Filaments Line Up, but Anode Cracks, Delaying Boron

As we prepare for our hydrogen-boron tests, we’ve made progress in ending an obstacle to higher fusion yield–the disruption of our plasma filaments. The filaments—dense threads of electric current, magnetic fields and plasma (current-carrying gas)—are the first step in increasing the density and temperature in our FF-2B experimental fusion device. Fusion requires both high density and temperature. For a long time, we’d been fighting the problem that the filaments formed at the beginning of our microseconds-long pulse, got disrupted before they are compressed into the dense plasmoid, where the fusion reactions take place. Disrupted or disorganized filaments lead to less symmetric compression, less density in the plasmoid and thus less fusion.

Filaments 1 | lpp fusion

Image of filaments from our experiments in 2019(upper left), May of this year (Upper right), and October of this year(bottom) show decrease in disruption.

We saw this process in our images of the filaments back in 2019 (top left image). In this image, taken with our ultra-fast ICCD camera, the plasma is moving down the inner electrode (the anode) which here is down and to the right. In the rear part of the sheath (to the left) the filaments , the bright vertical threads, are neatly arranged almost in parallel. But in front of the bright horizontal line, the filaments are a mess—disrupted into a tangle. It was this part of the sheath, unfortunately, that was compressed into the plasmoid.

We figured out that a big part of the problem was a relatively slow rise in current in the very first nanoseconds (billionths of a second) of the pulse. Too low current did not create the strong magnetic fields needed to produce the neat, organized filaments. We thought that our new switches, installed this spring, would speed the initial turn-on.

In May, we got the switches working and they did indeed speed up the turn-on. But the filament disruption was not substantially improved (image at upper right from May 4, 2023).

We identified some key problems in our initial conditions and fixed them, leading to our higher current and fusion yield reported in October of this year. Our recent analysis of the ICCD images (lower image from October 20, 2023) shows that we have indeed made big progress in preventing the filament disruption. In this image, we can see that the filaments remain nearly parallel to each other right up to the front of the sheath, where it counts. The horizontal line of disruption has essentially disappeared.  This was a big factor in increasing density and fusion yield.

But we still have a way to go. A glance at the image shows that the filaments this year are a lot fatter than those in 2019. That is not good, because the thinner the filaments, the smaller and denser the plasmoid that they twist into. So fat filaments limit fusion yield. We have some good ideas on how to fix this as soon as we get back to firing shots, such as fine-tuning the gas mix to speed the breakdown further.

Unfortunately, that will not be for a month or so, as we discovered that our anode had cracked and has to be replaced. We saw that a large leak occurred after our shot on Nov.20, but we could not identify the source of the leak for some time. With the help of some new equipment, we finally tracked it down and found that the source was a major crack in the anode, which had broken the vacuum seal at the top of the anode.

Fortunately, the crack clearly comes from a different source than the last one, back in 2020. The crack’s starting point far from the tip makes clear that it was due to stresses created by the electron beam that goes down the central hole in the anode, while in 2020 the crack came from the filaments of current at the tip. We think we can easily eliminate this failure mode in new anodes and we will be ordering two new ones—to have a spare—in early January.  This anode lasted three hundred shots compared with only one hundred for the first one, and we feel confident the next ones will last well over a thousand shots.

We don’t expect to get the new anodes before March. However, in the meantime we are going to do some shots with our old tungsten electrodes. While they produce a lot more impurities than the beryllium ones, we think some quick tests will give us data that will speed our work once we get the beryllium anodes back.

During December, LPPFusion’s Hassan completed the upgrades in the FF-2B device needed to use hydrogen-boron fuel. He completed and successfully tested a new heating system, required to keep the decaborane compound in a gaseous state. In addition, he completed the new fuel-handling system needed to feed the fuel into our vacuum chamber in a controlled way.

So, once we complete the tests with the new beryllium anode and get the conditions needed to run with the boron fuel, our machine will be ready for that fuel. The cracked anode does mean a delay in getting to hydrogen-boron, but not a long one.

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