Welcome to vapor deposition and ion implantation. This is common and discussed and seen on my video tapes of fusor operation. There is no way to avoid this, it seems. You can control it with shields and shaders, but they screw with the fields and wreck the spherical symmetry. It will continue to accumulate and is a function of runtime and currents.
Richard Hull
Created on Tuesday, June 05, 2001 12:07 PM EDT by Richard Hull