We started firing with a new insulator and the new beryllium anode and reprofiled cathode on Sept.10. These initial test shots were still with deuterium fuel, the last ones before our long-awaited hydrogen-boron shots. A key goal of these tests was to use the reprofiled cathode to reduce the number of filaments from 128 to 64. This would increase the current in each filament, allowing them to better compress the plasma and surive until the end of the rundown phase.
Sure enough, images taken by LPPFusion Research Scientist Dr. Syed Hasan with our ultra-fast ICCD camera clearly showed a reduction in the number of filaments between each pair of cathode vanes from eight to four. (Fig.1) Since there are 16 vanes, the total number of filaments is now at the targeted 64.
Figure 1. The new beryllium cathode produces 4 filaments between each set of cathode vanes (left) as opposed to 8 filaments with older electrodes (right.) This reduction in the number of filaments means each filament carries more electrical current and thus better compresses the plasma.
Images taken at the time of the pinch (maximum compression) showed that the plasmoid, where the fusion reactions take place, was about 1 mm in radius (Fig.2). This is much greater than the optimum radius, leading to lower than optimum density and fusion yield. Fusion yield average 0.12 J in the best five shots, four times better than with the last beryllium electrodes but still 30% less than the best five shots back in 2016.
Figure 2. This false color image of the plasmoid from shot 4, Sept.12, shows the brightest plasma in red, with some filamentation within the plasmoid visible. The purple objects at the bottom are the tips of the cathode vanes, which are black in the original image. The entire image is 3 cm across. The exposure time was 5 ns.
There are two reasons for this. First, the filaments are not yet strong enough to survive all the way to the end and compress into the plasmoid. Fortunately, with boron the magnetic forces on the nuclei will be stronger, due to boron’s five electrical charges, allowing the filaments to maintain themselves. Second, a tiny fragment of plastic accidentally got stuck inside the insulator. The volatile gases created as the plasma destroyed the plastic were still interfering with optimum pinching, creating multiple current sheaths that drained energy from the plasmoid. We expect that these remnant gases will be burned off in a few more shots.
We’ve now completed the deuterium shots and are very close to starting the boron shots. Dr. Hassan and Lerner have been tweaking the heating system and the filters needed to protect our pumps from any stray chemicals. We expect to burn boron very soon!
