Modeling Quench Protection Heater Delays in an HTS Coil

The purpose of this research was to investigate the efficiency of the state-of-the-art quench protection heater technology applied to a high-temperature superconductor (HTS)-based accelerator type magnet. The heater delays, i.e., the time delay between the heater activation and consequent normal zone initiation in the winding, were simulated using the 2-D thermal modelling tool CoHDA, which has been successfully used for low-temperature superconductor (LTS) coils. In addition to the quench onset criterion used for LTS, requiring the cable maximum temperature to reach the current sharing temperature (Tcs), a criterion accounting for the current redistribution within a cable was introduced in the model. The heater delays were analysed as a function of various heater parameters, their value ranges were based on the heaters in the recent LTS magnets: heater powers (PPH(t = 0) = 20-200 W/cm2), heating station lengths (down to 10 mm) and insulation thickness's (up to 150 μm Kapton). The simulated delays in the reference YBCO-cable (Top= 4.5 K, I = 5 kA, B = 20 T (parallel to cable's wide surface), Tcs = 16.5 K) were mainly between 20 and 100 ms. The heater efficiency seriously decreased for Tcsabove 20 K when the cable energy margin approached the energy provided by the protection heater. Although experiments are needed to confirm these results, it seems clear that heater based quench protection for HTS-based accelerator magnets requires significant technology developments.