G.B.J. Mulder

Quench characteristics of a two-strand superconducting cable and the influence of its length

The quench process of a multi-strand cable was investigated using the simplest system: two twisted wires. Several properties of the quench, such as the commutation of currents, the time scale, the resistance rate, and the maximum voltage, were determined experimentally or by calculation. Particular attention was given to the role of the cable length. Several samples with lengths varying from 1.5 cm to 12 m were made from an AC superconductor with CuNi matrix. In the experiment, the decay of the currents was measured after initiating a local normal spot in one of the wires. An important conclusion is that the quench propagation and stability of a cable depend on its length and can therefore be influenced by soldering it at certain intervals. >

On the inductive method for maximum current testing of superconducting cables

In order to test superconducting cables at high currents it is convenient to generate the required transport current inductively, i.e. by means of a superconducting transformer. The paper gives a survey of the devices in different laboratories that apply this technique to test cables above 20 kA. An existing test facility at the University of Twente, suitable for 50 to 200 kA, is treated in more detail. Specific aspects of such a facility are discussed, for example the design of the transformer, the methods to measure the current in the superconducting secondary circuit and the fabrication of joints with a sufficiently low electrical resistance.

Development of a thermally switched superconducting rectifier for 100 kA

A full-wave superconducting rectifier for 100 kA has been developed. Typical design values of this device are: a secondary current of 100 kA, a primary amplitude of 20 A, an operating frequency of 0.5 Hz, and an average power on the order of 100 W. The rectification is achieved by means of thermally controlled superconducting switches with recovery times of 150 to 300 ms. A description of the rectifier system is given. The first experiments, in which the rectifier was tested at up to 25 kA demonstrate reliable and fail-safe operation of the rectifier at lower current levels. It was, for example, successfully used to load and unload a 25-kA coil at a rectifier frequency of 0.4 Hz and an average power of 30 W. During tests without any load, it was found that the secondary circuit of the transformer quenches at about 60 kA. Therefore, it is unlikely that the rectifier in its present configuration will attain 100 kA.

Thermally and magnetically controlled superconducting rectifiers

The switches of a superconducting rectifier can be controlled either magnetically or thermally. The authors point out the differences between these methods of switching and discuss the consequences for the operation of the rectifier. The discussion is illustrated by the experimental results of a rectifier which was tested with magnetically as well as thermally controlled switches. It has an input current of 30 A, an output current of more than 1 kA and an operating frequency of a few Hz. A superconducting magnet connected to this rectifier can be energized at a rate exceeding 1 MJ/h and an efficiency of about 97%. >

Experimental results of thermally controlled superconducting switches for high frequency operation

As part of a study to develop thermally controlled switches for use in superconducting rectifiers operating at a few hertz and 1 kA, a theoretical model is presented of the thermal behavior of such a switch. The calculations are compared with experimental results of several switches having recovery times between 40 and 200 ms. A discussion is given of the maximum temperature T/sub N/ that occurs in the normal regions when the switch is in the resistive state. Once T/sub N/ is known, it is possible to predict the recovery time, activation energy, stationary dissipation and minimum propagation current. The calculated and measured results, in good agreement, show that T/sub N/ is approximately 12 K and largely independent of the thickness or material of the insulation layer. Mention is made of some problems, related to the room-temperature equipment which drives the rectifier, that so far have prevented the rectifiers from being used at their design specifications. >

A study of quench current and stability of high-current multi-strand cables having a Cu or CuNI matrix

This paper discusses the experimental results concerning maximum current and stability of two braided superconducting cables. The expected critical current of both conductors is 95 kA under self field conditions, at 4. 2 K. An essential difference is that one of these conductors has a pure CuNi matrix, the other a Cu matrix. The maximum current of the cables was measured as a function of the temperature and the ramp rate of the current. We observed a remarkable decrease of the current-carrying capacity with increasing current rate in both cables, independent of the matrix material. Furthermore, the stability of the cables was investigated.

A fast operating magnetically controlled switch for 1 kA

The power of fully superconducting rectifiers can be improved by increasing either the operating frequency or the transformer primary inductance [1]. The frequency is usually limited by the recovery time of thermally controlled switches. In order to achieve a higher switching speed, magnetically controlled switches are preferable [1,2]. This paper describes a magnetically controlled switch which can be used for currents up to 500 A at 25 Hz. The switch element, consisting of several Nb1%Zr multifilamentary superconductors, is placed between two concentric solenoids which generate the necessary magnetic field. The Nb1%Zr superconductor is well suited for this purpose because of its relatively low critical field (≃ 0.75 T) and high maximum current density (about 5.109A/m2below 0.3 T).

A new test setup to measure the AC losses of the conductors for NET

A description is given of a new test system currently under construction. The system will be used to measure the AC losses of subcables from Next European Torus (NET) conductors. A special feature of the test arrangement is that the losses will be determined while the sample carries a transport current and is at the same time subjected to a changing magnetic field in the transverse and longitudinal directions. Several aspects of the design, such as magnetic field, forces, and losses, are discussed.

A high-power magnetically switched superconducting rectifier operating at 5 Hz

Above a certain current level, the use of a superconducting rectifier as a cryogenic current source offers advantages compared to the use of a power supply at room temperature which requires large current feed-throughs into the cryostat. In some cases, the power of such a rectifier is immaterial, for example if it is to be used as a current supply for short test samples with low inductances. Usually, however, a rectifier is intended to energize large superconducting magnets, so the maximum power available becomes an important parameter since it determines the loading time. One method of increasing the power of a rectifier is to raise the operating frequency. In this respect, magnetically controlled switches with very fast switching times are preferable to thermally controlled ones. This paper reports on the design, as well as the experimental results of a magnetically switched full-wave superconducting rectifier. Once this rectifier is brought to its design frequency of 5 Hz, the average power delivered to the cryogenic load will be 500 W.

A convenient method for testing high-current superdconducting cables

Superconducting cables with critical currents in the range of 50 kA to 200 kA are presently under investigation in Twente. Therefore, a low-cost test facility has been developed in which the required current is generated inductively, allowing cables to be tested up to at least 200 kA. A superconducting transformer induces the current through the sample and provides the background magnetic field. The secondary part of the transformer consists of one turn of the test cable. A part of it can be varied in temperature by means of a heater. The data aquisition, analysis and storage is accomplished by means of a microcomputer system. So far, two cables having critical currents in the order of 100 kA have been successfully tested concerning their maximum current, stability and quench behaviour. The paper describes the test set-up and presents some first results.