Risto Mikkonen

A numerical model for stability considerations in HTS magnets

We propose that in an HTS application, stability is lost more likely because of a global increase in temperature caused by heat generation distributed over the whole coil than because of a local normal zone which starts to propagate. For consideration of stability in HTS magnets, we present a computational model based on the heat conduction equation coupled with Maxwells equations, whereby analysis can be performed by using commercial software packages for computational electromagnetics and thermodynamics. For temperature distribution inside the magnet, we derive the magnetic field dependent effective values of thermal conductivity, specific heat, and heat generated by electromagnetic phenomena for the composite structure of the magnet, while cooling conditions and external heat sources are described as boundary conditions. Our model enables the magnet designer to estimate a safe level of the operation current before a thermal runaway. Finally, as examples, we present some calculations of the HTS magnet with ac to review the effects of slanted electric field-current density E (J ) characteristics and high critical temperature of HTS materials.

Effective thermal conductivity in HTS coils

Abstract We present a numerical model to determine the effective thermal conductivity in high- T c superconducting (HTS) coils. The model, based on the heat conduction equation solved with the finite element method, will help us to compute the temperature dependence of the effective thermal conductivity in coils with varying proportions of epoxy in the total winding volume. The conductor parameters in the coils correspond to those in a real Bi-2223/Ag multifilamentary composite tape, where the anisotropy of the Bi-2223 material has been taken into account. We will furthermore discuss about the effects of geometry of the composite structure on effective thermal conductivity. Our result shows that in epoxy impregnated HTS coils at temperatures below 60 K, the effective thermal conductivity is over two orders of magnitude higher in the parallel direction with the conductor axis compared to the perpendicular direction.

Stability considerations of multifilamentary MgB2 tape

The risk of overheating arises if the stability of an MgB2 coil is lost. This is mainly due to the low thermal conductivity of the matrix metal, which is typically iron or nickel. Due to a strong chemical reaction, copper can be used in contact with MgB2 only in specific cases: in situ preparation route and low formation temperature. However, recent developments in the manufacture of MgB2 conductors have resulted in stabilized MgB2 tapes. In any case, the stability considerations of MgB2 conductors are of great importance. We studied computationally the stability of an MgB2/Ni/Fe/Cu tape manufactured by Columbus Superconductors. First, the effective material properties of the tape were computed. Based on these material properties, we computed the basic quench characteristics (minimum propagation zone, minimum quench energy and normal zone propagation velocities) for the tape at 15, 20 and 25 K. The tape unit cell model was used in computations. The computed stability results were compared with the measured ones of a monofilament MgB2/Cu/Ni tape and commercial LTS and HTS. According to the results, the basic quench characteristics computed fit between the ones of LTS and HTS materials.

A checklist for designers of cryogen-free MgB2 coils

Intensive research has been directed at MgB2 since its discovery in 2001, focusing first on the material properties and conductor development and recently also on coil demonstrations. The relatively cheap and easy fabrication makes MgB2 a tempting material for superconducting applications. It can also be operated in the vicinity of 20 K, at which the commercial LTS materials are still in the normal state. However, commercial breakthrough requires practical applications and demonstrations. Therefore, we built a solenoidal react-and-wind MgB2 coil consisting of 46 m of commercially available MgB2 /Ni/Fe/Cu tape manufactured by Columbus Superconductors. We tested the coil in a cryogen-free environment and measured the effect of repeated cooldowns and current ramp rate on the coil critical current. Also, temperature homogeneity in the winding was studied. Based on the test results we point out features which should be checked when cryogen-free magnet systems are designed or their performance is discussed. For example, the coil critical current and n value can depend notably on the current ramp rate.

Optimization of HTS superconducting magnetic energy storage magnet volume

Nonlinear optimization problems in the field of electromagnetics have been successfully solved by means of sequential quadratic programming (SQP) and the finite element method (FEM). For example, the combination of SQP and FEM has been proven to be an efficient tool in the optimization of low temperature superconductors (LTS) superconducting magnetic energy storage (SMES) magnets. The procedure can also be applied for the optimization of HTS magnets. However, due to a strongly anisotropic material and a slanted electric field, current density characteristic high temperature superconductors HTS optimization is quite different from that of the LTS. In this paper the volumes of solenoidal conduction-cooled Bi-2223/Ag SMES magnets have been optimized at the operation temperature of 20 K. In addition to the electromagnetic constraints the stress caused by the tape bending has also been taken into account. Several optimization runs with different initial geometries were performed in order to find the best possible solution for a certain energy requirement. The optimization constraints describe the steady-state operation, thus the presented coil geometries are designed for slow ramping rates. Different energy requirements were investigated in order to find the energy dependence of the design parameters of optimized solenoidal HTS coils. According to the results, these dependences can be described with polynomial expressions.

