New Insights Help Plan Next ExperimentsOctober 31, 2016
In all scientific research, experiments and observations give rise to new theoretical concepts, which in turn lead to the design of more advanced experiments. LPPFusion has gone through such a cycle in the past few months, with new insights revising our plans for the next experiments. In August, the expected availability of beryllium anodes in September (see next section) led Lerner to propose that the next experiments use a combination of beryllium anode and tungsten cathode. The idea was to test the expected great reduction in impurities from beryllium without waiting for the beryllium cathode, not expected until February. However, cogent reasons led to the rejection of this plan.
First, LPPFusion Chief Information Officer Ivy Karamitsos raised the concern that the vaporization from the tungsten cathode might contaminate the beryllium anode, making it difficult to use for pure beryllium experiments later on. This seemed a strong objection, especially as cleaning the beryllium anode is made difficult by the toxicity of beryllium dust. In addition, literature searches by LPPFusion volunteer Charles Loney turned up experiments that showed that contamination of tungsten by beryllium produced an alloy that had a lower melting point than either pure material—another strong reason to avoid mixing the two metals in one experiment. This is a major concern for existing and planned tokamaks which do use both metals.
Instead, 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.