Superconducting undulator increases peak magnetic field

Synchrotron radiation facilities provide insights into the world of very small structures like microbes, viruses or nanomaterials and rely on dedicated magnet technology, which is optimised to produce highest intensity beams.

Synchrotron radiation is one of the most intense sources of x-rays and infrared radiation, and is a powerful tool for research in both industry and academia. It provides unique insights into the world of biology, medicine, chemistry and physics, and equips industrial researchers with non-destructive means of analysing materials or components of future devices and machines.

The team at the ANKA synchrotron radiation facility at the KIT, alongside its industrial partner Babcock Noell GmbH (BNG), have successfully developed, installed, and tested a novel superconducting undulator in the ANKA storage ring during the 2014/2015 winter shutdown. The full-length, high-performance superconducting undulator, named “SCU15”, is a 1.5m long device that forces electrons on an undulating path, using a periodic magnetic field with 100 periods of 15mm each.

An undulator is an array of dipole magnets with alternating magnetic field direction, which forces the beam of high-energy electrons onto an “undulating” path, leading to the emission of intense and highly collimated electromagnetic radiation. In contrast to common undulators that use permanent magnets, SCU15 uses superconducting electromagnets, relying on the ability of some materials at very low temperatures to conduct extremely high electrical currents with zero resistance. The high currents allow creating high magnetic fields inside the device. Thanks to high-precision components, SCU15 fulfills stringent demands on the accuracy of the magnetic field. This makes SCU15 the first full-length superconducting device to reach higher peak magnetic fields than comparable cryogenic permanent magnet undulators.

The development of superconducting undulators started in Karlsruhe in the early 1990s and KIT holds several patents applicable to superconducting undulator technology (e.g., H. O. Moser, B. Krevet, and H. Holzapfel (1991), Patent DE 4101094 C1). As early as 2005, a first superconducting undulator demonstrator with 14mm period length and 100 periods (developed in collaboration with Accel GmbH) was installed and operated in the ANKA storage ring. Valuable lessons were learned from this demonstrator device. In particular, it was observed that the heat load generated by the electron beam resulted in a limited peak magnetic field, smaller than the one reachable by permanent magnet undulators. Thanks to the lessons learned by ANKA and the extensive experience of BNG in designing and fabricating conduction cooled systems, such as the solenoid system operating successfully at the Spallation Neutron Source, U.S. Oak Ridge National Lab, a novel  design was implemented in SCU15 to overcome this barrier.

“The installation and reliable operation of the SCU15 is a major milestone in a long-term research & development programme devoted to next-gen insertion devices, and it is a significant breakthrough in superconducting insertion device design”, stated Dr. Sara Casalbuoni, Head of the Insertion Device Research Group, ANKA.

Cristian Boffo, Head of Development, Magnet Technologies, BNG, added: “Novel technological approaches implemented in the design of SCU15 have led to a successful completion of the first milestone in our KIT-BNG collaboration, making the SCU15 a world-wide unique device”. The joint development was supported through funding by the Helmholtz Association, enabling ANKA to conduct a multi-year research and development programme in the area of superconducting insertion devices.

As the aptly-named Prof. Anke-Susanne Müller, one of the three directors of ANKA and Head of Accelerator Research Activities, KIT, points out: “With future devices on the drawing board, the encouraging results obtained with the SCU15 at ANKA paint a bright picture for the future implementation of SCU devices in current synchrotron radiation facilities and the next generation of low-emittance light sources.”