The size of a simple fusor needs to be limited to 8". Simple means high gas pressure, low recirculation. Simple means mechanical pump only and inexpensive operation.
Mean free path for ions shuts the size off at 8" with 6" being ideal.
If one intends to pump to 10e-8 torr, backfill with D2 and run at 10e-5 torr then meter sized chambers are just dandy.
By makinbg a chamber spherical you just have a big heat sink for electrons and don't have to worry about melting and outer spherical grid in a cylindrical case. Also the spherical chamber is the minimum volume for vacuum pump down. Why use a 36" cylindrical chamber and put in an 18" outside grid unless you have a 10" throated turbo pump!
A University with a full 70 kilobuck pumping station can have the luxury of wasted, dead volume at pump down. (They have hundreds of grad student lackys to burn off hours sucking down a chamber like this.)
The average amateur thinks all he needs is a vacuum pump and he can pump to outerspace vacuums in a few minutes. Vacuum technologists know that regardless of how many thousands the pumps cost, the lowered volume at pump down pays big rewards in headache reduction.
Richard Hull
Created on Monday, December 25, 2000 2:06 AM EDT by Richard Hull