J. Cornelis

Precise Hall measurements at 4.2 K in superconducting magnets

In recent years our Institute has acquired the know-how for wet-winding and testing NbTi superconductive coils. For the determination of the magnetic fields Hall sensors are found to be very convenient. To increase the precision on the Hall measurements a computer controlled Hall current chopping and reversing circuit has been used. The analysis of the data showed that the Hall voltage is essentially proportional to the magnet current. A small, less than 0.5 percent, oscillatory term appears which is clearly resolved above 2 tesla and which is reproducible during each cooling cycle. The amplitude of the oscillatory term is proportional to the magnet current intensity and the phase is a linear function of the reciprocal magnet current intensity. The oscillatory behaviour has been measured on different Hall probes, the fitting parameters being characteristic for each probe. Similar oscillations were measured on the current contacts of the Hall sensor, showing the Shubnikov-de Haas effect. The sensitivity of our Hall measuring device is 0.1 mT, The resolution is high enough to reveal, on the magnet axis, small remanent field differences of maximum 6.7 mT after the magnet had been energized to 8 tesla. The persistent field at the centre of the magnet is lower than 2 mT. The Hall measurements on the superconductive S.C.K./C.E.N. magnet allowed the determination of a precise relationship of field versus Hall voltage.

Magnetic moments measured at 78 K on an YBa2Cu3Ox superconducting ceramic disc

Abstract Static magnetic susceptibilities of an YBa(2)Cu(3)O(x) disc at 78 K, reveal the presence of magnetic moments, induced by low measuring fields. For higher fields the limited shielding capacity of the conductor allows flux penetration into the sample. The penetrated flux can be trapped in the disc yielding a permanent moment.

Amorphous Zr0.7Pd0.3 as a temperature reference near 2.5 K

Abstract The superconductive transition temperature of the high solidification rate material Zr 0.7 Pd 0.3 was used as a temperature reference to estimate the temperature deviation between the thermometer and the sample in the pan of a vacuum microbalance with 10 −9 N sensitivity. The latter system belongs to a set-up for measuring magnetic susceptibilities at low temperatures down to 2.2 K. The transition temperature of the amorphous Zr 0.7 Pd 0.3 sample was calibrated in a separate helium bath in good contact with the thermometer. In this bath the lambda transition point of liquid helium offered an additional check on the thermometer.

Quench experiments on superconductive coils

A set of four similar superconductive coils of 166 mm inner winding diameter were wet-wound with NbTi wire for subsequent application in various configurations: one single coil, a pair of coils or all four coils stacked on top of each other and connected in series. The height of the winding was 41 mm and the thickness 9 mm. The detailed observation of the time-dependence of the current and voltage with a data processing system yielded information on the time scale of the quench propagation. In the single coil and the pair of coils the time dependence of the quench resistance with time was derived from an analysis of the current and the coil voltages during the transient. With the four coils the time required for the current to decrease from 90 % to 10 % gets shorter with increasing quench current. Variations of 1.6 s to 0.3 s were observed in this configuration. At the maximum current a magnetic induction of at least 2.5 T was reached in a volume of over 2.5 litres. The self-inductance of this system was 1.28 Henry and the stored energy attained 22 kJ.

Magnetic susceptibilities measured on ceramic high-Tc superconducting discs☆

Abstract Static magnetic susceptibilities have been measured on ceramic high-Tc superconducting discs in magnetic induction fields between 1 and 9 mT. Data for a TlBaCaCuO ceramic compound at temperatures between 2.85 and 123 K are compared with those for YBa2Cu3O7 − x at 4.29 K. The susceptibility of the latter sample results in a critical induction field for flux penetration of 4 mT. For higher fields the flux penetrates the sample, resulting, in certain cases, in a permanent magnetic moment. The general magnetic behaviour of YBa2Cu3O7 − x is similar to that of the TlBaCaCuO compound. In the latter, persistent moments have been measured at 1 mT up to 80 K. Induced moments are observed up to 117 K, the critical temperature based on magnetic measurements.