On June 16, LPPFusion’s Focus Fusion-1 (FF-1) device resumed firing, after a hiatus of 19 months. It became the first dense plasma focus device of over 1 meg-ampere current to use monolithic tungsten electrodes, or to fire with pre-ionization. (Pre-ionization involves using a small current to smooth the way for a larger one.) So far, only a dozen shots have been fired, too few to reach any firm conclusions. However, very preliminary results indicate that impurities have dropped significantly with the new design, but not yet far enough for dramatic gains in fusion yield.
The resumption of firing was made possible by the successful full repair of the tungsten cathode and the flawless assembly of the new cathode into the FF-1 device. In late May, LPPFusion CIO Ivy Karamtisos bridged over remaining micro-cracks in the tungsten cathode rim with indium metal (video coming soon). The soft indium cold-welded onto the tungsten providing a low resistance path for the electric current to flow into the cathode. Next, Karamtisos, Chief Scientist Eric Lerner and Research Associate Mark Klapheke carefully applied indium to the entire cathode rim, and then bolted the steel connecting plate onto the cathode. The steel plate actually attached to the steel brace that had previously been glued onto the tungsten cathode, with the tungsten current contact and its soft indium coating squeezed between the two steel pieces. The results were extremely low electrical resistance, no more at any point than18 micro-ohms, and good mechanical strength. This ensures that the cathode can safely take up to 1.6 MA of current, enough for the planned experiments.
The lab team then mounted the cathode steel plate assembly into the experimental device. Using the new micrometer tabs, the team centered the cathode on the anode to a precision of 25 microns (one thousandth of an inch.) (Figure 1) After fully assembling the vacuum chamber, the team was able to achieve a low leak rate of only 35 microtorr per minute. (At this leak rate it would take 38 years to fill the vacuum chamber back to atmospheric pressure!)
While assembly was completed June 3, teething problems to be expected with restarting an experiment after a year and a half hiatus slowed final preparations for firing. Then the new triggering system needed for pre-ionization required testing and fine-tuning, the ICCD camera was in need of maintenance (and still is) and other minor problems had to be addressed. The first shot was finally achieved on June 16th. As expected it produced no fusion. This is because the first shot coats the insulator with a thin, discontinuous layer of metal. In further shots, this coating then guides the electric current in a thin sheath, necessary for the formation of the plasmoid —the dense knot of plasma that is heated to fusion temperatures. On the third shot, however, on June 19th, the first plasmoid and fusion reactions took place, inaugurating LPPFusion’s new experimental series.
Figure 1: The newly assembled tungsten cathode (outer ring of vanes) and anode (inner hollow cylinder) successfully assembled into the FF-1 device. The rectangular object below the electrodes is the silver activation detector—one of the main instruments for measuring the neutrons produced in deuterium fusion reactions. photo-Erin Lerner