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Register a new userLower Critical Field Measurement System based on Third-Harmonic Method for Superconducting RF Materials
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We develop a lower critical field (Hc1) measurement system using the third-harmonic response of an applied AC magnetic field from a solenoid coil positioned above a superconducting sample. Parameter Hc1 is measured via detection of the third-harmonic component, which drastically changes when a vortex begins to penetrate the superconductor with temperature increase. The magnetic field locally applied to one side of the sample mimics the magnetic field within superconducting radio-frequency (SRF) cavities and prevents edge effects of the superconducting sample. With this approach, our measurement system can potentially characterize surface-engineered SRF materials such as Superconductor-Insulator-Superconductor multilayer structure (S-I-S structure). As a validation test, we measure the temperature dependence of Hc1 of two high-RRR bulk Nb samples and obtain results consistent with the literature. We also confirm that our system can apply magnetic fields of at least 120 mT at 4-5 K without any problem of heat generation of the coil. This field value is higher than those reported in previous works and makes it possible to more accurately estimate Hc1 at lower temperatures.
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The multilayer thin film structure of the superconductor has been proposed by A. Gurevich to enhance the maximum gradient of SRF cavities. The lower critical field Hc1 at which the vortex starts penetrating the superconducting material will be improved by coating Nb with thin film superconductor such as NbN. It is expected that the enhancement of Hc1 depends on the thickness of each layer. In order to determine the optimum thickness of each layer and to compare the measurement results with the theoretical prediction proposed by T. Kubo, we developed the Hc1 measurement system using the third harmonic response of the applied AC magnetic field at KEK. For the Hc1 measurement without the influence of the edge or the shape effects, the AC magnetic field can be applied locally by the solenoid coil of 5mm diameter in our measurement system. ULVAC made the NbN-SiO2 multilayer thin film samples of various NbN thicknesses. In this report, the measurement result of the bulk Nb sample and NbN-SiO2 multilayer thin film samples of different thickness of NbN layer will be discussed.
A comment to the authors SRF Conference pre-print [1] was submitted by A. Gurevich to the arXiv [2]. In this response, we show that the arguments used in the comment are not valid. [1] arXiv:1309.3239 [2] arXiv:1309.5626
Q0 determinations based on RF power measurements are subject to at least three potentially large systematic effects that have not been previously appreciated. Instrumental factors that can systematically bias RF based measurements of Q0 are quantified and steps that can be taken to improve the determination of Q0 are discussed.
We report a strong effect of the cooling dynamics through $T_mathrm{c}$ on the amount of trapped external magnetic flux in superconducting niobium cavities. The effect is similar for fine grain and single crystal niobium and all surface treatments including electropolishing with and without 120$^circ$C baking and nitrogen doping. Direct magnetic field measurements on the cavity walls show that the effect stems from changes in the flux trapping efficiency: slow cooling leads to almost complete flux trapping and higher residual resistance while fast cooling leads to the much more efficient flux expulsion and lower residual resistance.
The temperature behaviour of the first critical field ($B_{C1}$) of superconducting thin film samples can be determined with high accuracy using an inductive and contactless method. Driving a sinusoidal current in a single coil placed in front of the sample, a non zero third harmonic voltage $V_{3}$ is induced in it when Abrikosov vortices enter the sample. Conditions to be satisfied for the quantitative evaluation of $B_{C1}$ using this technique are detailed. As validation test, different type II superconductors (Nb, NbN, MgB$_{2}$ and Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-d}$ under the form of thin films) have been measured. The comparison between experimental results, data presented in literature and theoretical predictions is presented and discussed.
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