Paul S. Swartz

A MODEL FOR MAGNETIC INSTABILITIES IN HARD SUPERCONDUCTORS: THE ADIABATIC CRITICAL STATE.

The isothermal critical‐state model of hard superconductors is extended to include the effects of heating when the applied field is changed suddenly and magnetic flux enters adiabatically into the bulk. We consider the following specific situation. A semi‐infinite slab of superconductor is cooled in a magnetic field lying in its surface plane. Next, the external field is raised isothermally by an amount Hs. This excess field decreases linearly to a depth δ= 10Hs/4πJc from the surface. Finally, the field is raised by an infinitesimal amount ΔH in a time short compared to the thermal diffusion time and long compared to the electromagnetic diffusion time. Each element of volume exposed to the changing field receives a thermal impulse proportional to the local‐flux‐change times Jc. This thermal impulse, in turn, lowers the critical current and allows more flux to penetrate. We find that if Hs exceeds some critical value Hfj, then the isothermal critical state is not the only allowed state of the superconducto...

Characteristics and a New Application of High‐Field Superconductors

This paper presents a simple physical and mathematical model and experimental support of the manner in which magnetic flux penetrates into a high‐field superconductor. Based on this model a new device using high‐field superconductors in their bulk form called a superconducting magnetic flux compressor is proposed. Present theory suggests that magnetic field strengths approaching the critical field strength of a high‐field superconductor (> 100 kOe) should be attainable using the apparatus and techniques described. It was found experimentally with the particular high‐field superconductors tested that compressed field strengths of 15 kOe could be achieved. Beyond this value anomalous magnetic flux jumping occurred that limited the field strengths obtainable. The characteristic conditions under which magnetic flux jumping is found are discussed. The experimental results also suggest that heat is generated when a high‐field superconductor is exposed to an alternating magnetic field well below the critical fie...

EFFECT OF FAST‐NEUTRON IRRADIATION ON MAGNETIC PROPERTIES AND CRITICAL TEMPERATURE OF SOME TYPE II SUPERCONDUCTORS

Experimental results are given for fast-neutron irradiation effects on niobium, Nb --Ta, Nb/sub 3/Sn, Nb/sub 3/Al, V/sub 3/Ga, and V/sub 3/Si. (R.E.U.)

Effect of Thermal‐Neutron Irradiation on the Superconducting Properties of Nb3Al and V3Si Doped with Fissionable Impurities

The superconducting critical temperatures and current carrying capacities of Nb3Al and V3Si doped with small amounts of uranium and boron and subsequently irradiated with thermal neutrons are reported. While the critical temperatures are substantially unaffected, the critical current densities are dramatically increased by the internal fission of uranium in both materials. Samples of Nb3Al containing 0.321 at.% uranium and V3Si containing 0.19 at.% uranium irradiated with 1.7×1018 neutrons/cm2 give superconducting critical current densities over 106 A/cm2 at 30 kOe. The boron‐doped samples, however, showed no effects arising from fission of boron. Two new experimental techniques have been used in this investigation. First, uranium and boron analyses as well as information on homogeneity were determined by use of the Price‐Walker nuclear track detectors. Secondly, the critical current measurements were made on 50‐mg samples of 70‐μ powders by a technique that measures the generation of odd‐harmonic voltage...

Evidence for the Negative Surface Energy Models of Superconductivity in Nb3Sn, Nb3Al, V3Ga, and V3Si

The magnetization and hysteresis of Nb3Sn has recently been found to be size‐dependent; when the sample size is reduced, the size‐dependent contribution to the magnetization is diminished. The magnetization loops of powdered Nb3Sn, Nb3Al, V3Ga, and V3Si are found to converge with decreasing particle size to the form associated with the negative surface energy models of superconductivity, but with some intrinsic hysteresis.