Advanced Diagnostics of Rutherford Cable Performance in Accelerator Magnets: Current redistribution & Quench development

The Large Hadron Collider at CERN is being upgraded to increase its luminosity by a factor of ten with respect to its initial design value. For this purpose, new superconducting magnets that employ Nb3Sn instead of Nb-Ti are to be installed. Nb3Sn poses new engineering challenges due to its brittle nature, which makes it more susceptible to damage during manufacturing and operation. Several of the new Nb3Sn magnets showed reduced or even degrading performance that was accompanied by anomalous signals captured with diagnostic equipment. Voltage taps as well as arrays of pick-up coils, called quench antennas, were used to study the magnet’s behaviour, both while it is ramping as well as during a so-called quench, where a magnet quickly transitions from the superconducting state to the normal state. To understand the anomalous behaviour, the effect of damaged strands in the superconducting Rutherford cable, from which the coils are wound, was studied with numerical models as well as with dedicated experiments on a heavily instrumented model coil in which damaged was intentionally introduced in a controlled way.

The existing simulation code THEA was used to study current redistribution effects during a magnet ramp. A new 3D thermal-electric PEEC-FEM model called RuNe (Rutherford Network) was developed during this thesis work, to simulate both early quench development and current redistribution during magnet ramp. To increase performance, RuNe employs hierarchical matrices to accelerate the computation of integral operators related to self-field and mutual inductance calculations.

By a combination of experiment and simulation, decaying voltages observed on current plateaus during magnet ramp were explained by the presence of local conductor damage in the Rutherford cable. Current redistributes around such a defect in a diffusion process, causing an evolving unbalanced current distribution in the cable. Such an unbalanced current distribution was also shown to be able to explain an increased normal zone propagation velocity observed in the cable, by effect of a defect-related propagation mode whereby the normal zone propagates along the overloaded strands. The model RuNe also gave greater insight into the physics of early quench development of both conductor-limited - and training quenches.

By combining measurement tools and strategies with efficient numerical models, a greater understanding and better diagnosis of conductor damage in magnets under test was achieved, even with limited data sets.