The redesigned dual switches are now working on LPPFusion FF-2B experimental fusion device. As our test-bed experiments had indicated, the 16 redesigned switches are firing together and providing more electrical current than any earlier designs. With initial switch testing complete, our next step is optimizing the conditions for much higher fusion yield.
The new switches provide more current mainly because there are two switches for each capacitor, rather than one switch, as in the design used prior to 2021. Doubling the number of switches reduces the amount of energy tied up in the magnetic field and thus allows more current to flow. The redesigned switches correct design errors that were made with the initial dual switches in 2021.
Painstaking assembly of the 16 switches took all of March. While the parts of the switches were all machine-made, assembly was by hand and had to be kept to tolerances of about 25 microns (one thousandth of an inch). This was not easy, but was achieved mainly by LPPFusion Research Scientist Dr. Syed Hassan. Inevitably a few assembly errors (some by Chief Scientist Eric Lerner) were uncovered in initial switch testing during April. However, by the end of the month, good switch functioning was achieved, with all 16 switches firing and with almost all firing within 20 ns of each other.
The first big goal of the switch effort was to achieve higher current. We compared the current produced by the dual switches with control shots taken in February with the old single switches, using identical conditions with the vacuum chamber. As seen in Fig. 1, the peak current increased by almost 25% with the new dual switches, almost exactly the design goal.
This increase is quite significant as the optimal amount of gas in the vacuum chamber increase roughly as the square of the peak current. This denser gas is then compressed during the operation of the plasm focus, leading to higher plasma densities in the tiny plasmoid where fusion reactions take place. Higher density in turn leads to higher fusion yield. Preliminary experiments have already confirmed that optimal fill pressure has at least doubled with the new switches, although we have not yet begun to optimize conditions to achieve higher fusion yield.
Figure 1. The dual switches (blue line) are producing almost 25% more current than the older single switches (orange line) with similar conditions in the vacuum chamber.
A second major goal was to reduce the oscillations in the current supplied by the switches. These oscillations were likely a main contributor to the disruptions of filaments in the current sheath of FF-2B. This disruption in turn reduced density in the plasmoid and thus fusion yield. The oscillations have in fact decreased to the lowest levels yet seen in our experiments (fig.2). In particular, the dip in current that occurs early in the pulse has almost entirely disappeared.
Figure 2. Oscillations in the current early in the pulse have dramatically decreased with the new, redesigned dual switches (blue) compared with the earlier version (orange) with the dips in current almost disappearing. Oscillations also decrease more quickly than in the old single switches(grey). The zero current is displaced from the axis for clarity.
Closely connected with the reduction in oscillations is the goal of turning on the current quicker. This is measured by the time to achieve a maximum rate of increase of current. Again, the turn-on time has improved considerably compared with the earlier dual switch design. Finally, we eliminated entirely the negative pulse of current at the start of the much large positive current pulse.
Our immediate next steps, to be taken in May, are to optimize the functioning of the device at the much higher fill pressure. This higher pressure alters how the gas in the chamber ionizes during the first nanosecond of the pulse. (Ionization is the process by which electrons are stripped from atoms, allowing the gas to carry current.) This ionization has to be extremely symmetrical if the sheath is going to compress to high densities. We can adjust the preionization current and added nitrogen to optimize “breakdown”—the time of rapid ionization. If we can at the same time achieve a quicker, as well as more symmetrical breakdown, we may be able to further reduce the oscillations, entirely eliminating the small dip in current that remains. We may also be able to further speed up the turn-on time.
Once breakdown is optimized, we can the adjust the amount of angular momentum fed into the sheath by our axial field coil, which changes the magnetic field at the start of the pulse.
We expect that this optimization process will lead to yields much higher than those achieved previously with either our current beryllium or the earlier tungsten electrodes. Specifically, we anticipate that we will be able in this process to prevent the disruption of the filaments that interfere with high fusion yields. We have not yet been able to observe the filaments, as our 14-year old ultrafast ICCD camera is reaching the end of its life and is not functioning reliably. (We need more money to replace it!) But we expect to get some good images during May.