A thermally switched magnetic shield which can both shield against and trap magnetic fields is described. The shield, consisting of a Nb-Ti sheet, can trap stably fields of up to 0.3T, and shield against fields of up to 0.35 + O.05T, when the in superconducting state It provides large attenuation of alternating magnetic fields in the presence of a steady trapped field. It is shown to significantly reduce the eddy current damping of the mechanical vibration of a superconducting sample in a magnetic field, by shielding nearby normal metal surfaces. A specific application to magnetic levitation of gravitational radiation antennae is discussed.
Application of flux trapping in hard superconductors to magnetic shielding and magnetic levitation J. Ferreirinho and D.G. Blair
It is known that the almost complete absence of the Meissner effect in some superconductors allows flux trapping to occur in simply connected samples)-3 In this paper we discuss applications of this property which arose in an experiment being conducted in conjunction with the University of Western Australia gravitation radiational detection project. The aim of the project is to develop a magnetically levitated cylindrical niobium bar antenna with a mechanical Q of order 10 s in the fundamental longitudinal vibration mode. For this purpose we are attempting to measure the dependence of the mechanical Q of Nb on temperature, metallurgical state and magnetic flux penetration.
In the presence of a magnetic field one is faced with the problem that the vibrations of the Nb sample are damped by eddy currents induced in nearby normal metal surfaces (including the copper cladding of the superconducting field coil). These currents arise because the vibration of the superconducting Nb sample modulates the steady levitation field of the coil. At high intrinsic Q values of the sample, and depending on the sample geometry, such damping can be very significant.
A superconducting Nb-Ti shield which greatly reduces this damping is described below. The shield is placed between the sample and the field coil (Fig. ). It can be thermally switched from the superconducting to the normal state in order to trap the levitation field on cooling back to the superconducting state. Thereafter, the normal metal surfaces below the shield are shielded from time varying fields due to the vibrating sample. We show that the shield strongly reduces the eddy current damping of the mechanical resonator.
Description of the apparatus
For the purpose of measuring the Q of Nb we have used 125 mm diameter, 3 mm thick, disc samples suspended on conical points as shown in Fig. a.