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 non-planar and 20 planar superconducting coils. These coils produce the magnetic field that is required to confine the plasma. Magnetic flux densities up to 3 T can be reached in the center of the plasma. Supercritical helium is used to cool down the coils to operating temperatures <; 4K. The currents in the coils can reach up to > 18 kA. A critical issue in all superconductors is the occurrence of quenches. That 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 up to several kV. Therefore, the electrical coil insulation versus ground has to be high-voltage 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 then Paschen-minimum conditions could be given. This paper describes some high-voltage test procedures and techniques employed to test and qualify the coil system against that scenario. Some techniques are used today for the routine coil tests. Some other turned out as inefficient for daily use. Focus is on the detection of insulation defects, but also on their localization for the sake of a later repair.