1Outer jacket
The cable’s tough outer skin, usually polyethylene. It protects everything inside from moisture, abrasion and installation stress, and looks much like the jacket of a conventional underground cable.
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2Outer cryostat
The outer of the two steel tubes that form the cryostat, the thermos flask of the cable. Together with the inner cryostat it seals the vacuum space that keeps outside heat away from the cold core.
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3Thermal insulation (vacuum)
A pumped-down vacuum gap wrapped with many layers of reflective foil. With almost no gas left to conduct heat, and foil to block radiation, only a trickle of warmth ever reaches the liquid nitrogen.
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4Inner cryostat
The inner steel tube of the cryostat. It contains the returning liquid nitrogen and is often corrugated, so the finished cable can still bend without kinking.
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5LN2 return
Liquid nitrogen on its way back along the cable, flowing between the outermost dielectric and the inner cryostat at roughly 65 to 77 kelvin (around minus 200 °C). Together with the supply flow in the core it forms a closed cooling loop that holds every tape below its critical temperature.
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6Copper neutral
A layer of copper braid or tape at earth potential, wrapped just outside the phase conductors. Because the three phases are nested on one axis their magnetic fields very nearly cancel, so a modest copper neutral is enough here to carry any imbalance or fault current and to hold the outside of the cable at zero volts. A single-phase AC cable, with no cancellation, would need a superconducting shield in this position instead.
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7Dielectric (Phase 3)
Wrapped insulation over the outermost phase conductor, commonly a paper and polypropylene laminate impregnated with the surrounding liquid nitrogen. It withstands the voltage between Phase 3 and the grounded layers above it.
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8HTS tapes (Phase 3)
Ribbons of high-temperature superconducting tape carrying the Phase 3 current, the outermost of the cable’s three concentric phase conductors. Held below their critical temperature, the tapes carry current with zero electrical resistance.
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9Dielectric (Phase 2)
Wrapped insulation between the Phase 2 and Phase 3 conductors. It withstands the full phase-to-phase voltage in a layer only millimetres thick, which is what lets three phases nest inside a single cable.
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10HTS tapes (Phase 2)
Ribbons of high-temperature superconducting tape carrying the Phase 2 current, the middle of the cable’s three concentric phase conductors.
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11Dielectric (Phase 1)
Wrapped insulation between the Phase 1 and Phase 2 conductors, impregnated with liquid nitrogen. Cooling the insulation this way is called a cold dielectric design, and it is what allows the phases to sit so close together.
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12HTS tapes (Phase 1)
Ribbons of high-temperature superconducting tape carrying the Phase 1 current, the innermost phase conductor. Thin metal-backed tapes coated with a REBCO or BSCCO superconductor are wound in spirals directly over the layer below.
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13Cryostat
The wall around the central coolant channel. It gives the Phase 1 tapes a smooth, stable surface to be wound onto and keeps the pressurised liquid nitrogen flowing along the core of the cable.
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14Former / LN2 core
The hollow centre of the cable. It acts as the former that the conductor layers are built up around, and its bore is the supply channel for the liquid nitrogen coolant, which flows out along the core and returns near the cryostat.
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