161 research outputs found
Pancake vortices
I describe the magnetic-field and current-density distributions generated by
two-dimensional (2D) pancake vortices in infinite, semi-infinite, and
finite-thickness stacks of Josephson-decoupled superconducting layers. Arrays
of such vortices have been used to model the magnetic structure in highly
anisotropic layered cuprate high-temperature superconductors. I show how the
electromagnetic forces between pancake vortices can be calculatated, and I
briefly discuss the effects of interlayer Josephson coupling.Comment: 16 pages, 4 figures, accepted in J. Supercond. for the Special Issue
celebrating Michael Tinkham's 75th birthday, typos [notably in Eq. (67)]
correcte
Field and current distributions and ac losses in a bifilar stack of superconducting strips
In this paper I first analytically calculate the magnetic-field and
sheet-current distributions generated in an infinite stack of thin
superconducting strips of thickness d, width 2a >> d, and arbitrary separation
D when adjacent strips carry net current of magnitude I in opposite directions.
Each strip is assumed to have uniform critical current density Jc, critical
sheet-current density Kc = Jc d, and critical current Ic = 2a Kc, and the
distribution of the current density within each strip is assumed to obey
critical-state theory. I then derive expressions for the ac losses due to
magnetic-flux penetration both from the strip edges and from the top and bottom
of each strip, and I express the results in terms of integrals involving the
perpendicular and parallel components of the magnetic field. After numerically
evaluating the ac losses for typical dimensions, I present analytic expressions
from which the losses can be estimated.Comment: 8 pages, 9 figure
Self-fields in thin superconducting tapes: implications to the thickness effect in coated conductors
Most applications of superconductors, such as power transmission lines,
motors, generators, and transformers, require long cables through which large
currents circulate. Impressive progress has recently been achieved in the
current-carrying capability in conductors based on high-temperature
superconductors. Coated conductors are likely the best examples, consisting of
very good quality thin layers of YBCO superconductor grown on top of a metallic
tape with some intermediate layers. However, there is an important problem for
achieving large currents: a large decrease in transport critical-current
density Jc when increasing film thickness has been observed in coated
conductors made by all available techniques. Here, we theoretically explain the
nature and the ubiquitous presence of this so-called thickness effect by
analyzing the self-field created by the transport currents in the
superconductor, assuming a realistic field-dependent Jc. This knowledge can
help finding new ways to improve transport current in thick superconducting
films.Comment: 7 pages, 3 figure
Self-field effects upon the critical current density of flat superconducting strips
We develop a general theory to account self-consistently for self-field
effects upon the average transport critical current density Jc of a flat
type-II superconducting strip in the mixed state when the bulk pinning is
characterized by a field-dependent depinning critical current density Jp(B),
where B is the local magnetic flux density. We first consider the possibility
of both bulk and edge-pinning contributions but conclude that bulk pinning
dominates over geometrical edge-barrier effects in state-of-the-art YBCO films
and prototype second-generation coated conductors. We apply our theory using
the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba)
as a function of a perpendicular applied magnetic induction Ba and show how
Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to
JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux
density zero at the strip's edge. However, Jc(Ba) is suppressed relative to
JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring
when Ba*/B0 is of order unity or larger.Comment: 9 pages, 4 figures, minor revisions to add four reference
Flux domes in superconducting films without edges
Domelike magnetic-flux-density distributions previously have been observed
experimentally and analyzed theoretically in superconducting films with edges,
such as in strips and thin plates. Such flux domes have been explained as
arising from a combination of strong geometric barriers and weak bulk pinning.
In this paper we predict that, even in films with bulk pinning, flux domes also
occur when vortices and antivortices are produced far from the film edges
underneath current-carrying wires, coils, or permanent magnets placed above the
film. Vortex-antivortex pairs penetrating through the film are generated when
the magnetic field parallel to the surface exceeds H_{c1}+K_c, where H_{c1} is
the lower critical field and K_c = j_c d is the critical sheet-current density
(the product of the bulk critical current density j_c and the film thickness
d). The vortices and antivortices move in opposite directions to locations
where they join others to create separated vortex and antivortex flux domes. We
consider a simple arrangement of a pair of current-carrying wires carrying
current I_0 in opposite directions and calculate the magnetic-field and
current-density distributions as a function of I_0 both in the
bulk-pinning-free case (K_c = 0) and in the presence of bulk pinning,
characterized by a field-independent critical sheet-current density (K_c > 0).Comment: 15 pages, 23 figure
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