Design of a 60 kA Flux Pump for Fusion Toroidal Field Coils

Abstract

High-temperature superconducting (HTS) magnets offer a novel pathway to magnetically confined fusion energy generation. Reactor-scale fusion devices require tens of kA of current to generate large toroidal fields for magnetic confinement.Operating HTS magnets at kA-current levels is difficult due to unavoidable joint resistances that require active driving of current. The large footprint and energy demand of a solidstate power supply and multi-kA current leads present significant constraints on the size, cost and efficiency of a fusion tokamak. An alternative approach is to use an HTS flux pump, which can generate and sustain persistent currents in HTS coils at the kAlevels required by fusion magnet systems. Here, we outline the design of a 60 kA transformer-rectifier flux pump for energizing and sustaining an 80 mH HTS fusion magnet. The flux pump is driven by an iron-core transformer with copper primary and HTS secondary windings. This is rectified by switching elements in the superconducting circuit into a uni-directional dc voltage output which charges the inductive load. To charge the load coil to 60 kA in 24 hours, an output dc voltage of 55 mV is needed. However, only 6 mV is required to maintain a long-term steady current of 60 kA. Such a device can significantly reduce the cost and footprint of future fusion reactor designs.

A proof-of-concept Bitter-like HTS electromagnet fabricated from a silver-infiltrated (RE)BCO ceramic bulk

“…Operation at low temperatures would require very large current leads, but these current-ratings are not unrealistic for a high-field facility. An alternative approach is to supply the current using an HTS flux pump [30][31][32][33][34][35].…”

Critical current asymmetry in HTS switches using iron-core electromagnets

“…This may cause excessive heating, and unintentionally damage the switch. Additionally, it was recently shown that the power efficiency of flux-flow HTS transformer-rectifier flux pumps is improved by the ratio of the self-field to the applied magnetic field critical current κ = I c0 /I c (B a p p,⊥ ) [26]. In this case, the reduction of I c0 is expected to have a direct impact on the power efficiency of the device.…”

“…Secondly, I c influences AC magnetic field switching, as the threshold field B th,⊥ decreases with larger fractions of I t /I c0 [16]. Finally, I c alters flux flow resistance by decreasing the current at which voltage is generated, while also limiting the power efficiency of switching [26]. Thus, understanding the critical current behavior of HTS switching elements using iron-core electromagnets is important for HTS flux-pump design.…”

A Noninductive Bifilar Coil to Design Compact Flux Pumps for HTS Magnets

“…Applications like flux pumping require relatively low voltage so it rules out electrical breakdown potential risk. For example, to charge the load coil of 60kA in 24 hour, a dc voltage of 55mV is required and only 6mV is needed to maintain a long-term steady current of 60kA [29].…”