The recently-completed set of experiments with LPPFusion’s FF-1 device, combined with discussions with colleagues at the International Center for Dense Magnetized Plasma (ICDMP) conference in Warsaw, have produced a greater understanding of the ways impurities are produced in our device and how to reduce them. LPPFusion’s research team has long identified heavy-metal impurities, produced by the erosion and vaporization of the metal electrodes, as the key obstacle to obtaining higher plasma densities and thus higher fusion energy yield. Experiments with tungsten electrodes have led to some reductions in impurities, but not enough. The new understanding of how the impurities are produced has helped to plan the next steps in getting rid of them.
When the measured voltage on the anode is plotted against the current supplied by the preionization power supply, even blowing nitrogen through the chamber (purging) can greatly change the plot. So can the very small current supplied by the trigger circuit. This indicates that dust particles are moving around, making it easier for currents to move from one electrode to the other.
Tungsten oxide is about 200 times easier to vaporize than pure tungsten, and we found from a literature search that tungsten nitride is not that much better than the oxide. We could measure the optical absorption of the deposits on our windows and from this determine that the total mass of impurity metal in the plasma had indeed declined by about 60% from our previous experiments with copper electrodes. This was progress, but not enough.
More insight came when LPPFusion’s Chief Scientist participated in the ICDMP conference in Warsaw in mid-October. This is a small conference of those working with the plasma focus device. One presentation by Czech researcher Monika Vilemova showed that when tungsten melts, tiny pores of gas burst open as bubbles, sending droplets of metal into the plasma to be vaporized. (All manufactured tungsten is porous and our electrodes are about 5% porous by volume.) This indicated that even for pure tungsten, erosion only requires heating to the melting point, not to the boiling point.
Other discussions pointed to a likely source of the electrodes’ second main region of erosion—the end of the anode. Previously, Lerner had blamed the anode’s heavy erosion on the electron beam that comes from the plasmoid. But when the anode was carefully measured during this disassembly, there was no evidence of the increase in erosion in the center of the anode that is expected from a beam. Perhaps, Lerner hypothesized, the erosion instead comes from high-energy particles leaking out of the plasmoid. But in Warsaw, Lerner saw anodes from the large PF-1000 plasma focus device that showed clear signs of erosion by the current filaments (dense vortices of current) prior to the formation of the plasmoid.
A leading expert in plasma spectroscopy at the conference, Dr. Hans Kunze, suggested that with heavy metal impurities like tungsten, recombination and line radiation might be very important because they increase proportionally to z4, where z is the number of charges on the ion. In the case of tungsten, z can be as high as 74, depending on how many electrons the ion has lost. Recombination radiation occurs when electrons that have been stripped from an ion get back together with it, emitting UV or x-ray photons in the process. Line radiation occurs when the electrons that are still bound to an ion drop down to lower energy levels (like lower stair-steps) after being excited to higher ones.
For deuterium, which has only one electron, such radiation is unimportant once the plasma in the device heats up and all the electrons are moving freely. But Lerner calculated that with even a bit of tungsten impurity, the recombination and line radiation can be enough to melt tungsten and cause the heavy erosion seen at the anode tips. Thus impurity breeds more impurity. The erosion occurs only at the tips, since the intensity of the radiation also depends strongly on plasma density, which increases at the anode tip as the plasma is compressed inwards toward the anode axis. Since the plasma filaments touch the whole top surface of the anode, this radiation hypothesis explains why there was no central concentration of the erosion damage. Finally, this hypothesis would directly explain how impurities limit plasma density, as the energy loss from the radiation would prevent the plasma sheet from reaching high density.
Next experiments with shorter tungsten cathode and anti-impurity procedures
The next experiments, now planned for late November, will use the tungsten cathode with a new shorter tungsten anode—which had been the original plan prior to the proposed, and now rejected, August shift. A shorter anode (10 cm instead of the present 14 cm) will be a first step towards increasing the current produced by FF-1, as it reduces the energy stored in the magnetic field. While a shorter electrode will also reduce the angular momentum—spin—that is needed for the tiny spinning plasmoid. We expect our axial field coil (AFC) to compensate for this. Its magnetic field will give the electrons moving toward the anode additional angular momentum.
