Doan N. Nguyen

Computation of Losses in HTS Under the Action of Varying Magnetic Fields and Currents

Numerical modeling of superconductors is widely recognized as a powerful tool for interpreting experimental results, understanding physical mechanisms, and predicting the performance of high-temperature-superconductor (HTS) tapes, wires, and devices. This is particularly true for ac loss calculation since a sufficiently low ac loss value is imperative to make these materials attractive for commercialization. In recent years, a large variety of numerical models, which are based on different techniques and implementations, has been proposed by researchers around the world, with the purpose of being able to estimate ac losses in HTSs quickly and accurately. This paper presents a literature review of the methods for computing ac losses in HTS tapes, wires, and devices. Technical superconductors have a relatively complex geometry (filaments, which might be twisted or transposed, or layers) and consist of different materials. As a result, different loss contributions exist. In this paper, we describe the ways of computing such loss contributions, which include hysteresis losses, eddy-current losses, coupling losses, and losses in ferromagnetic materials. We also provide an estimation of the losses occurring in a variety of power applications.

A new finite-element method simulation model for computing AC loss in roll assisted biaxially textured substrate YBCO tapes

This paper presents a new finite-element simulation model for computing the electromagnetic properties and AC losses in systems of YBCO (yttrium barium copper oxide) conductors on roll assisted biaxially textured substrates (RABiTS). In this model, the magnetic field dependent permeability and ferromagnetic loss of the substrates in RABiTS YBCO tapes are taken into account. The simulations were employed to simulate the AC loss in stacks of two parallel connected YBCO tapes. The simulation results are compared with the experimental data to check the validity of the simulation model. The result reveals an effective way of significantly reducing AC loss in YBCO tapes by stacking two RABiTS YBCO coated conductors with the appropriate relative tape orientation.

AC loss study of antiparallel connected YBCO coated conductors

Some applications of high temperature superconducting conductors require a non-inductive winding, which may be constructed from antiparallel connected YBCO (yttrium barium copper oxide) tapes. In the case of AC applications, this antiparallel winding changes the AC losses from that of an isolated conductor. This study focuses on the effect of the spatial separation and misalignment between conductors on their AC loss behavior for YBCO conductors on both rolling assisted biaxially textured substrate (RABiTS) and ion beam assisted deposition templates in an effort to fully understand the behavior of these conductors in real world applications. For RABiTS samples, the study was carried out for all three possible configurations (the so-called back-to-back, front-to-front and same-way configurations) to clarify the effect of the ferromagnetic substrate on the AC loss behavior in these conductor configurations. Numerical simulations were also employed in some cases to compare with and elucidate experimental observations.

AC losses in thin superconductors: the integral equation method applied to stacks and windings

In this paper we present a method for computing transport current ac losses in interacting thin superconductors. The method solves the integral equations for the sheet current density distribution and is specifically developed for those configurations where the symmetry of the current density distributions allows writing the equation in a self-consistent form, without the need for using an auxiliary 2D model to describe the interaction between superconducting tapes. This results in very short computation times and therefore the model can be very useful for optimizing the design of superconducting devices. The method has been tested for different cases of practical applications and the ac loss results have been compared with those obtained with analytical models and with experiments.

AC loss and critical current characterization of a noninductive coil of two-in-hand RABiTS YBCO tape for fault current limiter applications

The combination of noninductive winding and the two-in-hand tape configuration has been found to be very effective in reducing the AC loss of coils of RABiTS™ YBCO coated conductor for fault current limiting (FCL) applications. In this study, a noninductive coil composed of 11 turns of two-in-hand RABiTS™ YBCO tape was designed and constructed for characterizing and investigating the AC loss, critical current and inductance. The AC losses in this coil were also calculated by finite element modeling (FEM) using the COMSOL Multiphysics package. With the magnetic field dependent permeability and ferromagnetic loss of the substrate material taken into account, the simulation results are in good agreement with the experimental data. The simulations therefore were employed to study the effect on the AC loss behavior of the coil of the width of YBCO tape and of the spatial separation between the coil turns to suggest a more effective coil design.

