Speaker
Description
Steel-hull ships generate a magnetic field making them vulnerable to weapons equipped with magnetic sensors. To mitigate this threat, navies use ship deperming treatments to reduce the ship’s magnetization. We design a flat seabed coil with a racetrack shape to deperm a ship that remains stationary above the coil for quick operation. The required magnetic field generated by the coil is calculated from the magnetic property of ship’s steel and demagnetization factor caused by the shape of the ship. The mathematical model of a ship was a spheroidal shell, which consists of high-tensile steel, commonly used in civil ships. We calculated a required magnetic field of 2,370 A/m.
To generate this magnetic field including demagnetization factor, onto a target ship, three racetrack coils with a common center, each up to 1,200 m in length, are designed to carry 200 kA of electric current. This high current is transmitted via high-temperature-superconducting (HTS) cables, which reduce cable volume and power consumption due to their zero resistivity. We base the design on commonly available HTS material—Rare Earth Barium Copper Oxide (REBCO) tape conductor—considering the superconducting properties related to temperature, magnetic field and electric current.
The racetrack coil design incorporates parallel and series connections of the tape conductors to reduce the required current from the power supply. To minimize the cost of specialized winding machines, a cable−on−round−core (CORC) assembly is used, with multiple CORC cables bundled to form a racetrack coil conductor.
The magnetic field on the conductor is a key design parameter, along with the electromagnetic forces arising from high current. The electromagnetic force exerted on a tape conductor due to the magnetic fields generated by other tape conductors within a designed CORC bundle was analyzed. In this calculation, the electric current within the tape conductor is assumed to flow along the center of the tape. The magnetic field generated by neighboring tapes within the CORC is oriented circumferentially and induces a compressive force along the thickness of the tape, directed toward the center of the CORC. This compressive force is counteracted by the stiffness of the core material.
Next, we considered the electromagnetic forces between racetrack coil cables. The CORC bundles, as cable conductors, are placed inside a cryogenic pipe for refrigerant flow. The electromagnetic forces push the conductor toward the outer shell of the cable, and the pipe, supported by a plastic structure, resists this force. The stiffness of the support and thermal input from the outer shell satisfy the design requirements.
Additionally, parallel stress is induced along the length of the tape conductor due to the vertical electromagnetic force on its surface as described previously. This stress is critical in coils with uniform hollow shells, such as solenoids, where the electromagnetic force uniformly distributed across all elements. In the CORC bundle, the magnetic field acting on each tape conductor varies, leading to different stresses and strains across the cross-section. Considering the stiffness and stretch of the tape conductor and support materials, we discuss the structural integrity of the seabed racetrack coil under electromagnetic forces.
