Optimization of a permanent magnet quadrupole doublet for laser-accelerated proton bunches at the Centre for Advanced Laser Applications

Many applications of laser-accelerated ions require a delivery of the particles to locations remote from the plasma source. Due to the large bunch divergence, achieving experimentally relevant particle fluences at more than a few tens of cm distance requires a dedicated bunch transport system. The most compact solution is a doublet of permanent magnet quadrupoles. Since these quadrupoles have a fixed magnetic field gradient, their focusing properties depend only on their geometric positions and relative rotations which therefore require careful alignment. We performed comprehensive ion optical simulations to characterize gradients and fields of the individual magnets and to identify the sensitivity of the focus shape to various positioning parameters, especially relative distances and rotation. The simulations also allowed devising radiation and laser safety measures for the quadrupoles. Based on these results and thanks to a very stable and reproducible proton source, we could optimize the obtained focus experimentally to a symmetric star like shape. This optimization yielded a proton spot in which the central area of highest fluence had a minimum diameter of approximately 2 mm FWHM, as revealed with spatially resolved scintillator and radiochromic film measurements. Furthermore, we developed a low-material-budget ionization chamber to monitor bunch charge and performed first tests with the aim to quantify the proton focus dosimetrically. The implemented controls and monitoring tools allow now for planning of first application experiments with (sub-) mm proton bunches of (sub-) ns duration.