Development and Implementation of a RC Snubber to suppress breaker arcing present in high-stored-energy superconducting magnet systems

Arc suppression Snubbers offer safety in high energy superconducting magnet systems. Large superconducting magnets may have arcing across their breakers at the moment of slow dump initiation resulting from a sudden current redistribution in the powering circuit. The ATLAS Experiment at CERN experiences such arcing with its 7.3 H Toroidal magnet powering circuit. To discharge the magnets, breakers in series with the power supply are opened and the current is redistributed to a parallel branch called the Run Down Unit (RDU). The RDU utilises diodes and resistors that dissipate the stored magnetic energy over a period of hours when discharging from nominal current. Due to the physical layout, large size of the facility and other metallic components in the vicinity, the circuit featuring the power supply and RDU branches have undesirable and unavoidable parasitic inductance. This parasitic inductance causes a voltage spike over the main breakers at the moment of opening. The resulting arcing deteriorates the breaker contacts, resulting in increased operating temperatures and a more frequent need for maintenance. The arc suppression system developed takes the form of an RC Snubber (Resistor-Capacitor). The Snubber makes use of a 9.6 F bank of capacitors which offer the current a low-impedance path, thus smoothing out the current redistribution and suppressing the voltage spike. This initial charging period of the capacitors gives the current enough time to overcome the parasitic inductance, therefore allowing the RDU to smoothly ramp up without a sudden voltage spike. To test the concept, a 1/50th scale demonstrator was developed and showed successful results. The demonstrator had 1/50th the capacitance, however operated at the same current per capacitor that the full system sees. After the demonstrator, endurance testing of the electrolytic capacitors was performed to ensure no degradation of the charging characteristics for an equivalent of 20 years of operation. After these successful endurance tests, the full-size Snubber was manufactured and implemented with successful results on the ATLAS Toroidal powering circuit