Helical disclination lines in smectics a

It is shown that helical disclination lines exist in smectics A; they can play the role of focal conics for the smectic planes or be true disclination lines. In the latter case, they are a source of screw dislocation lines. Single helices are terminations of twist walls made up of screw dislocation lines. Double helices are equivalent to large screw dislocation lines.