We report temperature and magnetic-field dependence of flux pinning force in bulk MgB 2 with variable grain size. The samples are prepared by advanced methods, allowing minimizing effects of porosity, impurities, and inclusions of secondary phases. The effects of grain connectivity, flux-creep phenomena, and grain size on critical current density and flux pinning curves are analyzed. We have compared the field dependence of the pinning force for a range of samples with the predictions of theoretical models accounting for the effect of the grain size. There is qualitative agreement between grain-boundary pinning mechanism proposed by Hampshire and Jones ͓J. Phys. C 21, 419 ͑1987͔͒ and the experimentally observed grain-size dependence of pinning force in bulk MgB 2 .

The thermal stability of different Fe-and Ni-sheathed MgB 2 conductors has been studied experimentally and numerically, focusing on the estimation of the quench propagation velocities, v p , minimum quench energies (MQEs) and minimum propagating zones (MPZs). The measurements have been done at self-field and under adiabatic conditions, at variable temperatures and transport currents. Energy pulses were deposited to the conductor by passing rectangular current pulses through a graphite-based epoxy heater. The measured voltage around the heater together with numerical simulation allows the estimation of the minimum propagating zone. Moreover, v p was obtained by measuring multiple voltage taps and thermocouples attached along the conductor. The effect of the current sharing temperatures, I c (T ), of the superconductor, and thermal and electrical properties of the metal sheath have been analysed. The experimental results are in qualitative agreement with the simulated ones, obtained by solving the one-dimensional heat balance equation of the system.

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