It's not necessarily true that you need a big fusor to have that third grid. You want the third grid pretty close to the outer grid (or outer wall if this is your grounded electrode) to provide a source of ions close to the outer wall that experience the maximum potential fall to the center grid. I plan to try this with a 4 inch cylindrical device at pressures of one micron or less, the goal being to establish control of fusor current more or less independently of gas pressure and center electrode voltage by varying the temperature of the emitter and/or its voltage with respect to the outer shell. I plan to space the emitter about one centimeter or less from the outer wall. Later on, one might want a larger chamber to take advantage of the longer mean free path for ion recirculation, but I see no reason why one cannot establish the basic principles with a small device.
Cylindrical fusors are attractive for this kind of play, as they have ends for feedthroughs, etc that can be readily and relatively cheaply modified (a new end flange for my chamber is $54) for experimentation without chewing up the entire vacuum vessel, unlike a spherical chamber. I plan a 10" spherical device somewhere down the line, but I prefer to make my initial mistakes on a cheaper and more easily configurable prototype.
Richard Hester
Created on Sunday, December 24, 2000 7:54 PM EDT by Richard L. Hester