Contactless Inductive Flow Tomography (CIFT) is a flow measurement technique that is able to reconstruct the time-dependent three-dimensional velocity field in electrically conducting fluids, e.g., liquid metals, from magnetic field measurements. The paper describes the design of a specific CIFT measurement set-up for flow studies in liquid metal Rayleigh-Benard convection (RBC) in a large cylinder of aspect ratio (diameter/height) of Gamma = 0.5 filled with the ternary alloy GaInSn as model fluid. An optimized configuration for the CIFT excitation system and magnetic field sensor layout under consideration of the specific requirements for the application in turbulent RBC is determined by numerical simulations. The new experimental CIFT-RBC system resulting from the design process is constructed and a preliminary experiment at a Rayleigh number of Ra = 2.13 x 10(7) and a Prandtl number of Pr = 0.03 is performed and evaluated.

Contactless inductive flow tomography (CIFT) is a flow measurement technique allowing for visualization of the global flow in electrically conducting fluids. The method is based on the principle of induction by motion: very weak induced magnetic fields arise from the fluid motion under the influence of a primary excitation magnetic field and can be measured precisely outside of the fluid volume. The structure of the causative flow field can be reconstructed from the induced magnetic field values by solving the according linear inverse problem using appropriate regularization methods. The concurrent use of more than one excitation magnetic field is necessary to fully reconstruct three-dimensional liquid metal flows. In our laboratory demonstrator experiment, we impose two excitation magnetic fields perpendicular to each other to a mechanically driven flow of the liquid metal alloy GaInSn. In the first approach, the excitation fields are multiplexed. Here, the temporal resolution of the measurement needs to be kept as high as possible. Consecutive application by multiplexing enables determining the flow structure in the liquid with a temporal resolution down to 3 s with the existing equipment. In another approach, we concurrently apply two sinusoidal excitation fields with different frequencies. The signals are disentangled on the basis of the lock-in principle, enabling a successful reconstruction of the liquid metal flow.

In this paper, we present numerical and experimental results on helicity oscillations in a liquid-metal Rayleigh-Bénard (RB) convection cell, with an aspect ratio of 0.5. We find that helicity oscillations occur during transitions of flow states that are characterised by significant changes in the Reynolds number. Moreover, we also observe helicity oscillations at flow conditions where the temporal gradient of the change in the Reynolds number is significantly smaller than that of the helicity. Notably, the helicity oscillations observed during the transient double-roll state exhibit characteristics remarkably similar to those associated with the Tayler Instability.

In this paper we report on an experimental study focusing on the manifestation and dynamics of the large-scale circulation (LSC) in turbulent liquid metal convection. The experiments are performed inside a cylinder of aspect ratio $\varGamma = 0.5$ filled with the ternary alloy GaInSn, which has a Prandtl number of $Pr = 0.03$ . The large-scale flow structures are classified and characterized at Rayleigh numbers of ${Ra} = 9.33 \times 10^6, 5.31 \times 10^7$ and $6.02 \times 10^8$ by means of the contactless inductive flow tomography which enables the full reconstruction of the three-dimensional (3-D) flow structures in the entire convection cell. This is complemented with the multi-thermal-probe method for capturing the azimuthal temperature variation induced by the LSC at the sidewall. We use proper orthogonal decomposition (POD) to identify the dominating modes of the turbulent convection. The analysis reveals that a single-roll structure of the LSC alternates in short succession with double-roll structures or a three-roll structure. This is accompanied by dramatic fluctuations of the Reynolds number, whose instantaneous values can deviate by more than 50 % from the time-average value. No coherent oscillations are observed, whereas a correlation analysis indicates a residual contribution of the torsion and sloshing modes. Results of the POD analysis suggest a stabilization of the single-roll LSC with increasing $Ra$ at the expense of flow structures with multiple rolls. Moreover, the relative lifetime of all identified flow states, measured in units of free-fall times, increases with rising $Ra$ .

