A modified form of the critical-state model of type II superconductors is described which takes account of Hc1 and surface sheath currents. The penetrating flux, hysteresis loop characteristics, a.c. losses and rate of penetration of flux into the superconductor when exposed to a sinusoidal magnetic field are evaluated. The model implies zero power dissipation for peak applied fields less than Hc1+ΔH and rapidly increasing dissipation just above this region, asymptotically approaching that derived from the Bean-London model.

For pt.I see ibid., vol.9, p.1957, (1976). The gyrotropic waveguide, magnetized along the direction of propagation, is analysed by a conventional method which does not involve any initial assumptions regarding the state of polarization of waveguide modes. The problem is simplified by the use of complex helical coordinates in which the gyrotropic tensor assumes the diagonal form. The waveguide modes are shown to be elliptically polarized and dielectric boundary conditions are applied leading to a waveguiding condition which is outwardly similar to that applicable to isotropic dielectric guides but more complicated in content.

The effect of different surface treatments on flux penetration and dissipation at 50 Hz in bulk niobium samples has been quantitatively investigated. It is shown that for a sufficiently smooth sample surface the onset of a.c. dissipation occurs when the peak applied magnetic field exceeds Hc1+ΔH, in striking contrast to earlier results. A comparison of experimental results with a generalized critical-state model yields good agreement.

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Dielectric boundary conditions are applied to the gyrotropic waveguide magnetised along the direction of propagation. The secular equation, which defines the dispersion, is derived in the form of a 4*4 determinant which is used to compute the dispersion curves of a pure YIG guide on a GGG substrate with air above. Computed field profiles permit the classification of the normal modes into two families, one approaching TE characteristics and the other TM. The classification applies only to waveguide materials having a small off-diagonal tensor element delta (or small Faraday rotation).