Focusing of multi-MeV, subnanosecond proton bunches from a laser-driven source

Abstract

We report on our latest transverse focusing results of subnanosecond proton bunches achieved with a laser-driven multi-MeV ion beamline. In the frame of the LIGHT collaboration, a target normal sheath acceleration (TNSA) source based 6 m long beamline was installed. In the past years, the laser-driven proton beam was transported and shaped by this beamline. The particle beam is collimated with a pulsed high-field solenoid and rotated in longitudinal phase space with a radio-frequency cavity which leads to an energy compression with an energy spread of $(2.7\ifmmode\pm\else\textpm\fi{}1.7)%$ ($\mathrm{\ensuremath{\Delta}}\mathrm{E}/{\mathrm{E}}_{0}$ at FWHM) or a time compression to the subnanosecond regime. Highest peak intensities in the subnanosecond regime open up an interesting field for several applications, e.g., proton imaging, as injectors in conventional accelerators or precise stopping power measurements in a plasma. We report on achieving highest peak intensities using an installed second solenoid as a final focusing system in our beamline to achieve small focal spot sizes. We measured a focal spot size of $1.1\ifmmode\times\else\texttimes\fi{}1.2\text{ }\mathrm{mm}$ leading to $5.8\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\mathrm{protons}\text{ }\mathrm{per}\text{ }\mathrm{s}\text{ }{\mathrm{cm}}^{2}$ at a central energy bin of $(9.55\ifmmode\pm\else\textpm\fi{}0.25)\text{ }\mathrm{MeV}$, which can be combined with a bunch duration below 500 ps at FWHM.