Locating Electrical Faults in Superconducting Accelerator Magnets Using Time Domain Reflectometry

Identifying the location of an electrical fault (mainly short-circuit or circuit break) in close systems, such as superconducting magnet, is very difficult, especially when a problem appears only at cold conditions (no resistance). The time domain reflectometry (TDR) method could be useful notwithstanding that there is a commonly known limitation on the use of TDR to test the inductive circuit results of the high attenuation of incident and reflected pulses. This paper describes a method for locating electrical faults in superconducting magnets by means of measurements of the pulse response of magnet coil circuits (TDR principle). As a result of adapting TDR to test a circuit characterized by high values of equivalent capacitance and inductance, which is a winding of superconducting magnet, a study of the relationship between the frequency response of magnets and the injected pulse transient parameters is presented. The cutoff frequency of the investigated circuit is crucial information for pulse shaping. A TDR reflectometer with a pulse shaping circuit is described. The instrument consists of commonly available laboratory-measuring equipment: a pulse generator and a 50-MHz oscilloscope, which significantly facilitates the performance of the measurements. Test measurements were carried out for both the undamaged magnets and the magnets with various locations of coil shorts. Advantages and limitations of the proposed method are discussed.