AC-Loss Induced Quenching in Adiabatic Coils Wound with Composite Superconductors Having Different Twist Pitches

The effect of filament twist pitch length on AC-loss induced normal zone propagation in adiabatic coils wound with composite NbTi superconductor was investigated. Two test coils were constructed. They were identical in shape, dimensions, and other parameters, the exception being the conductor's filament twist pitch length. One had a twist pitch length of 10 mm and the other had a twist pitch length of 100 mm. Each coil was subjected to time-varying excitations, both transport current and external magnetic field. Measurements indicate that the AC-loss heating, as expected, is greater in the coil wound with 100-mm twist pitch length conductor than in the other coil. More importantly, they indicate that AC losses can significantly promote and accelerate normal zone propagation in adiabatic magnets and that filament twist pitch length may play a critical role in protection of the magnets. The experimental results are compared with simulation results. The simulation model includes the inter-filament coupling AC loss as a key component of dissipation in the quench process of a coupled multi-coil system. Excellent agreement between experiment and simulation validates the model. The agreement emphasizes the importance of coupling loss in normal zone propagation and suggests filament twist pitch length as a critical parameter for coil protection. Thesis Supervisor: Dr. Yukikazu Iwasa Title: Research Professor, Francis Bitter Magnet Laboratory, and Senior Lecturer, Department of Mechanical Engineering, MIT