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Register a new userFlux-transfer losses in helically wound superconducting power cables
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Minimization of ac losses is essential for economic operation of high-temperature superconductor (HTS) ac power cables. A favorable configuration for the phase conductor of such cables has two counter-wound layers of HTS tape-shaped wires lying next to each other and helically wound around a flexible cylindrical former. However, if magnetic materials such as magnetic substrates of the tapes lie between the two layers, or if the winding pitch angles are not opposite and essentially equal in magnitude to each other, current distributes unequally between the two layers. Then, if at some point in the ac cycle the current of either of the two layers exceeds its critical current, a large ac loss arises from the transfer of flux between the two layers. A detailed review of the formalism, its application to the case of paramagnetic substrates including the calculation of this flux transfer loss is presented.
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We investigate theoretically the dependence of magnetization loss of a helically wound superconducting tape on the round core radius $R$ and the helical conductor pitch in a ramped magnetic field. Using the thin-sheet approximation, we identify the two-dimensional equation that describes Faradays law of induction on a helical tape surface in the steady state. Based on the obtained basic equation, we simulate numerically the current streamlines and the power loss $P$ per unit tape length on a helical tape. For $R gtrsim w_0$ (where $w_0$ is the tape width), the simulated value of $P$ saturates close to the loss power $sim(2/pi)P_{rm flat}$ (where $P_{rm flat}$ is the loss power of a flat tape) for a loosely twisted tape. This is verified quantitatively by evaluating power loss analytically in the thin-filament limit of $w_0/Rrightarrow 0$. For $R lesssim w_0$, upon thinning the round core, the helically wound tape behaves more like a cylindrical superconductor as verified by the formula in the cylinder limit of $w_0/Rrightarrow 2pi$, and $P$ decreases further from the value for a loosely twisted tape, reaching $sim (2/pi)^2 P_{rm flat}$.
We use a model of vortex dynamics and collective weak pinning theory to study the residual dissipation due to trapped magnetic flux in a dirty superconductor. Using simple estimates, approximate analytical calculations, and numerical simulations, we make predictions and comparisons with experiments performed in CERN and Cornell on resonant superconducting radio-frequency NbCu, doped-Nb and Nb$_3$Sn cavities. We invoke hysteretic losses originating in a rugged pinning potential landscape to explain the linear behavior of the sensitivity of the residual resistance to trapped magnetic flux as a function of the amplitude of the radio-frequency field. Our calculations also predict and describe the crossover from hysteretic-dominated to viscous-dominated regimes of dissipation. We propose simple formulas describing power losses and crossover behavior, which can be used to guide the tuning of material parameters to optimize cavity performance.
Measurements of the ac response represent a widely-used method for probing the properties of superconductors. In the surface superconducting state (SSS), increase of the current beyond the surface critical current $I_c$ leads to breakdown of SSS and penetration of external magnetic field into the sample bulk. An interesting free-of-bulk system in SSS is offered by thin-walled superconducting cylinders. The critical state model (CSM) asserts the ac susceptibility $chi$ to exhibit jumps as a function of the external ac field amplitude $H_{ac}$, because of the periodic destruction and restoration of SSS in the cylinder wall. Here, we investigate experimentally the low-frequency (128-8192,Hz) ac response of thin-walled superconducting cylinders in superimposed dc and ac magnetic fields applied parallel to the cylinder axis. Distinct from the CSM predictions, experiments reveal that $chi$ is a smooth function of $H_{ac}$. For the explanation of our observations we propose a phenomenological model of partial penetration of magnetic flux (PPMF). The PPMF model implies that after a restoration of the superconducting state, the magnetic fields inside and outside the cylinder are not equal, and the value of the penetrating flux is random for each penetration. This model fits very well to the experimental data on the temperature dependence of the first-harmonic $chi_1$ at any $H_{ac}$ and dc field magnitude. However, in a certain temperature range the values of physical parameters deduced within the framework of the PPMF model are questionable.
We describe an experimental protocol to characterize magnetic field dependent microwave losses in superconducting niobium microstrip resonators. Our approach provides a unified view that covers two well-known magnetic field dependent loss mechanisms: quasiparticle generation and vortex motion. We find that quasiparticle generation is the dominant loss mechanism for parallel magnetic fields. For perpendicular fields, the dominant loss mechanism is vortex motion or switches from quasiparticle generation to vortex motion, depending on cooling procedures. In particular, we introduce a plot of the quality factor versus the resonance frequency as a general method for identifying the dominant loss mechanism. We calculate the expected resonance frequency and the quality factor as a function of the magnetic field by modeling the complex resistivity. Key parameters characterizing microwave loss are estimated from comparisons of the observed and expected resonator properties. Based on these key parameters, we find a niobium resonator whose thickness is similar to its penetration depth is the best choice for X-band electron spin resonance applications. Finally, we detect partial release of the Meissner current at the vortex penetration field, suggesting that the interaction between vortices and the Meissner current near the edges is essential to understand the magnetic field dependence of the resonator properties.
We experimentally investigate the vortex induced energy losses in niobium coplanar waveguide resonators with and without quasihexagonal arrays of nanoholes (antidots), where large-area antidot patterns have been fabricated using self-assembling microsphere lithography. We perform transmission spectroscopy experiments around 6.25 and 12.5 GHz in magnetic field cooling and zero field cooling procedures with perpendicular magnetic fields up to B=27 mT at a temperature T=4.2 K. We find that the introduction of antidot arrays into resonators reduces vortex induced losses by more than one order of magnitude.
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