Comparative Analysis of Impulse and Impedance Tests to Detect Short Circuits Within the W7-X Magnets

The magnet system of the Wendelstein 7-X stellarator (W7-X) consists of 50 nonplanar and 20 planar coils. In particular, the winding packs of the nonplanar coils are densely packed and compressed in the area of inter-layer joints and coil terminations, which brings some risk of inter-turn and inter-layer short circuits. The paper describes methods deducted from impulse and ac tests performed on the coils, which enable demonstrating the absence of short circuits and weak points of low inter-layer insulation resistance. In the impulse test, frequency and damping are compared for coils with and without short circuits and coils with an external low resistance bypass. With ac voltage, a drastically reduced impedance clearly indicates a short circuit. Therefore both tests represent excellent QA (quality assurance) tools for checking the uniformity of the electromagnetic performance of a serial production of magnet coils