Design and manufacturing of an HTS helical undulator demonstrator for compact FELs

Among all superconducting undulator geometries, a helical undulator provides a very compact geometry and further is more effective in producing synchrotron radiation, providing circularly polarized photons, which are widely usable for synchrotron radiation users. These characteristics make it a very attractive option for realizing more compact free-electron lasers, requiring a combination of short-period and high-field undulators to produce coherent light up to X-rays. To further increase the magnetic field at 4.2 K and achieve larger operating margins as compared with low-temperature superconductors, the application of a high-temperature superconductor (HTS) in the form of coated rare-Earth barium copper oxide tapes was investigated. This article presents the design and manufacturing work done on an HTS helical undulator prototype—the very first helical undulator design based on HTS tape winding to the best of the authors' knowledge. To provide proof of concept, a five-period short-model demonstrator was realized by a bifilar, no-insulation winding scheme from a single piece of HTS cable, wound with a period length of 13 mm and a magnetic gap of 5 mm. ... mehrFirst powering tests at 77 K, performed in liquid nitrogen, revealed stable operations up to the calculated critical current of 140 A and even above. Higher currents of up to 160 A showed the expected coil protection, enabling the current to bypass the appearance of normal conducting zones and redistribute without degrading the superconductor.