Spark Detection and Search for High-Voltage Paschen Leaks in a Large Superconducting Coil System

The plasma fusion experiment Wendelstein 7-X (W7-X) uses a system of 50 nonplanar and 20 planar superconducting coils. These coils produce the magnetic field that is required to confine the plasma. Magnetic flux densities up to 3 tesla can be reached in the center of the plasma. Supercritical helium is used to cool down the coils to operating temperatures below 4 K. The coils are specified for currents up to 18 kA. A critical issue in all superconductors is the occurrence of quenches. These are unwanted local transitions from superconductivity to normal conductivity. If that happens, the coil current has to be discharged as fast as possible into a dump resistor. However, the strong current change will produce a self-induced high voltage (HV) with values up to several kilovolts. Therefore, the electrical coil insulation versus ground has to be HV proof to avoid a high-current arc in any circumstance. Worst case scenario is a quench that is induced by a loss of thermal insulation after a leak in the helium supply lines, because Paschen-minimum conditions could be given. This article describes some HV test procedures and techniques employed to test and qualify the coil system against that scenario. Some of these techniques are used today for the routine coil tests. Some other turned out as inefficient for daily use. Essential for a safe machine operation is the detection of insulation defects, but also on their localization along the superconductor, for the sake of a later repair.