The LPPFusion research team has now obtained all the new parts needed for our revised small switches and has begun assembly of the switches. We’ve also completed the planned control tests, using the old larger switches. We are now expecting the first tests of the new switches during March.
The tests of the old switches succeeded in getting them to fire in good synchrony, as they had in 2016-17 when they were used with the tungsten electrodes. However, the switches functioning was notably different than it was seven years ago. The switches took twice as long to initiate the maximum rate of rise of the current as they did before . The turn-on time increased from 80 ns to nearly 160 ns. In addition, they now delivered only 30 kV, not the full 40 kV, to the electrodes. This slow turn-on resulted in a broader current sheath and larger, lower density pinches, with only a quarter the maximum fusion yield in 2023 as compared with 2016.
By a process of elimination, Lerner concluded that the primary reason for the old switches’ deterioration was the contamination of the switches with adhesives which he had mistakenly used in them in 2019. References on the use of sulfur hexafluoride, the gas used in the switches, warned that adhesives could degrade over time by exposure to the moisture in the atmosphere. This created compounds that could be vaporized and broken apart by SF6 breakdown products during the switch firing. Such a disassociation process robs energy from the switch currents, slowing the process of turning on and reducing delivered voltage.
Although the control shots could not provide a one-to-one comparison between the tungsten and beryllium electrodes, they did emphasize the need for avoiding all contamination in the new switches. The team is taking good precautions to avoid this and it should not be a problem going forward.
In other tests, the team obtained images of what the breakdown process looks like with the current state of the beryllium electrodes (see fig.2 ) The images, obtained by only firing the weak trigger pulse, not the full capacitor bank, showed good symmetry, indicating that the electrodes remain in a good shape and are ready for the shots with the new switches.
Fig. An image of the electrodes during a discharge of the weak trigger pulse alone shows the good symmetry of the breakdown. It is important to achieve good symmetry during the pinch and high density. Similar results will be expected with the full capacitor-bank current firing. The off-center look of the image is caused by the off-axis location of the viewing window at the bottom of the vacuum chamber. The violet plasma from the breakdown currents start from the cathode vanes on the outside and go to the central anode on the inside. The ring of light near the anode outlines the lower edge of the insulator.