  The magnetic flux trapping in type-II superconductor containing fractal
clusters of a normal phase, which act as pinning centers, is considered. The
critical current distribution for an arbitrary fractal dimension of the
boundaries of the normal phase clusters is obtained. It is revealed that the
fractality of the cluster boundaries intensifies the pinning and thereby raises
the critical current of a superconductor. The pinning gain coefficient as a
function of critical current for different fractal dimensions is calculated.
