Problem and solution with the longitudinal tracking of the ORBIT code

TL;DR

This paper identifies and corrects a bug in the ORBIT code's longitudinal tracking algorithm, ensuring it accurately preserves emittance during acceleration in high-intensity hadron synchrotrons.

Contribution

It reveals a flaw in the ORBIT code's longitudinal tracking and proposes a correction that aligns simulation results with physical principles.

Findings

The original ORBIT code's longitudinal tracking is erroneous during acceleration.

Correcting the code ensures longitudinal emittance is preserved as expected.

The improved code produces more accurate beam dynamics simulations.

Abstract

The ORBIT code has been widely used for beam dynamics simulations including injection and acceleration in high-intensity hadron synchrotrons. When the ORBIT's 1D longitudinal tracking was employed for the acceleration process in CSNS/RCS, the longitudinal emittance in eV-s was found decreasing substantially during acceleration, though the adiabatic condition is still met during this process. This is against the Liouville theorem that predicts the preservation of the emittance during acceleration. The recent machine study in the accelerator and the simulations with a self-made code demonstrate that the longitudinal emittance is almost invariant, which further indicates that the ORBIT longitudinal tracking might be incorrect. A detailed check-over in the ORBIT code source finds that the longitudinal finite difference equation used in the code is erroneous when applied to an acceleration process. The new code format PyORBIT has the same problem. After the small secondary factor is included in the code, ORBIT can produce results keeping the longitudinal emittance invariant. This paper presents some details about the study.