Study On The Ramping And Quench Characteristics Of No-insulation High Temperature Superconductor Coils

Quench protection has always been a problem for high temperature superconductor(HTS)coils.No-insulation(NI)coil removes the traditional turn-to-turn insulation so that the coil has a metallic contact between turns.During a quench operation of the NI coil,the transport current can “bypass” the local normal region through turn-to-turn contacts,which reduces the heat greatly generated on the normal region and prevents the quench propagation effectively.Therefore,the NI coil shows a great advantage on the thermal stability,self-protection features and quench recovery,which makes it an international heat spot in the field of applied superconductivity.Due to the absence of electrical insulation between turns,the NI coil shows a significant difference on charging,discharging and quench characteristics than its insulated counterpart.Some issues on applications have not be researched in detail.This paper is to study the charging,discharging and quench characteristics of 2nd generation HTS NI coils by simulations and experiments.The main contents are as follows:1)A novel distributed parameter equivalent circuit network model is developed for NI HTS pancake coils.In this model,each turn of the coil is subdivided into fine arc elements,each element is equivalent to distributed circuit parameter,so that the whole NI coil is equivalent to a circuit network.The model is validated by the charging and discharging test of NI HTS coils.2)The charging and discharging characteristics of a single NI HTS coil is studied using the above equivalent circuit network model.The current distribution inside the NI coil is calculated,and the mechanism of charging delay is elucidated;the factors affecting the charging time of NI coils are analyzed with the viewpoint of magnetic field induced;a finite element model based on H-formulation is coupled with the equivalent circuit network model and the ramping losses of NI HTS coils are analyzed.3)The charging and discharging characteristics of magnet consisting of multiple NI HTS coils are also studied.The charging and discharging process of NI coils on different locations are analyzed and compared.The distribution of current,coil voltage and losses are studied.Researches are also performed on the screening current and the field it induced inside NI coils on different locations.4)A quench model is developed for NI HTS coils based on the above equivalent circuit network model.The quench process induced by a local heat spot is simulated using the model.The results show that during a local quench the transport current is forced out not only the local normal region but also the along the entire turn with the local normal region.This prevents the development of local quench and enhances the thermal stability and self-protection features of NI coils.This analysis provides a theoretical proof for many previous experimental results that the NI coil has an enhanced self-protection features.Further studies show that the magnetic field induced at coil center and coil voltage present a considerable change in the beginning of the local quench,which may be a novel method for the quench detection and protection of NI HTS coilsThe research in this paper presents the ramping characteristics and the quench prorogation of NI HTS coils.It is very useful for the industrial application of NI HTS coil.