We investigated the integration of transient liquid-assisted growth (TLAG) approach for epitaxial YBa 2 Cu 3 O 7− x (YBCO) films by physical deposition methodologies (pulsed laser deposition (PLD)), as an additional opportunity for high-throughput growth of YBCO. As a prerequisite, highly flat and amorphous YBCO precursor films were deposited by PLD at temperatures below 400 °C on single-crystalline SrTiO 3 (STO) and LaMnO 3 (LMO)/STO, as well as industrial coated conductor architectures. Contrary to TLAG based on chemical solution deposition, where BaCO 3 elimination is a key factor for the YBCO growth, TLAG-PLD growth is controlled by the transformation of Ba–Cu–O (s) to a transient liquid. High-quality c -axis YBCO films were successfully grown on different substrates, as demonstrated by high-resolution x-ray diffraction and transmission electron microscopy. In-situ resistance measurements revealed that the growth rates around 1000 nm s −1 can be achieved, outperforming the capabilities of standard PLD growth of REBa 2 Cu 3 O 7 films by few orders of magnitude. Experimental conditions such as temperature, oxygen partial pressure, and heating ramp, were optimized to obtain critical temperature ( T c ) values up to 90 K. Critical current densities of 15 MA cm −2 at 5 K and 1.7 MA cm −2 at 77 K were obtained for YBCO films of 450 nm on LMO/STO.

Kurzfassung Mit der kontaktlosen induktiven Strömungstomografie (CIFT) lassen sich Geschwindigkeitsfelder in elektrisch leitfähigen Flüssigkeiten global bestimmen. Kenntnisse über den Strömungszustand in Metallschmelzen sind für industrielle Prozesse, wie das Stranggießen von Stahl, von immenser Bedeutung und können auch in der Grundlagenforschung nutzbringend angewendet werden, z.B. zur Analyse von konvektiven Flüssigmetallströmungen als Modellsysteme des Wärmetransportes. Das Verfahren beruht auf der präzisen Messung kleinster Magnetfeldänderungen durch geeignete Sonden und der nachfolgenden Rekonstruktion der Strömungsstruktur durch die Lösung eines linearen inversen Problems. In dieser Veröffentlichung geben wir einen Überblick über die Entwicklungen der letzten Dekade und diskutierten je einen Anwendungsfall für CIFT aus der grundlegenden und der angewandten Fluiddynamik.

Contactless inductive flow tomography (CIFT) can reconstruct the complex 3-dimensional flow structure of the large-scale circulation in liquid metal filled Rayleigh-Benard (RB) convection cells. The method relies on the precise measurement of weak magnetic fields induced by currents in the conducting liquid arising from the fluid motion in combination with primary excitation fields. The velocity distribution is reconstructed from the magnetic field measurements by solving a linear inverse problem using the Tikhonov regularization and L-curve method. A number of technical challenges have to be overcome to reach the desired accuracy of the measurement signals. In this paper we will describe our design of a new CIFT set-up for a large RB vessel with a diameter of 320 mm and a height of 640 mm. We outline the major factors perturbing the measurement signal of several tens of nanoteslas and describe solutions to decrease mechanical drifts by thermal expansion to a sub-critical level to enable CIFT measurements for high-Rayleigh number flows.

The time dependent behaviour of the large-scale circulation in a Rayleigh-Benard convection cell exhibits a rich set of different three-dimensional flow features like rotations or torsional modes. In this paper, the applicability of contactless inductive flow tomography to visualise these flow features in a cylindrical cell filled with GaInSn is investigated numerically. The simulated flow in the cylinder with a diameter of 320 mm and a height of 640 mm serves as a basis to investigate the quality of the reconstructed velocity field by contactless inductive flow tomography.

The effects of granularity on the angle and field dependence of the transport Jc in coated conductors (CCs) are discussed. The granularity is revealed by scanning Hall probe microscopy of the trapped field profiles in tapes with different architectures. It was found that pulsed laser deposited (PLD) YBCO film on a RABiTS NiW substrate has the most prominent granular morphology in its trapped field profiles. This is complemented by a peculiar behavior of the critical currents when the orientation of the applied field is within a certain angular range around the ab-planes: Jc(θ) becomes independent of the angle and Jc(B) exhibits a peak. These phenomena are explained by the physics of Abrikosov–Josephson vortices at grain boundaries (GB) that leads to a transition from a GB-limited to a grain-limited regime both in Jc(θ) and Jc(B). The effects of granularity are strongly suppressed by the chemical solution deposition route for growing YBCO films on NiW, and are almost absent in PLD-YBCO on ABAD-textured templates. These results identify the collective influence of the GB on the percolative current flow in CCs, which becomes most significant at low temperatures.

