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Register a new userMicrowave measurements of the high magnetic field vortex motion pinning parameters in Nb$_3$Sn
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The high frequency vortex motion in Nb$_3$Sn was analyzed in this work up to 12 T. We used a dielectric loaded resonator tuned at 15 GHz to evaluate the surface impedance $Z$ of a Nb$_3$Sn bulk sample (24.8 at.%Sn). From the field induced variation of $Z$, the high frequency vortex parameters (the pinning constant $k_p$, the depinning frequency $ u_p$ and the flux flow resistivity $rho_{ff}$) were obtained over a large temperature and field range; their field and temperature dependence were analyzed. Comparison with other superconducting materials shows that high frequency applications in strong magnetic fields are also feasible with Nb$_3$Sn. In the present work, we report the first measurements about the microwave response in Nb$_3$Sn in strong magnetic fields.
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We study mechanisms of vortex nucleation in Nb$_3$Sn Superconducting RF (SRF) cavities using a combination of experimental, theoretical, and computational methods. Scanning transmission electron microscopy (STEM) image and energy dispersive spectroscopy (EDS) of some Nb$_3$Sn cavities show Sn segregation at grain boundaries in Nb$_3$Sn with Sn concentration as high as $sim$35 at.% and widths $sim$3 nm in chemical composition. Using ab initio calculations, we estimate the effect excess tin has on the local superconducting properties of the material. We model Sn segregation as a lowering of the local critical temperature. We then use time-dependent Ginzburg-Landau theory to understand the role of segregation on magnetic vortex nucleation. Our simulations indicate that the grain boundaries act as both nucleation sites for vortex penetration and pinning sites for vortices after nucleation. Depending on the magnitude of the applied field, vortices may remain pinned in the grain boundary or penetrate the grain itself. We estimate the superconducting losses due to vortices filling grain boundaries and compare with observed performance degradation with higher magnetic fields. We estimate that the quality factor may decrease by an order of magnitude ($10^{10}$ to $10^9$) at typical operating fields if 0.03% of the grain boundaries actively nucleate vortices. We additionally estimate the volume that would need to be filled with vortices to match experimental observations of cavity heating.
Nb$_{3}$Sn is a superconductor of great relevance for perspective RF applications. We present for the first time surface impedance $Z_s$ measurements at 15 GHz and low RF field amplitude on Nb$_{3}$Sn in high magnetic fields up to 12 T, with the aim of increasing the knowledge of Nb$_{3}$Sn behavior in such conditions. $Z_s$ is a fundamental material parameter that directly gives useful information about the dissipative and reactive phenomena when the superconductor is subjected to high-frequency excitations. Therefore, we present an analysis of the measured $Z_s$ with the aim of extracting interesting data about pinning in Nb$_{3}$Sn at high frequencies. From $Z_s$ we extract the vortex motion complex resistivity to obtain the $r$-parameter and the depinning frequency $ u_p$ in high magnetic fields. The comparison of the results with the literature shows that the measured $ u_p$ on bulk Nb$_{3}$Sn is several times greater than that of pure Nb. This demonstrates how Nb$_{3}$Sn can be a good candidate for RF technological applications, also in high magnetic fields.
We investigate the effect of the anisotropy and of the directional pinning in YBa$_2$Cu$_3$O$_{7-x}$ films grown by pulsed laser ablation from targets containing BaZrO$_3$ at 5% mol. BaZrO$_3$ inclusions self-assemble to give nanorods oriented along the c-axis, thus giving a preferential direction for vortex pinning. The directionality of vortex response is studied at high ac frequency with the complex microwave response at 48 GHz, as a function of the applied field and of the angle $theta$ between the field and the c-axis. The complex microwave response does not exhibit any angular scaling, suggesting that the structural anisotropy of YBa$_2$Cu$_3$O$_{7-x}$ is supplemented by at least another preferred orientation. The pinning parameter $r$ shows evidence of directional pinning, effective in a wide range of angles around the c-axis (thus ascribed to BZO nanocolumns).
Demanding microwave applications in a magnetic field require the material optimization not only in zero-field but, more important, in the in-field flux motion dominated regime. However, the effect of artificial pinning centers (APC) remains unclear at high frequency. Moreover, in coated conductors the evaluation of the high frequency material properties is difficult due to the complicated electromagnetic problem of a thin superconducting film on a buffered metal substrate. In this paper we present an experimental study at 48 GHz of 150-200 nm YBa$_2$Cu$_3$O$_{7-x}$ coated conductors, with and without APCs, on buffered Ni-5at%W tapes. By properly addressing the electromagnetic problem of the extraction of the superconductor parameters from the measured overall surface impedance $Z$, we are able to extract and to comment on the London penetration depth, the flux flow resistivity and the pinning constant, highlighting the effect of artificial pinning centers in these samples.
Here we show that addition of Hf to Nb4Ta can significantly improve the high field performance of Nb$_{3}$Sn, making it suitable for dipole magnets for Future Circular Collider (FCC). A big challenge for the FCC is that a realistic production target for FCC Nb3Sn requires ~30% improvement over current conductor performance. Recent success with internal oxidation(IO) of Nb-Zr precursor has shown significant improvement in the layer J$_{c}$ of Nb$_{3}$Sn wires, albeit the complication of providing an internal O$_{2}$ diffusion path and avoiding degradation of irreversibility field($_{irr}$). We compare Zr and Hf additions to the standard Nb4Ta alloy of maximum H$_{c2}$ and H$_{irr}$. Nb4Ta rods with 1Zr or 1Hf were made into monofilament wires with and without SnO$_{2}$ and their properties measured over the entire superconducting range up to 31 T. We found that group IV alloying of Nb4Ta raises H$_{irr}$, though adding O$_{2}$ still degrades this slightly. As noted in Nb1Zr studies, the pinning force density F$_{p}$ is strongly enhanced and its peak value shifted to higher field by IO. A surprising result of this work is that we found better properties in Nb4Ta1Hf without SnO$_{2}$, F$_{pmax}$ achieving 2.35 times that of the standard Nb4Ta alloy, while the oxidized Nb4Ta1Zr alloy achieved 1.54 times that of the Nb4Ta alloy. The highest layer J$_{c}$ (16 T, 4.2 K) of 3700 A/mm$^{2}$ was found in the SnO$_{2}$-free wire made with Nb4Ta1Hf alloy. Using a standard A15 cross-section fraction of 60% for modern PIT and RRP wires, we estimated that a non-Cu J$_{c}$ of 2200 A/mm$^{2}$ is obtainable in modern conductors, well above the 1500A/mm$^{2}$ FCC specification. Moreover, the best properties were obtained without SnO$_{2}$, the Nb4Ta1Hf alloy appears to open a straightforward route to enhanced properties in Nb$_{3}$Sn wires.
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