55th example. Use of a rectangular beam, for space charge spreading.
Based on xmpl3d07, but a rectangular beam is used instead of a cylindrical beam.
The current is the same as in xmpl3d07. The source rectangle (ie the window) has a half-width 3 in the x direction and 1.5 in the r direction ( in xmpl3d07 the source disc has a radius 3).
The beam converges to a point at z = 0.
To add some variety, one of the repeated space-charge runs is re-defined as a rectangular beam.