Edge and top/bottom losses in non-inductive coated conductor coils with small separation between tapes

In this paper we consider two different finite-element models for computing ac losses in coils composed of coated conductors: a 2D model based on solving Maxwells equations by means of edge elements and a 1D model based on solving the integral equations for the current density in the tapes. The models are tested for a configuration of practical interest, a non-inductive solenoidal coil for fault current limiter applications. We focused our attention on the conditions when differences between the two models are expected to emerge, for example when the tapes are closely packed or when the dependence of the critical current density on the local magnetic field is taken into account. We present and discuss several cases, offering possible explanations for the observed differences of ac loss values.

Current Density Distribution in Multiple YBCO Coated Conductors by Coupled Integral Equations

In this paper we present a new model to compute the current density distribution and the AC losses in multiple YBCO coated conductors by means of coupled integral equations implemented in a finite-element (FE) environment. The model considers the superconducting tapes as 1D objects and allows overcoming the problem of the mesh of conductors of large aspect ratio, typical of 2D implementations; as a result, it is also much faster. The interaction between the different conductors is taken into account by means of a coupling of the electromagnetic variables. The magnetic field distribution in the space between the conductors is computed by coupling the 1D model with a standard 2D magnetic model, which uses the computed current density distributions as sources for the magnetic field. Results are compared with those obtained with standard 2D implementations and with experimental results.

Fast Numerical Computation of Current Distribution and AC Losses in Helically Wound Thin Tape Conductors: Single-Layer Coaxial Arrangement

In this paper, we introduce a very fast method to compute the current distribution in helically wound thin conductors when one or many of them are arranged in a symmetrical manner to form a single-layer power cable. The method relies on two different approaches to find the magnetic vector potential due to helically wound current sheets. By invoking relevant symmetry arguments associated with the geometry of the problem and neglecting the thickness of the tape conductors, we show that this 3-D problem can be reduced to a computationally small 1-D problem whose domain lies along the half-width of any of the constituting conductor. As a consequence, the proposed method is very efficient in terms of computational time, and it is more accurate than many previous 2-D methods that cannot take into account the twist pitch. Since the nonlinear resistivity of the superconducting material can easily be treated with this method, it can be used to find current and field distributions, as well as ac losses in high-temperature superconductor coils and cables made of coated tapes. To verify the validity of the proposed method, we performed experimental measurements of ac losses in two configurations of solenoid-type cables made of a sample of YBCO-coated conductor tape. Excellent agreement was observed between the experimental data and the simulation results.

Experimental and finite-element method studies of the effects of ferromagnetic substrate on the total ac loss in a rolling-assisted biaxially textured substrate YBa2Cu3O7 tape exposed to a parallel ac magnetic field

This paper presents a study of the total ac loss characteristics of a rolling-assisted biaxially textured substrate (RABiTS) YBa2Cu3O7 (YBCO) sample exposed to a parallel ac magnetic field. The results have shown that, for a given applied magnetic field and transport current, a RABiTS YBCO tape can generate very different magnitudes of ac loss, depending on whether the transport current and applied field have the same phase or opposite phase. The results of this study are very important for the optimization of the design of a RABiTS YBCO cable because they can suggest an appropriate arrangement of RABiTS tapes in a cable to minimize the cable ac loss. In this study, both experimental and finite-element method simulation approaches were employed. A modeling model that takes the magnetic field dependent permeability and ferromagnetic loss of the substrate into account reproduced well the experimental data for both self-field and total ac losses.

Status and Development of Pulsed Magnets at the NHMFL Pulsed Field Facility

The National High Magnetic Field Laboratorys Pulsed Field Facility at Los Alamos National Laboratory is one of few research centers in the world that can create and use ultrahigh pulsed magnetic field for scientific research. The facility houses several types of nondestructive pulsed magnets which can provide the peak fields ranging from 60 to 100 T for users. This paper will report the status of the user magnets and recent magnet developments to improve the quality and magnetic fields of our user magnets. In particular, this paper will present a new cooling technique that significantly reduces the cooling time without affecting the mechanical performance of the magnets. A possible upgrade of our facility with a new duplex magnet that is powered by a 4-MJ capacitor bank to deliver 80 T for users will be also reported. In addition, we will discuss some valuable lessons learned from magnet failures at our facility and possible further magnet design improvements to increase the magnet longevity or peak magnetic fields.