Quench Behavior of the HL-LHC Twin Aperture Orbit Correctors

A study was performed to understand the quench behavior and ensure adequate quench protection of the canted cosine theta (CCT) twin aperture orbit corrector magnet, a superconducting magnet under development as part of the high-luminosity upgrade of the Large Hadron Collider (HL-LHC). The cosine theta geometry features canted superconducting coils, which together produce a magnetic dipole field. The NbTi/Cu strands are placed in slots inside formers that maintain the shape of the coils. The presence of these formers affects the quench behavior of the magnet by preventing direct thermal contact between adjacent groups of strands. At the same time, a discharge of the stored energy over an external resistor results in significant eddy current heating inside the formers, which quickly brings the entire superconducting magnet to a normal state. A calculation model was developed that describes the electrical and thermal behavior of this type of magnet, and the results of this model are compared to experimental observations on a 0.5 m CCT model coil. It is found that the calculation results and experimental observations are generally consistent, although the simplified manner in which the eddy current heating is described in the model leads to a modest overestimation of the hotspot temperature. The calculation results indicate that a proposed quench protection configuration, featuring a discharge over a 0.7  $Omega$ energy extractor and a 0.05  $Omega$ crowbar, is sufficient to protect both the 0.5 m CCT model magnet and the 2.2 m CCT prototype magnet, resulting in hotspot temperatures of 63 and 193 K, and peak voltages to ground of 300 and 310 V, respectively.