7.2 Superconductor Technology in the Future
165
this reason, research on superconducting cable with a fault-current limiting function
has been carried out in the USA, and such a long cable was practically fabricated in
Korea recently.
The author has proposed a high-performance DC superconducting power cable in
which the longitudinal magnetic field effect described in Chap. 6 is applied [1]. The
longitudinal magnetic field effect was discovered more than 50 years ago. Although it
is a fantastic phenomenon from the viewpoint of technology that the critical current
density increases, no application of this effect has been proposed. It is especially
strange that this effect has not been applied in superconducting power cables, where
it offers the direct benefit of an increase in the critical current density. The essential
point is to apply a magnetic field parallel to the cable itself, and hence, the superconducting tapes in the outer shielding conductor are wound in one direction so
that the return current flowing in this conductor produces a longitudinal magnetic
field. Then, the superconducting tapes in the inner conductor are wound so that the
force-free structure is achieved (see Fig. 7.4). The magnetic field is almost directed
parallel to the cable axis on the innermost layer and its angle tends to increase on the
outer layers. Thus, the winding angle of the superconducting tapes is monotonically
increased for the outer layers. Although a detailed analysis is needed to design an
optimal structure, the principle is very simple. It may be anticipated that an appreciable effect will not be obtained, since the resultant longitudinal magnetic field is not
very strong. A relatively strong longitudinal magnetic field can be achieved, however,
since a longitudinal magnetic field is also produced by the inner conductor. Only
Fig. 7.4 Structure of
superconducting DC power
cable in which the
longitudinal magnetic field
effect is utilized. The arrows
show the directions of the
currents flowing in the
superconducting tapes
165
this reason, research on superconducting cable with a fault-current limiting function
has been carried out in the USA, and such a long cable was practically fabricated in
Korea recently.
The author has proposed a high-performance DC superconducting power cable in
which the longitudinal magnetic field effect described in Chap. 6 is applied [1]. The
longitudinal magnetic field effect was discovered more than 50 years ago. Although it
is a fantastic phenomenon from the viewpoint of technology that the critical current
density increases, no application of this effect has been proposed. It is especially
strange that this effect has not been applied in superconducting power cables, where
it offers the direct benefit of an increase in the critical current density. The essential
point is to apply a magnetic field parallel to the cable itself, and hence, the superconducting tapes in the outer shielding conductor are wound in one direction so
that the return current flowing in this conductor produces a longitudinal magnetic
field. Then, the superconducting tapes in the inner conductor are wound so that the
force-free structure is achieved (see Fig. 7.4). The magnetic field is almost directed
parallel to the cable axis on the innermost layer and its angle tends to increase on the
outer layers. Thus, the winding angle of the superconducting tapes is monotonically
increased for the outer layers. Although a detailed analysis is needed to design an
optimal structure, the principle is very simple. It may be anticipated that an appreciable effect will not be obtained, since the resultant longitudinal magnetic field is not
very strong. A relatively strong longitudinal magnetic field can be achieved, however,
since a longitudinal magnetic field is also produced by the inner conductor. Only
Fig. 7.4 Structure of
superconducting DC power
cable in which the
longitudinal magnetic field
effect is utilized. The arrows
show the directions of the
currents flowing in the
superconducting tapes