Wonderful simulation. Thanks for sharing it. A few questions and notes.
[list]
[li]I tried to make a bunch of circular walls. It seems that I could not make the radius smaller than 20. That was not a big problem.[/li]
[li]I ended up making a matrix of 9 adjoining circles. For some reason, several of the circles did not close. Did they need to be away from each other to close?[/li]
[li]I also did not figure out how to get it to save a particular setup. That would be nice, especially if I can edit the xml file.[/li]
[li]It would be nice if I can assign mass to charges[/li]
[/list] 
Thanks for all of what you do.
TM