Time-Varying Magnetic Field Coupled Noise Reduction in Low-Voltage Measurements in Superconductors

Noise generated by time-varying magnetic fields interferes in the measurements of low voltages particularly with superconductor materials. The measured values can alter significantly if it is not compensated appropriately or if suitable protocols are not followed. Measurements on inductive loads, like superconducting wires and coils, considerably generate high magnetic field in the surrounding, which couples electromagnetic noise through measurement leads and circuits. Twisted pair cables are adopted and used as a classical solution to avoid magnetically induced voltages. However, the magnetic field produced by large inductive load couples imposed electromagnetic interference (EMI) with the cable loop created by voltage taps, which are soldered over part of the load. The effects are severe when there is a big loop area due to large sample size or when there is a high rate of change in the magnetic field due to high-current ramp rates. A finite uncompensated flux is invariably linked in such cases. Three simple, yet effective, techniques were implemented for the cancellation of EMI noise which couples in a cable loop formed by voltage-measuring leads of superconductor sample in low-temperature experiments. In all these methods, the basic principles are to detect and measure signal portion and coupled-noise portion with dedicated voltage taps, use subtraction mathematics to cancel the noise portion, and derive actual voltage-drop signal across a superconducting load. These techniques were tested successfully on low-resistance joint of high-temperature superconducting Bi₂Sr₂Ca₂ Cu₃O₁₀ tape in a bath-cooled condition (77 K). EMI noise coupling was simulated with background field coil and a permanent magnet. This paper describes the detailed experimental procedure, results, and observations. The concept can be extended to DC joint-resistance measurement of Toroidal and Poloidal field coils of Steady State Superconduc- - ting Tokamak-1.