To reduce impurities from tungsten compounds, we will take a number of steps, based on our previous experiments. We will bake the moisture out of the chamber at only 60 C, preventing the formation of oxides during bake-out, and carefully purge all valves of trapped water. After bake-out, we’ll use flowing hydrogen heated by microwaves to react away remaining oxides. (We could not use the microwaves during the previous experiment, as our windows were already so coated with metal that they reflected the microwaves.) Since nitrides may be another problem, as LPPFusion Research Physicist Syed Hassan suggested, in this experiment we’ll run only with pure deuterium. We had used a mix mainly to reduce damage from the electron beam, and our new insights indicate that the beam is not the main cause of anode tip erosion.
In addition, we will use our ultra-low current, corona-discharge preionization from the start. This should avoid the kind of electrical breakdowns that initially generated a rough anode surface and the dust problem. Finally, the shorter anodes will allow the use of somewhat more deuterium gas. Shorter electrodes need a slower speed for the current sheet to get to the end of the anode. For the same energy input, more gas can be pushed to this slower speed. In turn, more deuterium will dilute whatever tungsten impurities still exist. Overall, we hope to reduce the fraction of impurities in the plasma by about 5-10 times. This should lead to comparable increases in plasma density and fusion yield.
Discussions at the Warsaw conference have also led to ways to improve the performance of our instruments. We intend to put the ICCD camera, which has worked only intermittently, on battery power to completely isolate it from electromagnetic noise sources. We will move our photomultiplier tubes outside of the experimental room, connecting them via optical fiber to plastic scintillators inside the room. As well, we will improve our photographic monitoring of the symmetry of the breakdown process at the beginning of the pulse.
Beryllium Anodes Arrive, Cathode Due in February
Once LPP Fusion receive the beryllium cathode from Hardric Lab in February, we will use the 10-cm beryllium anode to assemble our first electrodes with no heavy metals, and thus no heavy metal impurities. With only four electric charges each, the beryllium ions will have almost no impact on the plasma, so these electrodes will allow a complete test of the basic hypothesis that impurities have limited fusion yield in the plasma focus device
They will probably be able to do the nitrogen-mix experiments using the beryllium electrodes.
Mixes of nitrogen and deuterium will allow us to study mixed ions before our crucial experiments with hydrogen-boron fuel later next year.
The second, 7-cm anode will be used in later experiments to further increase peak current and plasma density. With these electrodes in place we also expect to upgrade the connections between the switches and the electrodes for additional increases in current.
Collaboration with Poland
In the same trip to Poland in October, Lerner also visited the Institute of Nuclear Physics of the Polish Academy of Sciences in Krakow. There he discussed with Dr. Marek Scholz and other researchers the Institute’s plans to initiate their own research into hydrogen-boron fusion using a plasma focus device. This is only the second such effort, after LPPFusion’s own work. Dr. Scholz explained that they currently plan such experiments to begin as early as 2018, perhaps six months after the planned initiation of hydrogen-boron experiments at LPPFusion’s FF-1 facility. The Krakow experiments will use the PF-24 device, which is very similar in many ways to FF-1, having a similarly-sized capacitor bank and similarly small electrodes. While PF-24 is at the moment running with only about half of the current now produced by FF-1, it is capable at full power of producing mega-ampere currents.
Given the common goals of hydrogen-boron fusion with a plasma focus device and the similarity in the devices, Lerner and Dr. Scholz agreed to remain in close contact. Already the collaboration has provided benefits to both efforts. From the Krakow team, LPPFusion learned of better methods of shielding our instruments from electromagnetic noise, while Lerner was able to point to ways that PF-24 could improve the functioning of their switching system. “We expect both teams will be able to learn a lot more from each other in the coming months,” said Lerner. “We look forward to a growing collaboration with this and other labs in achieving aneutronic fusion.”