Complex Impedance of the LHC Main Dipole Magnet in the Presence of Artificial Short Circuits

Interturn short circuits inside the coil-winding pack of superconducting magnets can lead to severe damage to the magnet during a quench or fast current ramps. Investigating their impact and finding ways to detect these nonconformities could save considerable run-time of an accelerator. However, interturn short circuits are challenging to identify as they cannot be measured with conventional resistive methods, and only a few measurement data are available of coils exhibiting a short. The complex impedance as a function of the frequency was measured on a spare Large Hadron Collider (LHC) main superconducting dipole at the CERN magnet test facility. External resistances ranging from 0.3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Omega$</tex-math></inline-formula> to 100 k<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Omega$</tex-math></inline-formula> were connected across an entire pole via the magnet's voltage taps to imitate an interturn short circuit. The effects of such artificial shorts on the complex impedance were evaluated. A physically derived lumped-element network model, reproducing the frequency-dependent electromagnetic behavior, was recently validated against measurements of a conform LHC main dipole. This model is used to simulate the presence of interturn shorts in the coil-winding pack of the LHC main superconducting dipole. Good qualitative agreement was found between the model and the experimental data. The model was then used to simulate the impact of short circuits that are difficult to realize in a laboratory setup. Results are presented for various scenarios, such as single-turn short circuits in different locations and with different short-circuit resistances, and the effects of such shorts on the electrical integrity and electromagnetic behavior are assessed.