Methodology Studies Of Contactless Magnetic Excitation For High-temperature Superconductor Magnet And Its Experimental Investigation

High-temperature superconducting(HTS)magnet has become a promising candidate to promote the upgrade of tranditional electrical devices,like high field magnets,motor coils etc.,owing to its wide range of operating temperature and high current-carrying capability as well as good in-field performance.Ideally,HTS magnet operating in persistent current mode(PCM)could provide stable high magnetic field with a very low cost.However,due to the still unavailable superconducting joint and flux creep in the HTS material,as well as dynamic resistance occurring when DC current-carrying HTS magnet is under the external alternating magnetic fields,current in a closed HTS magnet inevitably suffers a decay.In a consequence,the HTS magnet normally runs in open loop mode.A power supply has to be eternally engaged to maintain the current in the HTS magnet.By using a routine power supply method,current is injected into the HTS magnet through a couple of current leads that connect the cryostat and the ambient environment,consequently generating substantial losses and heat load.These losses and heat load significantly increases the cost and energy consumption of HTS magnet and limits achievable current capacity as well as energy density of the magnet,meantime seriously affects the thermal stability of it.To address this issue,this dissertation has proposed a prototype inspired by flux pump and wireless power transfer(WPT)technology,for contactless charging and compensating of the HTS magnet.Firstly,the dynamic resistance,which acts as the main cause of the current decay in HTS magnet,was investigated.By Bean model,the development and theoretical calculation of dynamic resistance occurring when a DC current-carrying superconductor under external alternating magnetic field was discussed.Meanwhile,the dependence of its value on the critical current,transport current as well as the external magnetic field has been clarified.Then,the critical current of a race-track HTS coil built with YBCO coated conductors(CCs),which plays the role of charging target,was accurately calculated taking account of the experimental measured anisotropic critical current of HTS tape.Based on the calibrated anisotropic critical current of HTS tape,a finite element model was established to reveal the evoluation of electrical field and current distribution asssciated with the dynamic resisitance.Taking HTS stacked conductors as the research object to simulate HTS coil,the characteristics of its dynamic resistance under different transport currents and applied magnetic fields were sufficiently studied.The results indicate that in order to improve the operating stability of HTS magnet,it must operates at lower temperatures.Uuder this premise,an HTS bridge control strategy based on self-current drive without introducing other power-dissipating active components is determined for thr proposed HTS magnet contactless charging system.Secondly,based on the Norris theory,the principle and features of measuring the transport losses in HTS CCs have been analyzed.A system,which could precisely measure the transport losses in HTS CCs has also been designed and established.To understand the experimental findings,a finite element model taking the real dimensions of the HTS CCs into account was developed.From the perspective of material properties and structural dimensions,the crucial factors that determine the AC losses of HTS CCs in a wide frequency band are clearly defined.It is found that differing from theoretical prediction,above a certain“transition frequency”,the AC losses per cycle no longer increase with the frequency.This is because the current flowing in the metallic material is not an induced eddy current as commonly thought,but rather a transport current driven by the power supply and inductively coupled to the surrounded magnetic field.Moreover,at high frequencies,the losses level of the magnetic substrate HTS CC is equivalent to that of the non-magnetic substrate HTS CC,the cost of adopting former one is lower,while the utilization of narrower copper stabilizatier can significantly reduce the AC losses.These findings,by supplementing the existing description of AC losses in HTS CC,could provide theoretical support for the material selection of the secondary HTS winding in the proposed HTS magnet contactless charging system.It also provides theoretical guidance and scientific basis for how to effectively reduce the losses level of HTS devices under different working conditions.Then,a basic circuit topology was built to theoretically analyze the process of how this system converts small primary alternating current into the charging current flowing in the load HTS coil.Furthermor,establishing a finite element model to numerically study the proposed charging method.In the model,the non-convergence caused by conventional E-J power law model was solved through modifying the resistivity of HTS bridge regarding the transport current exceeding the critical current.The results show that using superconductor with its critical current weakly infected by magnetic field and weaker V-I hysteresis characteristics to construct HTS bridges,and adding the "consumption resistance" in the charging loop can further improve the charging performance of this method.Finally,designing and building an experimental test platform for the proposed HTS magnet contactless charging method.The experimental test results show that,unlike other solutions,the performance of this method is weakly related with the applied frequency,and the HTS coil can be charged at a very low operating frequency,which has a positive effect on reducing the overall losses and improving the charging efficiency.By optimizing the topology of the primary and secondary magnetic couplers,the non-superconducting components can be removed from the cryostat eliminating the parasitic heat loads due to the power dissipating components inside the cryogenic environment.