measured with ∆ E/E = 3%. It is focussed onto a 2 mm spot and has a pulse duration of 170 ps. On the other hand, a collimated beam in the energy range (13.5±0.5) MeV containing 3×10 protons with a final pulse duration of 300 ps could be transported over a distance of 50 cm. References