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.