Improved quench characteristics of superconducting racetrack coil via shape modification

Superconducting racetrack coils have worse quench characteristics due to the different structure from solenoid coils. Racetrack coils also have round curvatures along their end portions; however, a large difference in their structures from solenoid coils exists in the straight portions, which is considered as a cause of the worse performance. In the round portions, there exists strong continuous constraint along the windings against the winding movement via the Lorentz force during coil excitation because of winding tension. On the contrary, in the straight portions, there is almost no constraining. Thus, the windings are apt to move more easily than at the round portions. As a result, especially in the case of a dry magnet, they show worse characteristics than their solenoid counterparts. For the compensation of this mechanism, we propose to improve the quench currents of racetrack coils. We have ground the straight sides of a racetrack coil bobbin into a rounded shape with a negligible effect on coil performance. By experiments, we confirmed that the proposed one had a higher quench current. Moreover, after we applied the proposed idea to the four field coils of an actual 30 kVA superconducting generator, we obtained very good output voltage and current waveforms.