LPPFusion’s research team has completed the control experiments with the tungsten electrodes and is moving swiftly to the next set of experiments with beryllium electrodes and hydrogen-boron fuel, now planned for September. The July test runs succeeded in optimizing the preionization circuit, allowing the doubling of fusion yield at higher fill gas pressure. As reported earlier, the team has been trying to resolve the problems created by the more difficult breakdown– the transition to plasma where electrons flow freely–at these high pressures. With deuterium, the higher pressures are needed to create higher densities in the plasmoid and thus higher fusion yields.
The tiny preionization currents, while only microamperes in strength, create a zone of ionization around the insulator that in turn surrounds the anode at the heart of the FF-2B fusion device. In this zone, currents can start to flow faster than in the outer gap between the anode and the cathode vanes (see Fig 1) because some electrons are already free to move, having been stripped from the neutral atoms as part of the preionization current. By making breakdown of deuterium into ions and electrons faster even with higher fill gas pressure, this zone also makes the breakdown more symmetrical, leading to a more symmetrical current sheath. This sheath in turn compresses to a denser plasmoid with more fusion reactions.
The ionization zone helps solve another problem that is caused by high pressure. Higher pressure makes it easier for current to jump directly across from the cathode vanes to the anode (the straight horizontal red arrow). Such a radial breakdown is undesirable, as it creates a separate current sheath that runs ahead of the main one, again disrupting a symmetrical, tight compression. Preionization counters this problem by smoothing the path along the insulator, making it easier for the desirable vertical breakdown to take up all the current, preventing radial breakdown.
Figure 1 A small preionization current (blue dots) makes it easier for the correct breakdown current to flow along the insulator (diagonal red arrow) rather than radially from the cathode vanes (horizontal red arrow). This improves symmetry and thus fusion yield.
On July 14, LPPFusion Chief Scientist Eric Lerner tested the new preionization circuit with a pulse from our trigger circuit. The trigger circuit generates 30 kA (30,000 amps) of current—far less than the nearly 2 MA (2 million amps) of current of the main bank, but enough to test the breakdown conditions created by the preionization current, which runs continuously for several seconds before the trigger circuit fires. We can see how symmetrical the kilo-amp breakdown is, indicating how symmetrical the megampere breakdown will be.
Almost perfect symmetry was achieved in the breakdown with 41 torr pressure (viewed in Fig. 2 A from below and a bit off-axis). This is a great improvement from the breakdown without preionization (Fig. 2 B) where the current starts at only a few bright spots.
Figure 2. Almost perfect symmetry is observed with a trigger shot and intermediate preionization current (2A), much better than with no preionization (2B) or with high preionization, breaking down to the roughest vane (2C).
However, when Lerner increased the preionization current much further, into the milliamp range, he found by July 25 that complete symmetry had not been achieved. With the higher preionization current, there was radial breakdown (Fig 2C), always to the same specific cathode vane. This indicated that the visible roughness on the tungsten cathodes was still making radial breakdown easier. One vane had a rougher surface than others, concentrating the electric field like tiny lightning rods and leading to the persistent asymmetry. Fortunately, this is a problem we won’t encounter with the beryllium cathode vanes in the next experiment set, as those vanes have remained completely smooth after over 400 shots. The reduced erosion was one big reason that we switched from tungsten electrodes to beryllium ones in 2019.
Using the intermediate level of preionization that produced the highest symmetry with the trigger shots, Lerner and Research Scientist Syed Hassan fired the full bank’s 2 MA on August 1. They were able, on the best shot, to double the fusion yield from earlier high-pressure shots in June to 0.15 J of energy with 27 torr pressure. But this was still only 60% of the record yield achieved with less current back in 2016. The effect of the persistent asymmetry was evident in these shots as the neutron pulse, which should show one peak, in this and other shots showed two peaks (Fig.3).
Figure 3. The neutron pulse emitted on shot 3, August 1, 2024 shows a double peak, with a smaller one 100 ns ahead of the larger one. This indicates two current sheaths arriving to form two plasmoids, greatly reducing the plasma density and fusion yield. All the shots in this series had similar pulse shapes.
Despite the remaining asymmetry, the tests demonstrated that preionization could clearly improve results at high pressure, one key aim of the control shots with the tungsten electrodes .
We are now preparing for important testing with pB11 with beryllium electrodes. We will report on more important results from last month’s tests in the next report, coming next week.
