Superconducting RF cavities are an essential part of particle accelerators. They have been made of Nb for more than 50 years. This is due to the good workability of Nb and the high critical temperature (Tc= 9.25 K), the highest of all pure metals, as well as the high critical field Hc. The RF performance of Nb has been continuously improved over the years and is already close to the intrinsic limit of the material [1]. Therefore, in recent years, the search for alternative materials with higher critical temperature Tc, low surface resistance and higher critical field Hc has been intensified. On the one hand, this should lead to improved acceleration gradients, but also the cryogenic efficiency can be significantly improved, since Nb cavities are operated at about 2 K [2].
Nb3Sn is a suitable candidate due to its much higher critical temperature (~ 18 K) and higher critical field Hc compared to Nb. Attempts to fabricate thin Nb3Sn films for cavities began as early as the 1980s [3,4]. The fabrication techniques have been continuously developed and improved in recent years. The most advanced method is by vapor diffusion, in which an Nb3Sn film is built up on an Nb substrate (or Nb cavity) [5-8]. Studies showed that the roughness of the surface of the films and local differences in chemical composition or non-uniform film thickness affect the superconducting RF properties in this process [9-11]. Instead of Nb, Nb3Sn can also be deposited on a Cu substrate as demonstrated by direct current magnetron sputtering (DCMS), where Cu has the advantage of better thermal conductivity compared to Nb. A disadvantage of the Cu substrate is the interdiffusion of Cu into the Nb3Sn layer during prolonged heat treatment [11].
High power impulse magnetron sputtering (HiPIMS) is an alternative method for thin film fabrication. This technique has already been used to produce thin Nb films on Cu substrate [12]. Similarly, attempts to deposit Nb3Sn on Cu substrate using bipolar HiPIMS have already been described [13].
In the present work, we report the characterization of the microstructure of Nb3Sn films deposited on Cu by bipolar HiPIMS, where a thin Ta interlayer was first deposited on the Cu substrate. The influences of the different fabrication conditions on the microstructure were analyzed by scanning electron microscopy (SEM), focused ion beam (FIB) and transmission electron microscopy (TEM). The Ta layer is expected to prevent the diffusion of Cu into the A15 layer and onto the surface during the generation of the Nb3Sn layer. During the sputtering process, Kr was used as a chamber gas at either low pressure (7 x 10-4 mbar) or high pressure (2.5x10-2 mbar). The Nb-Sn targets used for HIPIMS preparation were either stoichiometric (25 % Sn) or enriched (27 % Sn). The formation of the A15 phase took place either during the sputtering process at a temperature of 750 °C ("reaction during coating") or during a heat treatment at the same temperature after sputtering at lower temperatures ("reaction after coating"). Similarly, the effect of changing Ta layer fabrication conditions on the formation of the Nb3Sn grains was investigated. The penetration of magnetic vortices into the superconducting layer causes losses resulting in a reduction of the quality factor of the cavity. Flux penetration was investigated by SQUID magnetometry and Scanning Hall Probe Microscopy (SHPM) in order to find weak spots where vortices enter preferentially and relate them to microstructural features revealed by electron microscopy.
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The present work was part of the FCC Study by The European Organization for Nuclear Research (“CERN”), Addendum KE5389/ATS.