77th example, accelerator tube, as used in Van der Graaf accelerators.
See xmpl2d47 for the analogous 2D simulation (which is more accurate).
This is a simplified simulation, with a relatively small number of accelerator rings. No attempt has been made to optimise the design.
The tube has 16 rings. The overall acceleration voltage is 750kV, 50kV per ring. The particles are H2+ molecules. Their initial energy is 1640eV, which is the energy found in the 2D analogue xmpl2d47.dat to give approximate focusing at the end of the tube.
A higher density of segments has been used near the critical region at the first ring. Also a 'back plate' has been added near the source, to stabilise the field there.
The voltages and initial energy are those used in the 2D analogue, xmpl2d47.
The final image shows spherical aberration.