Amit Rastogi

Axial stiffness of journal bearings in zero-field and field-cooled modes

Superconducting journal bearings have been investigated for use in flywheel systems. We report on the zero-field cooled and field-cooled stiffness of these bearings. They are made up of radial magnet rings with alternating polarities, a pole pitch of 11 mm and a surface field of 0.1 T. Field-cooled stiffness of the journal bearings increased four times over the zero-field-cooled stiffness.

Axial and journal bearings for superconducting flywheel systems

Axial and journal bearings have been investigated for use in superconducting flywheel systems. Our test rig comprises of an Evershed type magnetic bearing used to levitate a 35 kg rotor. The stabilizing forces are provided by superconducting axial and journal bearings. In this study we focus on the vertical stiffness measurements and explore the use of journal bearings. The journal bearing consists of radial magnets with alternating polarities. Our results indicate that this type of journal bearing can effectively stabilize the rotor. Spin-down test shows a linear behavior.

In-plane magnetization and hysteresis losses in YBCO thick films

We present our observations of hysteretic magnetization of c-axis oriented thick (1-2 /spl mu/m) YBCO films in parallel fields. For other orientations of the field, the magnetization is reported to be mainly due to the c-axis component (M/sub c/sin/spl theta/) because of a large aspect ratio of the film. But our observations at low fields (/spl les/100 mT) did not follow such behavior. Moreover, the in-plane magnetization is affected by an artifact introduced during the deposition of such thick films. We discuss our results in the framework of Beans model after removing the artifact.

Meissner-levitated micro-systems

Advanced silicon processing techniques developed for the Very Large Scale Integration (VLSI) industry have been exploited in recent years to enable the production of micro-fabricated moving mechanical systems known as Micro Electro Mechanical Systems (MEMS). These devices offer advantages in terms of cost, scalability and robustness over their preceding equivalents. Cambridge University have worked for many years on the investigation of high temperature superconductors (HTS) in flywheel energy storage applications. This experience is now being used to research into superconducting Micro-Bearings for MEMS, whereby circular permanent magnet arrays are levitated and spun above a superconductor to produce bearings suitable for motors and other micron scale devices. The novelty in the device lies in the fact that the rotor is levitated into position by Meissner flux exclusion, whilst stability is provided by flux pinned within the body of the superconductor. This work includes: the investigation of the properties of various magnetic materials, their fabrication processes and their suitability for MEMS; finite element analysis to analyse the interaction between the magnetic materials and YBCO to determine the stiffness and height of levitation. Finally a micro-motor with the above principles is currently being fabricated within the group.

Angular dependence of hysteretic magnetization in micron-thick YBCO films

Abstract We investigated the magnetization ( M ) hysteresis loops of micron-thick, c -axis-oriented YBa 2 Cu 3 O 7− δ (YBCO) films in low magnetic fields (≤0.1 T) applied nearly parallel to the film surface. As is expected from the film geometry, M strongly depends on the angle θ between the film and the applied field, and most of the magnetization comes from the perpendicular magnetization parallel to the c -axis, which is due to the shielding current flowing in the ab plane. However, there is considerable contribution from the magnetization along the ab plane when the field is oriented very close to the film surface. We also found interesting sample dependence in the observed M ( θ ) behavior.

Failure analysis of superconducting bearings

The dynamics of superconductor bearings in a cryogenic failure scenario have been analyzed. As the superconductor warms up, the rotor goes through multiple resonance frequencies, begins to slow down and finally touches down when the superconductor goes through its transition temperature. The bearing can be modelled as a system of springs with axial, radial and cross stiffness. These springs go through various resonant modes as the temperature of the superconductor begins to rise. We have presented possible explanations for such behaviour.

Numerical Simulation of Magnetic Fields in YBCO High Temperature Superconducting Flywheel Model

Calculations of the fields in YBCO superconductors do not fit easily into standard finite element methods designed for ferromagnetic materials. A new calculation procedure, which can be called the Trapped Field Magnet (TFM) method, is proposed in this paper to predict the magnetic flux distributions and the stiffness of a flywheel model. The experimental results in field-cooled condition showed that the proposed procedure could be used to calculate the magnetic flux distribution of hard superconductors

Longitudinal and transverse magnetizations in micron-thick Y-Ba-Cu-O films in nearly parallel magnetic fields

We have studied the magnetization in c-axis oriented-micron-thick Y-Ba-Cu-O (YBCO) films in magnetic fields oriented nearly parallel to the ab plane. The orientation dependence of the magnetic moment is dominated by shape anisotropy effects and this moment is contributed by its component along the c-axis for most of the cases. However, when the applied field is oriented very close to the ab plane, it is found to have a hysteretic component contributed by in-plane magnetization due to flux penetration along the ab plane. We measured longitudinal (along the field), and transverse (orthogonal to the field) moments to elicit in-plane magnetic response. At high fields, the penetrating flux in the ab plane decreases the critical current density in the ab plane, which results in the reduction of c-axis magnetic moment.

Orientation Dependence of Magnetization Near ab Plane in Micron- thick YBCO Films

We have studied the orientation dependence of magnetization for the applied magnetic field oriented close to the ab plane in YBCO films of micron thickness. Using a SQUID magnetometer, the longitudinal (parallel to the applied field direction) and the transverse (orthogonal to the applied field) components of the magnetization were measured. We have observed that most of the magnetization at any given orientation comes from that along the c-axis, due to the screening currents in the ab plane. However, magnetization along the ab plane cannot be neglected when the field is oriented very close to theab plane.