The potential curvature in z dimension at the trap center with respect to z/w. For small w that w = 0.5a, the curvature is relatively small. For very large w that w = 2.5a, the curvature is reduced near the center of the segment. When w/a equals to 1.5 or 2.0, the potential curvature equals to zero at z = ±w/2, which is not good for ion transport. Thus, a good choice for the width is w = a.