A numerical model for stability considerations in HTS magnets

Abstract-Due to the wide spectrum of current sharing temperatures in an HTS magnet, estimating the energy required to quench the magnet is a complicated task. On the other hand, quenching an LTS magnet for quench characterization purposes with a heater is straight-forward due to the small temperature margin, and correspondingly low minimum quench energy (MQE). To estimate the required energy for LTS magnet, the analytic concept of MQE can be utilized. In this paper we propose that only numerical simulations can give adequate estimates to the MQE of an HTS magnet for measurement purposes. Further, due to the high enthalpy margin, the utilization of spot heaters with short energy pulses becomes questionable. We present in detail the effect of heater's pulse length to the MQE when a strip heater is utilized for quenching. In addition, the effect of the heater area on MQE is studied. We consider the model of a REBCO coil to be constructed and tested in a European project EuCARD-2. According to the results: 1) MQE increases almost linearly for pulse lengths between 100 ms and 500 ms. 2) When the heater area is enlarged, the required energy per area saturates to a certain value related to the coil's enthalpy margin. 3) MQE obtained with a traditional analytic approach based on a minimum propagating zone (MPZ) underestimates considerably the numerically obtained MQE.Index Terms-high temperature superconductors, minimum quench energy, quench simulation, stability analysis, superconducting magnets

“…where f i is volumetric fraction of the material i and ρ i (T ) resistivity of the material i. Superconductor resistivity was computed with the power law [23], [24] …”

Section: A Fem Simulationmentioning

confidence: 99%

Modeling of Minimum Energy Required to Quench an HTS Magnet With a Strip Heater

Haro

Stenvall

Nugteren

et al. 2015

“…where ρ ef f is the effective resistivity of the cable, f i volumetric fraction of the material i and ρ i (T ) resistivity of the material i. Superconductor resistivity was computed with the power law [27], [28] …”

Section: A Fem Simulationmentioning

confidence: 99%

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

Haro

Stenvall

Nugteren

et al. 2015

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 aluminum water cooled shield, which surrounds the targets, has been removed for clarity. and it could be argued (see [5] for example) that an unexpected quench is highly unlikely to occur in most HTS magnets operating around 20 K.…”

Section: The Design Of the Cooling Systemmentioning

confidence: 99%

A Wide Split, Cryocooler Refrigerated High Temperature Superconducting Magnet, for Industrial Coating Applications

Hales

Hickman

Cooke

et al. 2008

We present a description of the design, construction and operation of a magnet system comprising of 2 HTS coils separated by 164 mm, that can encompass a plasma coating module. The coil system comprises of two winding packages of 3 pancake pairs of BSCCO (Bismuth Strontium Calcium Copper Oxide)/Ag tape. These are mounted such that they are in optimum thermal contact with an Edwards Coolstar Coldhead, model 6/30, which is attached to a Cryodrive 3.0 compressor unit. This allows operation of the magnets down to 20 K. The windings provide superconducting ampere turns for an iron core. The main objective of this project is to explore the replacement of high energy density permanent magnets with electromagnets in order (a) to achieve higher fields and (b) to be able to vary the field in time during a given coating process.