![Image](http://www.broadbit.net/download/polywell/Spherical-design-B-field-near-X-axis.png)
While the magnitude of field is zero at the exactly the axis, I am not sure whether a cusp forms here. What do you think?
From my point of view the interesting question is the one highlighted above: if there is some ion focusing into the center, does it increase bremsstrahlung and fusion rate at same density exponent?