Aki Korpela

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.

Optimization of SMES magnet volume with electromagnetic and mechanical constraints

A tool for solving a nonlinear optimization problem by means of sequential quadratic programming (SQP) and finite element method (FEM) is presented. In this paper both the electromagnetic and mechanical design objectives are considered when optimizing the volume of a 0.2 MJ conduction cooled Nb/sub 3/Sn superconducting magnetic energy storage (SMES) magnet, which is under construction at Tampere University of Technology. The results show that the combination of SQP and FEM provides a useful computational tool for the design of the superconducting applications.

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.

Mathematical model of voltage–current characteristics of Bi(2223)/Ag magnets under an external magnetic field

We have developed a mathematical model, which enables us to predict the voltage–current V(I) characteristics of a solenoidal high-temperature superconductor (HTS) magnet subjected to an external magnetic field parallel to the magnet axis. The model takes into account the anisotropy in the critical current–magnetic field (Ic(B)) characteristic and the n-value of Bi(2223)Ag multifilamentary tape at 20 K. From the power law between the electric field and the ratio of the operating and critical currents, the voltage on the magnet terminals is calculated by integrating the contributions of individual turns. The critical current of each turn, at given values of operating current and external magnetic field, is obtained by simple linear interpolation between the two suitable points of the Ic(B) characteristic, which corresponds to the angle α between the vector of the resulting magnetic flux density and the broad tape face. In fact, the model is valid for any value and orientation of external magnetic field, and is only limited by the validity of the electric field power law as a function of operating current. Electric fields of individual turns of the model magnet, which consists of 22 pancake coils, have been analysed for different values of operating current and external magnetic field. The voltage distribution on individual pancake coils and the overall voltage between the magnet terminals have also been analysed. Finally, the influence of external magnetic field on the V(I) characteristics of the magnet have been studied for different values of operating current. We report on a new and rather unexpected behaviour of the HTS magnets at different operating conditions, together with a theoretical explanation.

Design of a 0.2 MJ conduction cooled Nb/sub 3/Sn SMES system

A superconducting magnetic energy storage (SMES) system can be used to develop methods for improving power duality where a short interruption of power could lead to a long and costly shutdown. The commercialized /spl mu/-SMES concepts are based on NbTi technology. Nb/sub 3/Sn has excellent superconducting properties but unfortunately it is extremely brittle and usually the coils are made with the time consuming wind and react method. On the other hand, a Nb/sub 3/Sn magnet enables a coil operating temperature around 10 K. Considerable advances in cryocooler technology have been made during the past ten years-pushed partly by the progress of HTS technology. Therefore a Nb/sub 3/Sn coil with a reliable cryocooler unit could be a preferable alternative when designing different kinds of magnet systems. Based on these viewpoints a cryogen free Nb/sub 3/Sn SMES system operating at 10 K has been designed in order to compensate a short term loss of power.

Relation between different critical current criteria and quench current in HTS magnets

Abstract In practice, the critical current of an HTS magnet is the current at which a thermal runaway (quench) occurs. The stability analysis required to determine the quench current, Iq is often a time consuming numerical problem. Usually the short sample critical current, Ic is measured by using some electric field criterion. In trained LTS magnets Iq can be estimated from Ic due to a steep electric field (E)–current density (J) characteristic. For HTS magnets the situation is more complicated due to the anisotropy and slanted E(J)-characteristics. In this paper the theoretical maximum of Iq, I q max of conduction cooled HTS magnets is computationally compared with different critical current criteria at 20 K. Computations are based on the Ic data measured with a Bi-2223/Ag tape. Two electric field criteria, 1 and 0.1 μV/cm, are applied to the magnets by investigating both the maximum electric field in a single turn of the coil and the voltage between the magnet poles. The critical currents obtained by these criteria are compared with I q max in several coil geometries of a solenoidal conduction cooled HTS magnet having the wire length of 2, 5 and 10 km. I q max is determined from the balance between the available cooling power and the total loss power generated inside the magnet. The objective is to enable a magnet designer to determine a safe operation current for an HTS magnet without performing a detailed stability analysis.

Two ways to model voltage?current curves of adiabatic MgB2 wires

Usually overheating of the sample destroys attempts to measure voltage?current curves of conduction cooled high critical current MgB2 wires at low temperatures. Typically, when a quench occurs a wire burns out due to massive heat generation and negligible cooling. It has also been suggested that high n values measured with MgB2 wires and coils are not an intrinsic property of the material but arise due to heating during the voltage?current measurement. In addition, quite recently low n values for MgB2 wires have been reported. In order to find out the real properties of MgB2 an efficient computational model is required to simulate the voltage?current measurement. In this paper we go back to basics and consider two models to couple electromagnetic and thermal phenomena. In the first model the magnetization losses are computed according to the critical state model and the flux creep losses are considered separately. In the second model the superconductor resistivity is described by the widely used power law. Then the coupled current diffusion and heat conduction equations are solved with the finite element method. In order to compare the models, example runs are carried out with an adiabatic slab. Both models produce a similar significant temperature rise near the critical current which leads to fictitiously high n values.