L.J.M. van de Klundert

The thermal conductivity of argon at elevated densities

Synopsis The thermal conductivity coefficient λ of compressed argon has been measured with a parallel plate method described previously1). Measurements were carried out at 0°C, 25°C, 50°C and 75°C and at densities up to 680 amagat (pressures up to 2400 arm.). In addition values are given for the ratio K = λ/ηcυ, where η is the viscosity coefficient and cυ the specific heat at constant density. The experimental data for λ and η are confronted with the values predicted from the theory of Enskog.

Observation of a periodic pattern in the persistent-current fields of the superconducting HERA magnets

During an experimental study of the time dependence of a HERA dipole, it was found that the sextupole field exhibits a sinusoidal structure along the axis of the magnet. A similar periodic structure was found for the main dipole field with the help of a nuclear magnetic resonance probe. The wavelength of the periodic pattern is compatible with the transposition pitch of the Rutherford-type cable in the magnet coils. The effect is related to superconducting currents in the cable because it vanishes when the magnet is warmed up above its critical temperature. The structure was found to exist in all HERA dipoles measured afterwards ( approximately 10) and also in a superconducting coil without iron yoke. With a specially developed 2-cm-long pickup coil, it was found that all accessible multipole components in dipole and quadrupole magnets are modulated along their axis.<<ETX>>

Design and operation of a protection system for transformers with superconducting windings

Power transformers with superconducting windings need a protection system to prevent damage to the low-loss superconducting winding by an abnormally high current. The generally accepted protection technique which uses auxiliary coils has been analysed using a network representation. The current distribution between main and auxiliary coil is expressed in terms of geometrical parameters. Experimental data on current transfer and main coil recovery in a test transformer are presented and a method of obtaining a very low auxiliary coil current is suggested.

Optimal neutron Larmor precession magnets

Spectroscopic techniques based on Larmor precession of particle spins require that for all trajectories of a diverging beam the path integral of the modulus of the magnetic field must be a constant. The amount of precession performed by each spin is then a function of the particle energy only. For cylinder magnets this homogeneity condition can be expressed as a variational problem. An analytical solution is presented for this variation problem. This solution describes the optimal field shape (OFS) to obtain the best possible homogeneity for a given magnet length. In practice the ideal shape can be obtained by superposing a series of solenoids of different lengths but the homogeneity is generally not good enough so that in-beam correction coils are needed that include corrections for the line integral differences caused by the finite-beam divergence. The solution is presented together with a method to implement it in practice using discrete in-beam current distributions. The resulting magnet has a homogeneity of 10/sup -6/, so that the Larmor precession angle is still well defined after 10/sup 4/ turns. >

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. >

The effect of transverse loads up to 300 MPa on the critical currents of Nb/sub 3/Sn cables (for LHC)

In the framework of the development of an experimental 10-T Nb/sub 3/Sn dipole coil for the LHC (Large Hadron Collider) at CERN, the effects of transverse stress on Rutherford-type Nb/sub 3/Sn cable were investigated. For this purpose a special facility was designed and put into operation in which the voltage-current behavior of short pieces of Nb/sub 3/Sn cables can be investigated in a background field of up to 11 T and an applied stress of 300 MPa. The repulsive Lorentz force of 250 kN, generated by a set of superconducting coils, is used to impress the cable over an area of 20*42 mm/sup 2/ maximum, in the presence of a transport current of up to 40 kA. The testing equipment is described, and the first results of the observed critical current degradation of two Nb/sub 3/Sn cables are discussed.

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. >

The critical current in a NbTi tape measured in different directions of magnetic field and the current reduction due to the self field

With reference to the application of NbTi tape in a superconducting thermal switch, the critical current of a 20- mu m-thick NbTi tape was measured in several directions of the magnetic field. The critical current was found to behave strongly anisotropically, due to the deformation of the NbTi. The tape is extrasensitive to the component of the magnetic field perpendicular to the surface. Without an external field this component of the self-field reduces the critical current far below its intrinsic value. A one-dimensional model can describe the reduction of critical current due to the self-field in a thin tape. >