Quantitative evaluation of a longitudinal rms emittance decrease using nonlinear space charge force in an intense deuteron linac

Radio frequency (rf) bunchers are often utilized to control a longitudinal bunch length in low- and medium-energy transport lines. However, nonlinear rf fields in the bunchers deform a longitudinal phase space distribution, which causes emittance growth leading to beam quality degradation and unexpected beam losses. This paper describes a decrease in the longitudinal rms emittance in a high-intensity deuteron linac, where the longitudinal phase space deformed by a nonlinear rf field in the buncher is self-linearized by a longitudinal nonlinear space charge force. To quantitatively explain this phenomenon, we develop two formulas to express deformations of phase space and forces acting on the beam. We perform particle tracking simulations with the racein code using a 125 mA, 5 MeV deuteron beam at the Linear IFMIF Prototype Accelerator (LIPAc) and validate the two formulas by applying them to LIPAc. The quantitative evaluations of the deformations with the formulas lead to a deeper understanding of beam physics and are also expected to be useful in mitigating phase space deformation.