Magnetic field dependent stability and quench behavior and degradation limits in conduction-cooled MgB₂ wires and coils

Long lengths of metal/MgB₂ composite conductors with high critical current density (J_c), fabricated by the powder-in-tube process, have recently become commercially available. Owing to its electromagnetic performance in the 20-30 K range and relatively low cost, MgB₂ may be attractive for a variety of applications. One of the key issues for magnet design is stability and quench protection, so the behavior of MgB₂ wires and magnets must be understood before large systems can emerge. In this work, the stability and quench behavior of several conduction-cooled MgB₂ wires are studied. Measurements of the minimum quench energy and normal zone propagation velocity are performed on short samples in a background magnetic field up to 3 T and on coils in self-field and the results are explained in terms of variations in the conductor architecture, electrical transport behavior, operating conditions (transport current and background magnetic field) and experimental setup (short sample versus small coil). Furthermore, one coil is quenched repeatedly with increasing hotspot temperature until J_c is decreased. It is found that degradation during quenching correlates directly with temperature and not with peak voltage; a safe operating temperature limit of 260 K at the surface is identified.