Analysis of an Internal Electrical Short in an LHC Orbit-Corrector Magnet

A few beam orbit-corrector circuits of the Large Hadron Collider (LHC) show an unusual behavior during operation. These circuits can be ramped up to their nominal current, however their superconducting magnets quench when they are ramped down using the same ramp-rate. Furthermore, after the quench the measured circuit current increases above its initial value for a few tens of milliseconds. This transient is analyzed with the STEAM-LEDET program, which allows multiphysics transient simulations including electro-magnetic coupling between the turns, 3-dimensional thermal diffusion in the coil windings, ohmic loss, and inter-filament coupling loss. Simulation results show that this atypical magnet behavior can be fully explained by an internal turn-to-turn short. In fact, due to the strong magnetic coupling of the shorted turn to the rest of the coil, whenever the magnet transport current is changed, a current-change with opposite polarity is induced in the shorted turn. During circuit ramp-down, the current through the shorted turn reaches values significantly higher than in the rest of the coil. The simultaneous increase of the superconductor current density due to the induced current and of its temperature due to local ohmic loss in the short can lead to a quench. Subsequently, the current in the shorted turn rapidly reduces, and its energy transfers by inductive coupling to the rest of the coil, causing a temporary increase of its current. With the limited amount of available diagnostic signals, the simulation tools and the related analysis prove crucial in characterizing the short-circuit key features and help assessing the consequences of such a transient on the magnet operation.