Magnetic shielding for magnetically levitated vehicles

The magnetic shielding required for magnetically levitated vehicles is discussed. Shielding is needed to protect passengers in a vehicle from stray dc magnetic fields coming from the superconducting dipoles carried by the vehicle. In addition, the superconducting dipoles must be shielded against various ac magnetic fields. Here we consider shielding of ac magnetic fields generated by the propulsion windings for the case of a linear-synchronous-motor active guideway. Protection for passengers against magnetic fields is necessary as there are at present no data upon which to base a human tolerance of magnetic-field intensity. It is shown here that in a magnetically levitated vehicle where the space to be shielded is large and where the weight of shielding is a prime concern, active shielding, in which compensation dipoles generate a canceling magnetic field, is superior to passive shielding, which uses iron plates. At the floor of the passenger cabin of a proposed vehicle, the magnetic field, without compensation, is as high as 0.03 T. Compensation dipoles, comparable in size to and placed 0.5 m above the main dipoles, increase the total conductor requirement by about ⅓ to bring the magnetic field at the floor of the passenger cabin below a level of 0.005 T. To achieve the same result by passive shielding, an iron plate or equivalent up to 2 cm thick must cover the floor of the passenger cabin, resulting in at least a 10 000-kg increase in the total weight for a 100-passenger vehicle. We therefore strongly recommend active shielding. At 100 Hz, a peak ac field of more than 10
^-5
T cannot be tolerated in the superconducting dipoles of a vehicle as hysteresis and eddy-current losses within the conductor become excessive. The ac field reaching the main dipoles from the propulsion windings in the guideway can be as high as 10
^-3
T without shielding. An aluminum sheet of 2-mm thickness kept at 4.2 K attenuates the field and makes ac superconductor losses negligible while keeping the eddy-current losses in the sheet to an acceptable level.