Calculation of undulator section wakefield at the SHINE

Abstract

The wakefield is always an important issue in a free electron laser (FEL) facility. A longitudinal wakefield could lead to an energy spread of an electron bunch, and a transverse wakefield could lead to a deterioration in quality of an electron bunch. As a result, the performance of the FEL will be decreased under these impacts. Due to the discontinuous structures and the resistive wall of the chambers in the undulator section, the wakefield is ineluctable. The Shanghai high-repetition-rate XFEL and extreme light facility (SHINE) is under construction with three undulator lines as planned. These three lines have the similar framework, thus we consider the first undulator line (FEL-I) as a representation. FEL-I contains 40 undulators and many other devices in the inner segments between undulators which might generate a strong wakefield. In this paper, we give the theoretical and simulation results of the resistive wall wakefield in the vacuum chambers. In addition, the simulation result of geometry wakefields and the overall undulator section wakefield are given, which is as strong as predicted. Lasing simulations with and without wakefield are also given to show the impact of the wakefield. In order to counteract the damage to the FEL performance caused by the wakefield, we employ an undulator tapering scheme and the simulation result shows a great performance.