Investigation of self switching flux pump for high temperature superconducting magnets

Student thesis: Doctoral Thesis

Abstract

The rapid development of second-generation (2G) high-temperature superconducting (HTS)coated conductors (CCs) has made it possible to manufacture 2G HTS coils with enormouspotential for a wide range of applications, including magnetic resonance imaging (MRI) magnets, electrical propulsion systems (HTS machines), magnetic levitation trains, and energy storage (SMES). While these coils can be operated using either DC or AC current, challenges such as properly magnetizing an HTS coil under DC conditions and reducing losses under AC conditions still need to be addressed before their widespread use in scientific and industrial settings. Typically, high-current power supplies power these coils through current leads, which can complicate insulation between cryogenic and room temperature environments. Fortunately, HTS flux pumps provide an alternative method of energizing superconducting magnets without the need for direct electrical contacts, reducing resistive heating and heat leakage from current leads at room temperature. Recent developments in flux pumps for HTS magnets have made it possible to charge kA levels of current without the need for thick current leads. This thesis aims to provide a comprehensive investigation of charging an HTS magnet to operate it in a persistent current mode, and presents a novel perspective on controlling the magnetic field in HTS magnets via flux pumping.First a two-dimensional (2D) model of a single turn high-temperature superconducting (HTS) coil was developed using a well-established H-formulation, which was iteratively refined to eliminate numerical errors from the solution. The resulting model provides insights into the self-rectifying flux pumping mechanism, which was subsequently validated experimentally. The 2D model also enables the estimation of the over-critical current voltage (also known as the flux flow voltage) across the HTS tape, which acts as a stable voltage source for injecting current into the HTS magnet. This results in the quantization of the bridge voltage, enabling precise flux injection into a fully superconducting circuit. A higher stable dc voltage can be achieved across the terminals of the HTS magnet using a bifilar coil as a bridge (bridge – the HTS tape short-circuits the terminals of the magnet and the secondary coil), the results are verified experimentally. The influence of the HTS tape and bifilar coil acting as a bridge across the HTS magnet is investigated. The results showthat the bifilar bridge gives higher stable dc voltage to charge the HTS magnet to its criticalcurrent values and leads to the compact geometry making it suitable for adoption to complexgeometries like rotor magnets in HTS machines. HTS flux pumps can charge the magnet and compensate for any current decay, enabling quasi-persistent operation of HTS magnets. To operate an HTS magnet in the persistent current mode, a jointless HTS magnet is constructed that offers zero joint resistance, allowing it to operate in persistent current mode. However, when used in applications like rotors of fully superconducting machines, it continuously experiences a background magnetic field in the form of magnetomotive force coming from the stator. The external alternating field can cause a gradual decay of the magnetic field. Therefore, this work presents a closed-loopfeedback control for field modulation in HTS magnets to operate in persistent current mode.This method eliminates the need for continuous flux pumping and allows for the injection and reduction of current in increments of 0.5 A. This flux modulation can enable a stable magnetic field for HTS magnets. Finally, the thesis investigates critical aspects of the flux pumping in HTS magnets operating at 30 K, marking an advancement in the field of HTS magnet technology as previous flux pumps have only been reported to operate at higher temperatures. These results provide insight into achieving a stable magnetic field in HTS magnets via flux pumping and outline the methods to compensate for current decay in HTS magnets operating in the persistent current mode – opening new pathways to high-field, low-cost HTS magnets.
Date of Award29 Sept 2023
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorMin Zhang (Supervisor) & Weijia Yuan (Supervisor)

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