Hot Spot Temperature in an HTS Coil: Simulations With MIITs and Finite Element Method

Abstract

Abstract-MIITs, a zero-dimensional concept to study hot spot temperature, has been previously used to estimate hot spot temperatures and quench heater delays in NbTi and Nb3Sn magnets. However, quench behaviour is completely different in high temperature superconducting (HTS) magnets due to the slow normal zone propagation velocity and high temperature margin. Because MIITs concept does not take into account thermal diffusion in the magnet, opposite to the finite element method (FEM) analysis, the difference of these concepts is studied in this paper. Here we have taken the approach to compute the hot spot temperatures for a future HTS magnet, designed to be built from REBCO Roebel cable, with MIITs and FEM simulations. The magnet protection is accomplished with a dump resistor, and the effect of quench detection threshold voltage on the hot spot temperature has been studied. Furthermore, the inductance of the magnet increases with the magnet length. Thus, there exists a maximum inductance of the magnet which should not be exceeded to be able to protect the magnet only with a dump resistor. The hot spot temperatures with different values of inductance are also studied in this paper. Our simulations show, that the hot spot temperatures computed with MIITs are from 60 K to 150 K higher than those of FEM analysis. Thus, MIITs concept seems unreliable when considering hot spot temperatures in HTS magnets protected with only dump resistors. However, MIITs concept might be a usable tool when comparing different magnet designs. If 400 K is the upper limit for the hot spot temperature and the protection scheme includes only a dump resistor, the length of the investigated magnet can be increased to only such value that the magnet inductance is at most 50 mH.

The Influence of Metal Plates on Quench Protection of High Temperature Superconducting Pancake Coils

“…In the seven cases with different materials, the current of the coil with metal plates decays with a similar rate at the very early stage of the discharging process. The integral of the square current of the coil can estimate the increase in hot spot temperature in the coil during the discharge process [9]. A parameter η is defined to describe the ratio of the integral of the square current of the coil with and without metal plates.…”

“…sive joule heat can be generated at local quench zones, resulting in a high hot-spot temperature in the HTS coils. The operating current of the HTS magnet must be discharged before the hot-spot temperature exceeds the safety limit [9]. Therefore, fast discharge is key for successful quench protection of HTS magnets.…”

Numerical Study on Mechanical Responses during Quench Protection in High-Temperature Superconducting Coils

“…The selection criteria for the dump resistor should consider the following two factors: the dissipation velocity must be high, and the discharge voltage of the coil must be less than the safety value. An MIITs computation is used to select the value of the dump resistor [ 40 , 46 ]: where F is the MIITs; C ( T ) is the heat capacity; ρ is the material resistivity; J ( t ) = I ( t )/ A and I ( t ) = I 0 exp(- R d / L × t ). T HS is the hot spot temperature.…”

“…No-insulation (NI) HTS coil is one of the popular solutions for quench protection, which is self-protection [ 43 , 44 ]. The dynamic current distribution, transient voltage, temperature profiles, and mechanical behavior of HTS coils have been addressed by different research groups [ 45 , 46 , 47 , 48 , 49 , 50 , 51 ]. Although much research on the mechanical behavior of HTS magnets in the process of winding, excitation, cooling, and quenching has been conducted, the reliability and safety evaluation of the superconducting structures during quench protection is still rarely involved.…”

HTS Accelerator Magnets and Conductor development in Europe

“…The ring imparts to the coil the radial force it receives from the restraining cylinder; however, most important, these rings are a well coupled inductor, to quickly extract the energy out of the coil following a quench and avoid an excessive hot spot temperature due to low quench propagation velocity. The University of Tampere (Fin) has developed a model to evaluate the quench propagation for our HTS coils from basic properties [69,70], which greatly complemented the CERN e.m. numerical model.…”