BTW the Farnsworth Fusor benefits from "spherical convergence" of ion vectors.
The vectors are self-focused and not chaotic. Farnsworth/ Hirsch found the fusion threshold is lowered by a factor of 4 due to spherical convergence, allowing substantial neutron production at far lower voltage potential than colliding beams. Polywell borrowed the idea http://www.askmar.com/Fusion_files/Polywell%20Ion%20Focus%20Concept.pdf
