Numerical analysis of the end-field with correction coils for the superconducting undulator at IHEP

Superconducting undulators (SCUs) are developing fast around the world. An R&D project is undergoing at Institute of High Energy Physics (IHEP) in China to produce a prototype of 15 mm period length planar undulator with superconducting technology. For undulators, the first and second magnetic field integrals along the longitudinal axis should be zero to avoid disturbing the beam trajectory. By using the finite element program OPERA-3D, we built the SCU model to study the magnetic field distribution on axis. We designed two correction coil pack on each end of the SCU core. The end-field can be adjusted flexibly with different correction currents. Our study in this paper reveals that the magnetic field integrals are highly dependent on the end-field. The relationship between the end-field integrals and the correction currents are given based on polynomial fitting. The optimal correction currents are obtained via numerical analysis.