Unbalanced Impedance of the Aperture Coils of Some LHC Main Dipole Magnets

During the Large Hadron Collider operation in 2010 and 2011, it was observed that about 50% of the main dipole magnets exhibiting two apertures showed unbalanced dynamic-impedance behavior not well understood. When a main dipole circuit is switched off, voltage waves with a frequency of 28 Hz travel along the chain of magnets. The difference in voltage drop across the two halves of the magnets can cause a spurious trigger of the quench detection system leading to undesired firing of quench heaters. In order to investigate this phenomenon, dedicated measurements of the impedance in the frequency range 1-20 kHz are performed on four selected magnets in the Large Hadron Collider tunnel. As expected, separate measurements of the frequency transfer function of the two apertures of certain magnets show a difference in the ac behavior of the apertures, particularly in the frequency range 10-200 Hz. On the contrary, other apertures present very similar frequency transfer functions. The measured frequency transfer functions are compared with results obtained using an equivalent electrical model developed with Wolfram Mathematica. A possible explanation for the unbalance effect is the presence of induced currents of different amplitude in one aperture as compared with the other.