We studied the impact of soft ferromagnetic permalloy (Py) on the shielding currents in a strong-pinning superconductor—YBa2Cu3O7−δ with Ba2Y(Nb/Ta)O6 nano-precipitates—by means of scanning transmission x-ray microscopy. Typically and in particular when in the thin film limit, superconductor/ferromagnet (SC/FM) bilayers exhibit isotropic properties of the flux line ensemble at all temperatures. However, in elements with small aspect ratio a significant anisotropy in flux penetration is observed. We explain this effect by local in-plane fields arising from anisotropic magnetic stray fields originated by the ferromagnet. This leads to direction-dependent motion of magnetic vortices inside the SC/FM bilayer. Our results demonstrate that small variations of the magnetic properties can have huge impact on the superconductor.

Recent efforts in the development of YBa2Cu3O7-x (YBCO) coated conductors are devoted to the increase of the critical current I-c in magnetic fields. This is typically realized by growing thicker YBCO layers as well as by the incorporation of artificial pinning centers. We studied the growth of doped YBCO layers with a thickness of up to 7 mu m using pulsed laser deposition with a growth rate of about 1.2 nm s(-1). Industrially fabricated ion-beam textured YSZ templates based on metal tapes were used as substrates for this study. The incorporation of BaHfO3 (BHO) or Ba2Y(Nb0.5Ta0.5)O-6 (BYNTO) secondary phase additions leads to a denser microstructure compared to undoped films. A purely c-axis-oriented YBCO growth is preserved up to a thickness of about 4 mu m, whereas misoriented texture components were observed in thicker films. The critical temperature is slightly reduced compared to undoped films and independent of film thickness. The critical current density J(c) of the BHO- and BYNTO-doped YBCO layers is lower at 77 K and self-field compared to pure YBCO layers; however, I-c increases up to a thickness of 5 mu m. A comparison between films with a thickness of 1.3 mu m revealed that the anisotropy of the critical current density J(c)(theta) strongly depends on the incorporated pinning centers. Whereas BHO nanorods lead to a strong B vertical bar vertical bar c-axis peak, the overall anisotropy is significantly reduced by the incorporation of BYNTO forming a mixture of short c-axis-oriented nanorods and small (a-b)-oriented platelets. As a result, the J(c) values of the doped films outperform the undoped samples at higher fields and lower temperatures for most magnetic field directions.

An outstanding current carrying performance (namely critical current density, Jc) over a broad temperature range of 10-77 K for magnetic fields up to 12 T is reported for films of YBa2Cu3O7-x with Ba2Y(Nb,Ta)O6 inclusion pinning centres (YBCO-BYNTO) and thicknesses in the range of 220-500 nm. Jc values of 10 MA cm-2 were measured at 30 K - 5 T and 10 K - 9 T with a corresponding maximum of the pinning force density at 10 K close to 1 TN m-3. The system is very flexible regarding properties and microstructure tuning, and the growth window for achieving a particular microstructure is wide, which is very important for industrial processing. Hence, the dependence of Jc on the magnetic field angle was readily controlled by fine tuning the pinning microstructure. Transmission electron microscopy (TEM) analysis highlighted that higher growth rates induce more splayed and denser BYNTO nanocolumns with a matching field as high as 5.2 T. Correspondingly, a strong peak at the B||c-axis is noticed when the density of vortices is lower than the nanocolumn density. YBCO-BYNTO is a very robust and reproducible composite system for high-current coated conductors over an extended range of magnetic fields and temperatures.

