  We consider a smectic-A* in a capillary with surface anchoring that favors
parallel alignment. If the bulk phase of the smectic is the standard
twist-grain-boundary phase of chiral smectics, then there will be a critical
radius below which the smectic will not have any topological defects. Above
this radius a single screw dislocation in the center of the capillary will be
favored. Along with surface anchoring, a magnetic field will also suppress the
formation of a screw dislocation. In this note, we calculate the critical field
at which a defect is energetically preferred as a function of the surface
anchoring strength and the capillary radius. Experiments at a few different
radii could thus determine the anchoring strength.
