Emittance growth due to energy spread in a laser-driven proton beamline

Abstract

Collection of particles from a laser-driven accelerator into a conventional beamline requires careful manipulation of the beam to handle the wide energy spread, large angular divergence and short pulse lengths characteristic of laser-driven acceleration. Accurate simulation of the first few millimetres is crucial in order to optimise the collecting elements such as solenoids or quadrupole magnets. Experiments and simulations both show significant emittance growth in this collection region. We have developed a multi-particle beam dynamics model using impact-t that includes the characteristic exponentially-decaying laser-driven energy spectrum and accurately models the emittance growth due to the large energy spread. In this paper we present theoretical relationships between energy spread and emittance growth, and compare these with the simulated emittance growth. Results show excellent correspondence between theory and simulation. The effect is negligible for low-energy proton accelerators, but is important for electron accelerators and for proposed high-energy laser-driven proton accelerators.