Racetrack coil instability resulting from friction heat generation at fixtures

A series of experiments on the instability resulting from mechanical disturbance at the coil surface of a small racetrack coil is described, along with a preventive measure against its instability. Epoxy-impregnated racetrack coils sometimes experience premature quenches due to frictional heat produced by coil slides at fixtures. The first experiment confirmed coil slides during coil charging. These slides were about 10 mu m, an equivalent of 20 mJ in fractional heat generation. One effective preventive measure against this mechanical disturbance is the utilization of a thermal barrier method. The thermal barrier is an insulation layer at the interface between the coil and the fixtures. The second experiment examined the thermal barrier effect on the stability margin on the racetrack coil. A thicker insulation layer substantially increased the coil stability margin. The margin increased from 105 mJ to 200 mJ by thickening the insulation layer from 0.36 mm to 1.00 mm. The two experiments showed that the racetrack coil was stabilized if the thickness of the insulation layer exceeded 0.20 mm. Charging a racetrack coil with a 0.36 mm-thick insulation layer confirmed this criterion.<>