This is in reply to Dave's general request related to electron multipacting.
Farnsworth recognized a use for the long noted problem in some UHF systems of electron knotting, which he termed multipacting. For a short while Farnsworth and later, ITT made a small multipactor vacuum tube! (use unknown and now a collector's item)
The multipaction scheme is simple in theory. If one switches the plus and minus connections (alternating current) at just the right speed in any vacuum geometry where electrons are emitted from a hot cathode towards a plate system, a bunching or electon knot can be created. A form of electrostatic, physical, electron resonance is created, hopefully, with a single, well controled, nodal point
This was first noted and caused great damage in early UHF tubes, mainly power output tubes as grids and plates and even some cathodes were melted or damaged coupled with effective shutdown of the device, even if not damaged. The problems were solved by mostly geometric considerations. (making the devices smaller, internally.)
Farnsworth sought to create a physical, spherical, electron knot or virtual cathode in the center of his "Mark I" device, (1960-62). It was hoped this massive current locked wad of electrons would accelerate deuterons to itself and, if missing the electrons, would collide with the usual infalling deuterons from the opposite direction and fuse. (gridless fusor)
This is a tricky business, at best, and while sounding theoretically simple is very difficult to physically achieve. While multipacting accidentally happened to detrement in vacuum tubes, there was not the additional requirement of symetrical co-acceleration of positive nuclei into the knot thrown into the mix.
This latter demand is the sticking point.
There was a single tube that was designed, hand blown and constructed to Farnsworth's personal specs in 1959 Just after the program's inception. Unfortunately, it was such a horrid kludge that it was never used or tested according to George Bain. He did note that the tube lab at Pontiac Street spent over three months on the tube, discarding several models as Farnsworth almost daily had a design change. The tube lab quickly learned to drag its feet on Farnsworth requests, but it had to do so strategically as the Admiral was instantly prepared to crush and fire anyone impeding Farnsworth's progress.
Like many of Phil's brain storms, this early fusor tube turned out to be a brain fart and was never utilized, but occupied a place in a cabinet or on a shelf along with many similarly conceived and constructed bretheren.
Stories similar to Bain's were told by every team member that I have ever interviewed.
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.