Characterization of Nb3Sn and multilayer thin films for SRF applications

Basic R&D on materials for superconducting radio frequency (SRF) applications requires RF
measurements on samples with high resolution. The Quadrupole Resonator (QPR) at Helmholtz-Zentrum Berlin is a dedicated test cavity, enabling RF characterization of samples in a wide parameter space of temperature and RF field strength at three frequencies. Thereby, the instrument covers typical real-life accelerator conditions without being limited to it.
Within the scope of this thesis, the QPR measurement capabilities were continuously expanded
and improved, allowing calorimetric measurements of RF surface resistance at all three operating frequencies. Furthermore, magnetic penetration depth, critical temperature (Tc) and the RF critical field can be studied. From those directly measured values, superconducting and normal conducting quantities such as DC critical fields, Ginzburg-Landau parameter, mean free path and normal state resistivity can be derived, yielding a multi-parameter characterization of the investigated sample.
In this work, two superconducting coatings were characterized: One single layer of Nb3Sn and
a NbTiN on Nb multilayer structure. Nb3Sn is one of the most promising alternative materials
to niobium and already showed high quality factors in coated research cavities. The sample
investigated here underscores the potential of this high-Tc material for low surface resistance.
Ultimately, this could enable low-loss continuous-wave particle accelerators. However the SRF
performance failed to reach theoretical values, presumably due to off-stoichiometric areas that
were subsequently identified using surface analysis. Furthermore, the RF quench field measurement indicated practical difficulties in reaching the superheating critical field of Nb3Sn with the current coating process.
Multilayer structures of superconducting thin films are expected to increase the superheating
critical field of a bulk superconductor, corresponding to higher maximum achievable accelerating gradients. With the second sample characterized in this work, the theoretical descriptions of magnetic penetration depth and superheating field for multilayer structures can be confirmed. The observation of non-monotonic surface resistance as a function of temperature reveals additional contributions that are not included in current theories and opens a promising field for future studies.