Beam emittance growth due to the strong space-charge field at low energy of a high-intensity ion linac and its mitigation using an octupole magnetic field

Abstract

In the low-energy region of a high-intensity ion linac, a strong space-charge field causes a rapid beam emittance growth over a short distance of only a few meters. The beam emittance growth leads to beam loss and machine activation, raising a serious issue for regular maintenance of the accelerator component and beam power ramp-up. We studied the mechanism of beam emittance growth due to the space-charge field based on three-dimensional particle-tracking simulation and theoretical considerations. Numerical simulations of the high-intensity H⁻ (negative hydrogen) linac at the Japan Proton Accelerator Research Complex shows that the nonlinear terms in the space-charge field directly cause beam emittance growth and beam halo formation. We also propose a method to mitigate the beam emittance growth by using an octupole magnetic field, which arises as one of the nonlinear terms in the space-charge field. By applying this method in the simulation we have succeeded in mitigating the beam emittance growth.