Coil Stress Research Articles

Analysis of thermo-electromagnetic stresses in high field tokamak resistive coils

The problem of the accurate computation of thermo-electromagnetic stresses in a toroidal field coil for Tokamak machines will be an important task in the design of future high field Tokamak machines. The high value of flux density, and the pulsed current operation of these machines, imply critical working conditions and require a correct modelling of the thermal and electrical diffusion phenomena. In this paper are presented the thermo-electromagnetic model developed for this aim and the results obtained by implementing this model in the code COMPELL. These results are then used to derive input data for the structural analysis code, ANSYS, thus providing the means for a rather detailed analysis of the structure.

High field superconducting solenoid for the LASA in Milan

The preliminary design of a 19-T superconducting high-magnetic-field facility for the Laboratory for Accelerators and Applied Superconductivity is presented. The device uses NbTi, Nb/sub 3/Sn, and V/sub 3/Ga coils giving high field homogeneity in the center regions, and two cryostats that allow separate opening of the NbTi coil and the Nb/sub 3/Sn-V/sub 3/Ga coils. The main design parameters of the facility are reported. Coil stability and stresses are considered, and superconducting cable parameters and magnetic-field characteristics are discussed. >

Stress Analysis of the NET-II Toroidal Field Coil Based on the SIN Conductor Concept

This paper concerns the investigation of the structural integrity of a toroidal field coil within the NET-II Tokamak environment. Results of global coil and local conductor stress analysis, using the finite-element method, are presented and discussed. Effects of in-plane and out-of-plane loads due to toroidal and poloidal magnetic fields are included. Fatigue considerations and preliminary design implications are outlined.

Mechanical properties of superconducting coils

Experience with the present generation of Tevatron superconducting magnet coils has made apparent several areas of possible improvement of coil structure. Implementation of the structural changes; i.e., cabling, insulation, adhesive, etc., requires a new determination of coil mechanical properties. In particular, measurements to enable prediction of coil stresses from the dimensions of coil substructures and prediction of residual stresses as a result of magnet storage and operational conditions. Theoretical uncertainties, because of the complex geometries, make empirical measurements desirable. We will measure the relationship between assembly stress and the component dimensions for several coil structures. From this data and the allowable design stress levels, component dimensions can be set. We have also measured the effect on stress levels of storage for protracted periods at high ambient temperatures and of operational occurrences such as a forced warm up which will substantially elevate temperatures for short periods. This data makes it possible to predict the number and severity of such occurrences that can be tolerated.

Analytical stress calculations for magnetic field coils with anisotropic modulus of elasticity

Analytical solutions for the radial and tangential stresses in the midplane of long magnetic field coils are derived, for both constant current density and a 1/r current distribution. The elastic anisotropy is contained in the solution as the ratio of the axial and tangential modulus of elasticity. The critical tangential stress is largest at the inner surface for coils with radial transmission of stress; this is greatly reduced if the coil is softer in the radial direction. An outer reinforcing cylinder becomes quite ineffective in this case. The average axial stress in the midplane is calculated for inclusion in Timoshenko's criterion. The authors also calculate the critical stresses in polyhelix coils with equally distributed stress among the layers. These have a modest advantage over radially soft coils only for a large number of layers.

The circumferential stress in large high field magnet coils

The circumferential stress in large high field magnet coils

Basic Features of EXTRAP Concept

The potentialities and properties are discussed of a recently suggested axially symmetric, quasi-steady, closed, high-beta magnetic confinement system. A toroidal pinch of non-circular cross section is immersed in the strong, inhomogeneous polodial vacuum field from a set of ring-shaped conductors, all carrying currents in the opposite direction of the plasma current. This "EXTRAP" concept is expected to have some important advantages, as compared to magnetic bottles based on a main torodial field component. Concerning plasma equilibrium, there are closed local and average guiding centre orbits, beta values of order unity should become available, there is no magnetic surface splitting limit, cyclotron radiation should be kept at a comparatively low level, and bootstrap operation may become possible. Concerning plasma stability, the system should not become sensitive to magnetic island formation and asymmetries, and the short magnetic connection lengths as well as the strong vacuum field gradients should favour stabilization of flute-type, kink, and other magnetohydrodynamic modes. Concerning reactor technology, high power densities are expected to be reached, superconducting coils should not become necessary, the coil stresses are moderate, and repair and replacement are facilitated by the coil geometry. Before a final judgement can be made on this concept, however, extensive experiments and further theoretical analysis are needed.

Mechanical stresses in solenoid coils

The stresses in toroidal magnet coils of anisotropic structure are analysed. Consideration is given to shear forces and to the correct choice of the elastic constants of a composite coil. It is concluded that for many practical purposes some of the earlier approximate analyses may be adequate.

Stress calculations for high magnetic field coils

A computer program is described which enables calculations to be made of all components of the stress in uniform current density coils used for producing intense magnetic fields. Particular reference is made to wire-wound pulsed field coils, but the technique is appropriate to all types of solenoids including superconductors. The effect of the elastic properties of the insulation is taken into account and shown to be important.Three separate types of coil are considered: unreinforced coils; coils reinforced with external cylinders; and coils reinforced with both external and internal cylinders. Charts are presented which give the maximum stress differences ΔS=Sθ-Sz and hence the maximum field strength for different geometries and cylinder thicknesses.Experimental results are presented for tests on a particular pulsed field solenoid wound from copper wire. These results are shown to be in agreement with the predicted values.

Cryogenic difficulties

A 3.0 T MRI magnet using MgB2 superconductors with conduction cooling for dedicated mice imaging was designed. The magnet configuration is determined by the central magnetic field strength, maximum magnetic field, magnetic field uniformity, uniformity volume, stray field, and coil costs. Linear programming was used to optimize the superconducting magnet of the MRI system. The optimized superconducting magnet can generate a central field of 3.0 T with field homogeneity better than 10 ppm in a 50 mm diameter of spherical volume (DSV). Actively shielded coils with opposite current densities were adopted to reduce the stray field. Finite element methods were used to analyze the mechanical stress and strain in the MgB2 coils during winding, cool-down from room temperature and electromagnetic excitation. The analysis results showed that the strain on the coils is within the design criteria. In this paper, the coil optimization design, electromagnetic and stress analysis were given.

Field Preheating for Large Turbine Generators [includes discussion

Preheating of the field windings of large turbine generators is a procedure for warming up and expanding the coils somewhat during the start-up operation by heating them with direct current while the machine is at a relatively low speed. This is done to reduce the mechanical stresses in the coils and the abrasion effects on the coil insulation resulting from thermal expansion. This paper presents an analysis of the various forces which act on the coils in normal service, including a shortening effect in the cylindrical field body caused by the rotational stresses. It proceeds then to show how the coil stresses and insulation abrasion can be reduced by preheating. The procedure recommended is based on the transient heating characteristics of the rotor and requires a relatively short period of time which can normally be co-ordinated with practical turbine starting cycles. With suitable equipment, preheating can be carried out quite easily and gives promise of prolonging the useful life of generator field windings. This paper is concerned only with fields employing conventional ventilation as used for many years. In the future, preheating may well be applied to fields employing direct cooling, at least, in cases where this improved cooling is capitalized to the point where the temperature rise of the winding is comparable with present practice. However, the preheating cycle for directly cooled fields will probably require some modification on account of the more rapid heating of the winding.