Achieving low cost, safe, reproducible and high performance superconducting thin films of YBa2Cu3O7-δ is essential to bring this material to the energy market. Here, we report on the chemical solution deposition of YBa2Cu3O7-δ nanocomposites from environmentally benign precursors with a low-fluorine content. Preformed ZrO2 nanocrystals (3.5 nm) were stabilized in a methanolic precursor solution via two strategies: charge stabilization and steric stabilization. Counter-intuitively, charge stabilization did not result in high quality superconducting layers, while the steric stabilization resulted in highly reproducible nanocomposite thin films with a self-field Jc of 4-5 MA cm 2 (77 K) and a much smaller decay of Jc with magnetic field compared to YBa2Cu3O7-δ without nanocrystals. In addition, these nanocomposite films show a strong pinning force enhancement and a reduced Jc anisotropy compared to undoped YBa2Cu3O7-δ films. Given the relationship between the nanocrystal surface chemistry and final nanocomposite performance, we expect these results to be also relevant for other nanocomposite research.

The critical current I c (B) of YBa 2 Cu 3 O 7-δ (YBCO) coated conductors can be increased by growing thicker superconductor layers as well as improving the critical current density J c (B) by the incorporation of artificial pinning centers. We studied the properties of pulsed laser deposited BaHfO 3 (BHO)doped YBCO films with thicknesses of up to 5 μm on buffered rolling-assisted biaxially textured Ni-5 at% W tape and alternating beam assisted deposition textured Yttrium-stabilized ZrO 2 layers on stainless steel. X-Ray diffraction confirms the epitaxial growth of the superconductor on the buffered metallic template. BHO additions reduce the film porosity and lower the probability to grow misoriented grains, hence preventing the J c decrease observed in undoped YBCO films with thicknesses >2 μm. Thereby, a continuous increase in I c at 77 K is achieved. A mixed structure of secondary phase nanorods and platelets with different orientations increases J c (B) in the full angular range and simultaneously lowers the J c anisotropy compared to pristine YBCO.

Nanostructural modifications, in particular nanoparticle additions, have been shown to have great success in improving Superconducting material performance, such as REBa2Cu3Oy (REBCO) superconducting films [1-3] and iron pnictide films [4]. For REBCO coated conductors (CCs), NPs disrupt the superconducting parameter locally while maintaining crystallinity unperturbed elsewhere, thus pinning vortices at the NPs and therefore preventing dissipation. To be effective, the NP size has to be tuned, and the density needs to be higher for greater enhancement. We show how an economically viable method, namely trifluoroacetate metal organic deposition (TFA-MOD), can be tuned to obtain both small size and high density of NPs while maintaining the crystallinity of the REBCO matrix. We achieve these goals in two ways: by changing the nanoparticle material and by modulating the precursor chemistry with the result of constraining the NPs spatially. We get significant improvement of the in-field critical current density (Jc) for NP-doped REBCO CCs over a broad temperature range. The enhancements are seen not only in Jc but also in the reduction of the effects of thermal fluctuations (flux creep) at all magnetic fields measured. Detailed microstructural and superconducting properties for nanocomposite REBCO CCs will be presented.

Achieving low cost, safe, reproducible, and high performance superconducting thin films of YBa2Cu3O7-delta is essential to bring this material to the energy market. Here, we report on the chemical solution deposition of YBa2Cu3O7-delta nanocomposites from environmentally benign precursors with a low fluorine content. Preformed ZrO2 nanocrystals (3.5 nm) were stabilized in a methanolic precursor solution via two strategies: charge stabilization and steric stabilization. Counter-intuitively, charge stabilization did not result in high quality superconducting layers, while the steric stabilization resulted in highly reproducible nanocomposite thin films with a self-field J(c) of 4-5 MA cm(-2) (77 K) and a much smaller decay of J(c) with magnetic field compared to YBa2Cu3O7-delta without nanocrystals. In addition, these nanocomposite films show a strong pinning force enhancement and a reduced J(c) anisotropy compared to undoped YBa2Cu3O7-delta films. Given the relationship between the nanocrystal surface chemistry and final nanocomposite performance, we expect these results to be also relevant for other nanocomposite research.