Commissioning and optimization of superconducting undulator in the horizontal state

Taking advantage of a higher field than a conventional permanent magnet undulator, superconducting undulators (SCUs) have been rapidly developed in recent years for applications at synchrotron radiation sources and free-electron lasers. The Institute of High Energy Physics, Chinese Academy of Sciences, has developed a 1.5-m long planar SCU with a short period of 15 mm. This SCU has been commissioned and optimized in the horizontal position. In contrast to the vertical position, where the undulator coils are immersed in liquid helium for superconductivity, the horizontal test requires the assembly of the complete device, including the cryostat, current conductor, cryocooler, vacuum chamber, and undulator coils. This test reflects the operation status and measures the SCU's characteristics under working conditions. Although experiences from the vertical test provided good guidance for horizontal optimization, special care needs to be taken for effects such as gravity. After improving field uniformity through iterative measurements, the rms phase error of the SCU is reduced to 7.3° at a current of 400 A. Additionally, the functions of the end correction coils to mitigate field integral errors have been verified. This paper also demonstrates and analyzes other phenomena and interesting observations during the commissioning process. It discusses important issues that impact field quality and measurement accuracy and suggests further improvement measures for an even better field.