FF-1’s tiniest plasmoid; dramatic leak reductionMarch 11, 2013
Over the past few months, LPP’s experimental team has been trying to improve the symmetry of the compression that creates the plasmoid, so that the plasmoid will become smaller and denser. Higher density will make the fusion fuel burn faster and produce more energy output. Up until now, the core of our plasmoids, which are shaped like the sugar glaze on a doughnut, was no smaller than 300 microns in radius. Although this sounds pretty tiny, our goal was to get it down to 50 microns radius, with much higher density. We know that this is possible, as other researchers using similar plasma focus devices have observed and measured plasmoids this small. We also know that other researchers have achieved ion densities up to a few thousand times higher than what we have achieved, (hundreds of milligrams/cc vs our 0.1 milligram/cc) so we know that this too is possible.
A few shots after we got a record beam, on shot 7 of February 28, we also imaged our smallest plasmoid yet, shown in Figure 2, with a core radius of only 200 microns. This image was taken several ns before the point of maximum compression, so the plasmoid has not fully formed and the smallest radius is probably somewhat smaller than 200 microns. The plasmoid core is seen forming at the narrowest “waist” of the pinch column, before the current has twisted itself up into the fully formed plasmoid. Like the “Big Beam,” we interpret this smaller plasmoid as the result of improved symmetry in our compression, due to our progress with the vacuum system.
Leaks squeezed down by 100-fold
When 2013 began, FF-1 was beset by persistent leaks. These leaks were allowing oxygen to be present during our shots, so that the copper on the anode was rapidly oxidized in uneven patterns. Since copper oxide is an insulator, the current filaments had to cut through this oxide layer to reach the copper below. In the process the filaments would wander around, getting closer to each other in some places and farther in others. This in turn led to asymmetric compression and the “early beam” phenomenon, where energy would be released in filament collisions before compression was complete.
By early March, with the help of consultants and investors, LPP Chief Scientist Lerner and Lab Coordinator Derek Shannon had cut the leak down from 30 milliTorr/min at the beginning of January to only 0.3 milliTorr/min, by the beginning of March. First we got help from our new consultant, Brian Bures, who has had years of experience with small plasma focus devices. Then, we used an idea suggested earlier by LPP investor Rudy Frisch, who is a mechanical engineer. He suggested putting a Teflon restraining ring around the rubber O-ring that seals to the anode, forcing it to have a good seal when it is compressed. That got us a good seal before we fired, but a large leak re-opened after the first shot.
Another investor, Walter Rowntree, came to the rescue by acquiring on LPP’s behalf a Residual Gas Analyzer, a sensitive instrument that analyses and identifies the gas in the chamber. Using the new RGA, Shannon rapidly identified the main leak gas as isopropyl alcohol. We had been using the alcohol to check for leaks and it had gotten trapped in a cavity in the anode, bursting out when heated by the current in the anode. Draining the cavity solved the problem.
We are not quite through with leaks as we still have some oxygen in our chamber. But, as with previous engineering challenges such as arcing and high voltage switching, we expect that our growing understanding of the issues will enable us to solve the remaining leaks soon.