Collection and focusing of laser accelerated proton beam by an electromagnetic quadrupole triplet lens

To make use of laser-accelerated protons in real-world applications, specially-designed beamlines are required to control and improve the quality of the beam. At Peking University, the Compact LAser Plasma Accelerator (CLAPA) has been built, and is now coupled with an image-relaying beamline. The primary collection component of this beamline is an electromagnetic quadrupole triplet lens. As this lens is the first and the most crucial step for successful beam transport, we have systematically studied the focusing effect on the laser-driven protons, in terms of emittance, beam profile, and collection efficiency. We found that the normalized rms emittance of the protons can increase significantly, from the order of a fraction of mm⋅mrad to several mm⋅mrad, due to the dispersion of the phase space ellipse caused by the joint effect of the wide energy spread and the nonlinear field at the edge of the quadrupoles. The focusing profile and efficiency are also sensitive to the position and angular deviations of the injected proton beams. With fine adjustment, we found that the collection efficiency can be improved from lower than 10% up to 29%, for protons at the reference energy for CLAPA.