I. Hiltunen

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.

The effect of sample holder and current ramp rate on a conduction-cooled V –I measurement of MgB2

In a voltage–current measurement of a superconductor the sample must be kept at a constant temperature. Otherwise, the measured critical currents and n values are distorted. However, when conduction cooling is applied and currents are high keeping a constant temperature is difficult, or even impossible, to achieve. We measured voltage–current characteristics of a conduction-cooled multifilamentary MgB2 tape in the self-field above 23 K. Simultaneously, the sample temperature was monitored. Due to the losses at the contact resistances, the sample holder and the sample, considerable sample warming was detected already below the critical current. Therefore, at low ramp rates, the critical current and the conductor n value cannot be determined from the measured voltage–current characteristic. However, when the ramp rate was increased, the critical currents and the n values levelled out. The required increase depended on the sample holder. We studied two sample holders and scrutinized the effect of thermal and electrical conductivities on the distorted critical currents and n values.

Measured Performance of Different Solders in Bi2223/Ag Current Leads

In superconducting magnet systems the current leads are usually divided into two parts. Normal metal like brass or copper is often used in the first part from the room temperature to the temperature of the radiation shield. The second part down to the magnet is made of high temperature superconductors (HTS). HTS leads can reduce the conductive heat load because they have poor thermal conductivity. Since HTS wires are lossless with direct current and have fair tolerance to the magnetic field only Ohmic losses are generated in the contact resistances at the current terminals. Thus, efficient current leads require that appropriate solders are used to reduce the contact resistances. In this paper, the thermal and contact resistances as well as thermal conduction losses of Bi-2223/Ag current leads are experimentally investigated using indium- and tin-based solders at operation temperatures between 20 and 77 K. Finally, the measured data is utilized to design an efficient HTS current lead to the current range of 0-1000 A.

CRYOGENIC DESIGN OF THE ALUHEAT PROJECT

ALUHEAT project aims to build a cryogen-free superconducting induction heater. In this paper, the cryogenic design, which includes heat loss calculations, mechanical analysis, optimization of current leads and description of cooling conditions, is presented in detail. The induction heater consists of two magnets in separate cryostats and the billet to be heated is rotated between the cryostats. Short distance between the magnet and cryostat wall, large forces between the magnets, and the need to rotate the billet create application specific requirements that must be taken into account. Finite element analysis is used to design the cryostat to withstand the forces created by the vacuum and magnetic field. Finally, thermal interface, radiation shield, magnet support and apparatus to rotate the billet are designed and all components assembled into the complete system.

Influence of Self-Heating on Measured

Knowledge of the voltage current characteristics, and especially n-values, is very important for the development of superconducting applications such as NMR-magnets and fault current limiters. n-values are usually determined by fitting the power law into a measured voltage-current-characteristics (V(I)) of a sample. However, the sample warms due to resistive losses even at subcritical currents, and thereby, measuring accurate characteristics at overcritical currents has proven to be difficult. Previously, we have developed a mathematical method to determine the critical current and n-value of a poorly cooled short sample. In this paper, we test this method with two samples of different materials in different cooling conditions. V(I) -curves of a MgB2 tape were measured at 30 K in vacuum environment and of a Bi-2223/Ag tape at 73 K in gas cooled environment to verify the effect of warming during the sample characterization. With high currents and slow current ramps the sample warmed up the most distorting the n-value significantly. However, when correction method was applied acceptable results were obtained from all samples.

n

Superconducting magnets operate at low temperatures, and therefore, even small heat pulses can ruin their stable operation. For example, resistive joints or changes in the operation current generate heat which must be extracted to prevent a quench. In impregnated magnets, the transverse thermal conductivity inside the coil has a vital influence on the heat extraction, and it dominates the 3D quench propagation. In this study, the transverse heat conductivity is measured from the cross-section of a small epoxy impregnated MgB2 coil at temperatures between 10 and 35 K. Finally, the results are analysed and compared with the results of a computational model based on heat conduction equation solved with the finite element method. The results show that effective thermal conductivity is over two orders of magnitude higher in the parallel direction with conductor axis compared to the perpendicular direction. The measured effective thermal conductivities at 20 K parallel to the broad tape face and perpendicular to the broad tape face were 1.55 W/mK and 0.31 W/mK, respectively. The fill factor of the measured coil was 60.5% for the whole conductor.

-Value

In particle accelerators quasi-DC superconducting magnets are used to keep particles in desired tracks. The needed rapid field variations of these high energy magnets require large energy bursts. If these bursts are taken from and fed back to the utility grid, its voltage is distorted and the quality of the electricity degrades. In addition, these bursts may decrease operation life time of generators and extra arrangements may be required by the electricity producers. Thus, an energy storage is an essential component for a cost-effective particle accelerator. Flywheels, capacitors and superconducting magnetic energy storage (SMES) are possible options for these relatively large and high power energy storages. Here we concentrate on AC-loss of a pulse SMES aiming to demonstrate the feasibility of NbTi SMES in a particle accelerator. The designing of a SMES requires highly reliable AC-loss simulations. In this paper, calorimetric AC-loss measurements of a NbTi magnet have been carried out to consider conductors suitability in a pulse SMES. In addition, the measured results are compared with AC-loss simulations.