Short-Period Superconducting Undulator Coils With Neutral Poles: Test Results

The success in creating new bright magnetic structures of synchrotron radiation (SR) sources allows generation of subnanometric-emittance electron beams. This has motivated many SR centers to upgrade the old structures to increase the brightness of the SR sources. Smaller emittance imposes higher requirements on the generators of undulator radiation. With the advent of new materials, the production of undulators on permanent magnets has progressed greatly. Superconducting undulators are less common, but as sources of undulator radiation they are much more promising than undulators with permanent magnets. This paper presents test results of a prototype superconducting undulator magnet using active and neutral poles with a period of 15.6 mm and an operational field of ~1.2 T. A pole gap of 8 mm provides a vertical aperture of 6 mm for an electron beam. In contrast to the traditional type of undulator with vertical racetrack coils, in this type of undulator the coils are of the horizontal racetrack type. The magnet design consists of the aluminum-alloy frame, into which individual poles are inserted. The design can significantly improve the manufacturing accuracy and, ultimately, reduce the phase errors. The coils are made of NbTi/Cu superconducting wires, which will be indirectly cooled by cryocoolers in own cryostat to the temperature of liquid helium.