Since 2014, the LPPFusion research team has theorized that heavy metal impurities have limited fusion yields by disrupting the dense filaments that formed early in each shot of the FF-1 plasma focus device. Because the filaments are the first stage of compressing the hot plasma, disrupting them before the main compression phase that forms the plasmoid leads to lower final density and thus less fusion. However, while the team had plenty of indirect evidence for this theory, they did not have concrete proof that the filaments did in fact form and then dissipate.
But now analysis of ICCD photographic images taken of the early stages of the pulse demonstrate for the first time that the filaments do indeed form in FF-1 (as they do in other plasma focus devices) but that they then begin to expand and merge into each other. The filaments, seen coming off the anode near the end of the insulator, first form around 350 ns into the shot and are very distinct at 400 ns. But already at 500 ns are beginning to merge together. The tungsten impurities vaporized from the anode glow very brightly, outlining the filaments. But the tungsten ions also cause high resistance and rapid heating of the filaments, causing their expansion. (See video) By the end of the image sequence, the filaments are starting to merge together.
The new images provide confirmation of the LPPFusion theory, and thus build confidence that, without the heavy metal impurities, the filaments in the upcoming beryllium-electrode experiments will be preserved. In addition, the new set-up of the ICCD mirrors will let the team quickly monitor exactly what is happening at all stages of the pulse. This will allow them to better optimize conditions, getting to higher fusion yields faster.
This video shows a sequence of ICCD photographs taken from similar shots of FF-1. The numbers on each frame show the time elapsed, in nanoseconds, since the start of the pulse. The region shown of the anode, close to the end of the insulator is a cm wide. The filaments (bright stripes) first form and become more distinct, then at the end of the sequence, begin to merge together, becoming destroyed. The bright regions near the lower end of the sheath are heavily contaminated with tungsten vaporized from the anode. Note in the last frame that the filaments still seem well-organized in the fainter upper regions, with less impurities, but are merging in the lower, brighter and more contaminated region. This shows the effect of impurities.