Superconducting magnet systems are the enabling technology for several research fields, e.g., experimental high-energy physics and fusion. Advanced superconducting magnet systems are strongly needed to achieve ever-higher beam energy in particle accelerators. They are also extensively used in plasma confinement for fusion. The energy stored in a magnet converts to heat when the magnet is quenching, i.e., a state change from superconducting to normal. The temperature increase and the high turn-to-turn voltage developed in a quench may degrade or damage the magnet. Thus, one of the key issues for the successful operation of superconducting magnets is the quench detection and protection. This thesis discusses the self-field quench behavior of Bi2Sr2CaCu2O8+x (Bi2212) short samples and coils with the purpose of discovering key critical quench conditions that cause loss in critical current (Ic) of the conductor.
Bi2212 tapes and round wires are investigated in short sample quench experiments. The experiments are conducted by means of heater induced quenching, and V(t) and T(t) data during a quench is recorded. The minimum quench energy (MQE) and normal zone propagation velocity (NZPV) are determined for both conductors. Using the collected data, and measuring the Ic of samples after quenching, critical values for energy deposited into the conductor via Joule heating (E), maximum temporal temperature gradient (dT/dt|max), maximum spatial temperature gradient (dT/dx|max) and maximum temperature (Tmax) that cause loss in Ic are determined. These quench conditions are varied in order to determine specifically what the critical values for each are. It was determined that while the tape short samples exhibited higher stability than the round wires, their NZPV was excessively slower and critical quench conditions significantly smaller.
Coil quench experiments where conducted on round wires due to their proven resilience to quench induced degradation during short sample experimentation. In similar fashion to short samples, MQE and NZPV were determined for coils, as operating conditions and conductor batch varied. Coil quench experiments where also conducted via means of “hot-spot” generation using a heater. It was concluded that the coils exhibited larger stability, yet slower NZPV than short sample round wires. The critical quench conditions that do not cause loss in Ic to the conductor agreed with those determined in the short sample round wire experiments. This allows for the conclusion that critical quench conditions are intrinsic to the conductor and not dependant on operating conditions.
The dependence of quench behavior on sample Ic and current sharing temperature (Tcs) is also observed. It is noticed that with smaller Tcs, NZPV increases and stability decreases. Furthermore, inhomogeneous Ic and Tcs along the length of the conductor allow for inhomogeneous quench behavior. In turn quench conditions are difficult to predict, and vary between samples.