High-Temperature Superconductor Application to Undulators for Compact Free-Electron Lasers

Short-period high-field undulators are essential components for the production of brilliant coherent light up to X-rays in compact light sources, e.g. synchrotrons or free-electron lasers (FELs). The compactness should come with a small footprint and thus lower costs, having to operate at lower energies. On the other side of the energy spectrum, future lepton colliders like CLIC or FCC-ee demand high-field damping wigglers for the production of low-emittance beams in order to increase the collision luminosity. The requirement of high magnetic fields for such insertion devices (IDs) already stimulated further development from permanent magnet wigglers and undulators to superconducting IDs. Here, the state-of-the-art technology Nb-Ti already reached its performance limits and is commonly operated at 4.2 K or lower, making its operation and cooling complex and expensive. Therefore, the above-described accelerators may benefit from the application of high-temperature superconductors (HTS), like $\textit{Re}$BCO (rare-earth barium copper oxide): On the one hand, HTS makes magnetic field amplitudes in the range of 2 T or higher feasible for short undulator periods of 15 mm or smaller with magnetic gaps of 6 mm at 4.2 K or lower; this may also scale up to longer wiggler periods like 50 mm. On the other hand, larger temperature margins will make operations at higher temperatures like 20 K or higher achievable, which may relax cryogenic requirements and thus reduce costs. Within the scope of this thesis, the application of $\textit{Re}$BCO in the form of coated superconducting tape to superconducting undulators for compact FELs was investigated for the three most common coil geometries (vertical and horizontal racetracks as well as a helical undulator). The parameter space in terms of the undulator period length $\lambda_\text{u}$, the magnetic flux density amplitude, and the magnetic gap was studied by means of electro-magnetic calculations. This revealed an operation range of $\lambda_\text{u} \leq$ 20 mm, where the application of $\textit{Re}$BCO may be significantly superior to the current Nb-Ti technology. Consequently, modular coil models for all three geometries were designed with a $\lambda_\text{u}$ of 13 mm and studied for their electro-magnetic performance and mechanical durability. To deal with the high engineering current densities of 2 kA/mm$^2$ or more, a non-insulated (NI) winding approach was chosen. Subsequently, this thesis presents the technical design of a vertical racetrack (VR) coil and the very first design of a helical undulator wound with HTS tape, to the author's knowledge. Important characteristics of coated $\textit{Re}$BCO superconducting tapes for coil manufacturing were analyzed by bending and heating experiments as well as critical current measurements in self-field and external field at 77 K. Furthermore, winding and splicing techniques were developed for coil manufacturing. Three VR prototype coils were manufactured and powered successfully at 77 K in liquid nitrogen, proving the feasibility and stability of the NI coil design, by operations with up to 300% of the critical current. Powering tests of two VR coils at 4.2 K showed safe operations at high engineering current densities of 2.3 kA/mm$^2$ and generated magnetic fields of up to 1.5 T at 3.5 mm distance from the magnetic iron poles. The stability of the tested VR coils was once more demonstrated by operations beyond 3.6 kA/mm^2 with produced magnetic fields in the region of 2 T. Furthermore, the very first HTS helical undulator demonstrator was manufactured as a five-period short model wound with $\textit{Re}$BCO superconducting tape. The first powering tests at 77 K successfully proved the principle of this extremely compact and high-field undulator design. In summary, the potential of HTS for superconducting undulators has been demonstrated successfully as well as the feasibility to build and operate such designs with modular coils.