Protection of HTS magnets

The paper discusses protection issues for HTS magnet. For protection, the HTS magnet must rely on an active technique. Closed-form expressions of the matrix metal operating current density based on overheating and internal voltage criteria for protection, under very simplifying assumptions, are presented. Perhaps the most important conclusions of these criteria are that: (1) HTS (and LTS) magnets must be wound with composite conductor having a significant portion of its overall cross section occupied by normal metal generally of high electrical conductivity and (2) HTS windings must possess “high” NZP velocities to make the resistive zone occupy as large a fraction of the winding volume as possible. The paper also derives an analytical expression, under another set of simplifying assumptions, to determine the minimum resistive voltage level, dictated by the maximum hot-spot temperature set at 150 K, required to initiate an active protection process. Remarkably, this minimum detection voltage is nearly independent of the matrix metal current density, Iop/Am. For a set of operating parameters used in a numerical example, a computed minimum detection voltage, at Iop/Am = 5 × 104 A/cm2, is ∼30 mV, which, considering it must be discerned in the presence of extraneous voltage signals likely to be present in real world operating conditions, would be non-trivial. To satisfy the overheating criterion at a level of Iop/Am, which keeps the winding overall current density “viable” and at the same time to raise the minimum detection resistive voltage, the winding volume occupied by the resistive state must be expanded. The paper concludes with discussion of challenging new areas of research for protection of HTS magnets.