Figure 3: Projection of the singular flow onto the planes and in the cerebrospinal fluid/water case. The trajectories are plotted in black (solid line) and in red (dashed line). The control fields of the dashed extremals diverge. The trajectories have been plotted up to the explosion of the field (the absolute value is then larger than ). The horizontal and vertical dashed lines represent the - and -axes of the Bloch ball. Note the contrast that can be obtained between the two spins, which illustrates the role of the singular flow in this problem.