A systematic study of superconducting properties of niobium films sputtered on the inner wall of radiofrequency cavities is presented. The measured quantities include in particular the response to 1.5 GHz microwaves, the critical temperature, the penetration depth and the magnetic penetration field. In addition to films grown in different gas discharges (Xe, Kr, Ar and Ar/Ne mixtures) and to films grown on substrates prepared under different conditions, the study also includes bulk niobium cavities. The surface resistance is analysed in terms of its dependence on temperature, on RF field and, when relevant, on the density of trapped fluxons. A simple parameterisation is found to give a good fit to the data. Once allowance for the presence of impurities and defects is made by means of a single parameter, the electron mean free path, good agreement with BCS theory is observed. The fluxon-induced losses are studied in detail and their dependence on RF field, on temperature and on the density of trapped fluxons is analysed. The residual resistance is observed to be essentially uncorrelated with the other variables, suggesting that it is dominantly extragranular. In occasions very low residual resistances, in the nΩ range, have been maintained over a broad range of RF field, indicating the absence of significant fundamental limitations specific to the film technology in practical applications such as the production of accelerating cavities for particle accelerators.

http://cds.cern.ch/record/433395/files/est-1999-002.pdf

Study of the surface resistance of superconducting niobium films at 1.5 GHz

We consider the nonlinear dynamics of a single vortex in a superconductor in a strong rf magnetic field ${B}_{0}\phantom{\rule{0.2em}{0ex}}\mathrm{sin}\phantom{\rule{0.2em}{0ex}}\ensuremath{\omega}t$. Using the London theory, we calculate the dissipated power $Q({B}_{0},\ensuremath{\omega})$ and the transient time scales of vortex motion. For the linear Bardeen-Stephen viscous drag force, vortex velocities reach unphysically high values during vortex penetration through the oscillating surface barrier. It is shown that penetration of a single vortex through the ac surface barrier always involves penetration of an antivortex and the subsequent annihilation of the vortex-antivortex pairs. Using the nonlinear Larkin-Ovchinnikov (LO) viscous drag force at higher vortex velocities $v(t)$ results in a jumpwise vortex penetration through the surface barrier and a significant increase of the dissipated power. We calculate the effect of dissipation on the nonlinear vortex viscosity $\ensuremath{\eta}(v)$ and the rf vortex dynamics and show that it can also result in the LO-type behavior, instabilities, and thermal localization of penetrating vortex channels. We propose a thermal feedback model of $\ensuremath{\eta}(v)$, which not only results in the LO dependence of $\ensuremath{\eta}(v)$ for a steady-state motion, but also takes into account retardation of the temperature field around a rapidly accelerating vortex and a long-range interaction with the surface. We also address the effect of pinning on the nonlinear rf vortex dynamics and the effect of trapped magnetic flux on the surface resistance ${R}_{s}$ calculated as a function of rf frequency and field. It is shown that trapped flux can result in a temperature-independent residual resistance ${R}_{i}$ at low $T$ and a hysteretic low-field dependence of ${R}_{i}({B}_{0})$, which can decrease as ${B}_{0}$ is increased, reaching a minimum at ${B}_{0}$ much smaller than the thermodynamic critical field ${B}_{c}$. We propose that cycling of the rf field can reduce ${R}_{i}$ due to rf annealing of the magnetic flux which is pumped out by the rf field from a thin surface layer of the order of the London penetration depth.

Dynamics of vortex penetration, jumpwise instabilities and nonlinear surface resistance of type-II superconductors in strong rf fields

/pdf/superconducting-rf-materials-other-than-bulk-niobium-a-4qj2n6er8g.pdf

Superconducting RF materials other than bulk niobium: a review

The first niobium-coated copper cavities were produced at CERN in the early 1980s. The sputter coating technology was chosen as a basis for development, first in the pure diode configuration and subsequently in the magnetron configuration, which was then adopted for the successful series production of the LEP and LHC cavities. In parallel, an intensive R&D effort was undertaken at CERN and other Laboratories in order to understand the advantages and limitations of this technique. Some highlights of the present understanding will be given. Several new developments in the coating technique are being pursued around the world, which will be discussed together with their motivations.

/pdf/20-years-of-experience-with-the-nb-cu-technology-for-3na91xsxta.pdf

20 Years of experience with the Nb/Cu technology for superconducting cavities and perspectives for future developments

We present the design and realization of cylindrical dielectric resonators operating in the 40–60 GHz frequency range, designed for the measurement of the surface resistance and of the surface reactance shift in High-Tc Superconductors (HTS) thin films in a dc magnetic field. The resonators are single tone, based on the TE011 mode, and multiple tone, the latter allowing in principle to exploit the simultaneous determination of the surface impedance at different frequencies. As an application example, we report the temperature and field dependencies of the effective surface impedance of some cuprate superconductors thin films. The results are compared with those obtained through the use of a standard metal cavity with a similar Q-factor and operating in the same frequency range. The comparison highlights a superior stability and a higher sensitivity, resulting in an increase of about two orders of magnitude in the resolution of the surface impedance measurement. By contrast, the dissipative part of the superconducting transition (above T/Tc = 0.97) is better studied with the metal cavity. We also present measurements in the vortex state in YBaCa2Cu3O7−δ and Tl2Ba2CaCu2O8+x that show significantly different physics.

Dielectric Resonators for the Measurement of Superconductor Thin Films Surface Impedance in Magnetic Fields at High Microwave Frequencies

High-frequency experiments depend sensitively on homogeneous and inhomogeneous “defects” in the normal and superconducting state. As homogeneous effects, the intrinsic scattering time is of great theoretical importance above 0.1 THz in the surface impedanceZ. Of practical importance are the planar defects, “the weak links (WL),” which interrupt the rf shielding currents and thus enhanceZeff. In the superconducting state, the Josephson currentjcJ crosses the WL in parallel with the normal, leakage currentjbl. The latter explains the observed, finite rf residual lossesRres(T∼0) quantitatively and as a function of material parameters, temperatureT, fieldH, and frequencyΩ for Nb, NbN, and cuprate superconductors. With increasing field,Z deteriorates likeH2 up toHcIJ, where Josephson fluxons (JF) penetrate into the WL yieldingδZ∫ αH. Above HcIJ≈0.1–10 mT, JF dynamics dominatesZ with hysteresis losses and reactive components. The nonlinear JF effects are enforced by thin-film edge enhancements limiting the performance of various devices by enhanced dissipation, reactance, and flux noise. A method is presented which is able to separate electron dynamics at the WL from their strength and distribution.

J. Halbritter

Papers

Granular superconductors and their intrinsic and extrinsic surface impedance