Lawrenceville Plasma Physics Nuclear Fusion – Plasmoid Ion Density Tripled but 100 times less due to metal impurities in filaments

The density of the fusion-producing plasmoid is the key factor that must be increased for LPP to
the scientific feasibility of net energy production from Focus Fusion—net energy meaning more energy out than is lost in making that energy. In the past month’s experiments, LPP’s research team has demonstrated the near tripling of ion density in the plasmoid to 8×10^19 ions/cc, or 0.27 mg/cc. At the same time, fusion energy output has moved up, with the best three shot average increasing 50% to one sixth of a joule of energy.

The greater increase in density than fusion energy is expected, because as compression improves and the plasmoid gets smaller, its lifetime also decreases. So while density improves roughly as 1/r3, where r is radius, lifetime decreases proportional to r and energy output increases roughly as the product of the two, or 1/r2.

The higher density was determined by combining measurements of the total fusion energy and ion temperature derived from our neutron detectors, and measurements of plasmoid size from our ICCD-camera images. The LPP team has moved the camera to a new position, looking up close to the axis of the electrodes instead of side-on as previously. Our very first image from this direction shows our smallest plasmoid yet observed with a core radius of only 150 microns and core length of about 1.5 mm.

While the yield and density improvements show we are moving in the right direction, they are still well below what the LPP team theoretically expects for our present peak current of 1.1 MA. Yield is low by a factor of 10 and density by a factor of nearly 100. If we can get yield up to our theoretical expectation of over 1 joule, our scaling calculations tell us that with higher current we can make it all the way to the 30,000 J that we need to demonstrate scientific feasibility. We’ve long concluded that this gap between theory and results is caused by the “early beam phenomenon” which is itself a symptom of the current sheath splitting in two, feeding only half its power into the plasmoid.

Disruption of the filaments. The cathode plate (right) at 7.5” in diameter is shown for context below, with rods, insulator, and anode in a, and alone with close-close up area outlined in b. In c, the close-up of the cathode plate runs from the tungsten teeth at right to a copper rod at left. Bright blue marks trace the paths of filaments from 60 shots, showing that the filaments at this point are only about 150 microns in radius. (The abrupt change in the blue marks’ brightness is due to a change in the tungsten surface.) Note how the filament paths spread out and eventually are disrupted as they approach the band of evaporated silver and copper near the rod.

So with oxidation way down and the leaks fixed, where were these metal impurities coming from? We had to take the electrodes apart to find out, and then analyze the dark deposits we found with an x-ray spectrometer (provided by local company NJ Plating). That pointed to the likely culprit: tiny amounts of silver and copper were being vaporized by micro-arcing at the base of the cathode rods, where silver-coated copper washers were located. While the amount vaporized was tiny—about 0.2 milligram per shot—the current sheath only has a mass of 2 milligrams, and we calculate disruption can occur with even 60 micrograms contamination.

In the next shot series, we will replace the washers with indium wire which has worked elsewhere on our electrodes to entirely eliminate even the tiniest arcing. We will also silver-plate the cathode rods as we have done with the anode. Over the longer run, we are looking at ways to have a single-piece cathode made out of tungsten or tungsten-copper in order to eliminate the rod-plate joint altogether. These steps should get rid of the filament disruption for good, enabling results to catch up with theory.

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