Quench analysis of MgB2 coils with a ferromagnetic matrix

If a superconducting wire includes a ferromagnetic constituent, then magnet design requires a novel approach. We have previously reported on computing the critical current of coils that include ferromagnetic matrix material. Now we introduce a quench analysis in MgB2 solenoid and racetrack coils that utilize conductors with a ferromagnetic matrix. The computer code that was developed uses the commercial software MATLAB and FEMLAB. The code is also capable of simulating quench with coils wound of low-temperature superconductor (LTS) and high-temperature superconductor (HTS). The enhancement to this code compared to the earlier reported quench programs is that the non-linear magnetic behaviour due to the ferromagnetic matrix is taken into account. So far, the code does not take into account the conductor AC losses during current decay and the possible quench back due to the thermal interface.

Stability and quench of a HTS magnet with a hot spot

Stability and quench experiments were performed on a cryocooler refrigerated Bi-2223/Ag magnet with an artificial hot spot. The hot spot was realized by making a poor, high-resistivity joint between the current terminal and the bottom pancake coil of the magnet. According to the experiments the magnet could tolerate considerable power dissipation at the hot spot for long times still avoiding a thermal runaway. Processes preceding the quench occurred much slower if compared to quenches in typical LTS windings. For a deeper understanding of the measured results computer simulations were done by using a model based on the heat conduction equation coupled with Maxwells equations and solved using the finite-element method. Simulated temperature distributions inside the magnet showed that considerable temperature differences exist between different parts of the magnet, despite the slowness of the processes. Before a quench there were no large temperature gradients around the hot spot. During the quench the hot spot temperature increased rapidly but the temperature of other parts of the magnet increased only with a delay.

Magnetic separation of industrial waste waters as an environmental application of superconductivity

Open gradient magnetic separation enables continuous operation of the separator without the need of matrix-elements. Such a separator consisting of liquid helium cryostat and an interchangeable pair of NbTi and Nb3Sn coils were designed and constructed for testing the separation of various substances. As an environmental application, synthetic and genuine steel mill waste waters were purified with the separator. Synthetic waste water contained dissolved Cr while genuine steel mill waste water contained Cr, Fe, Ni and Mo both in particle and in ionic form. Chemical treatment was needed to make the dissolved metals susceptible for magnetic field; this was achieved with the help of an adsorbent and ferromagnetic magnetite. Separated sludge was collected by attached pipeline. Cleaned waste water was first pH-treated and then analyzed by AAS-spectroscopy. Results showed that open gradient separator is a promising alternative for matrix-type separators.

Quench current in conduction-cooled HTS magnets

The quench current of a superconducting magnet, Iq, is the current at which a thermal runaway occurs. In trained LTS magnets Iq can be estimated from a short sample critical current, Ic, due to a steep electric field (E)–current density (J) characteristic. Anisotropy and the slanted E(J)-characteristic make the situation more complicated in HTS magnets. Furthermore, the Iq of a conduction-cooled magnet depends strongly on the geometry of the thermal interface. Several criteria, such as the average electric field of 0.1 μV cm−1 and the maximum electric field of 1 μV cm−1 have been suggested for Iq of an HTS magnet. However, in order to determine Iq accurately a detailed stability analysis is required. In this paper different Iq criteria for conduction-cooled HTS magnets are computationally compared at the operation temperatures of 4.2, 20 and 77 K. Computations are based on the Ic data measured with a Bi-2223/Ag tape. 150 different solenoidal magnets having the wire length of 2, 5 and 10 km have been studied. The effect of the thermal interface geometry on Iq has also been investigated. Rules of thumb for the quick estimation of Iq at the given operation temperature are suggested.

Stability considerations of a high-temperature superconductor tape at different operating temperatures

Abstract Stability of a Bi-2223/Ag multifilamentary composite conductor against fast transport current ramps was studied by using a numerical model. The model was based on the two-dimensional magnetic diffusion and heat conduction equations. Calculations were carried out both in an adiabatic mode and pool boiling modes in liquid helium, hydrogen and nitrogen. When estimating the heat load (AC losses), real temperature dependent current density–electric field characteristics were used. The results computed by the finite element method are presented and discussed with special emphasis on differences of the stability considerations between high-temperature and low-temperature superconductors.