Observations Strengthen TheoryDecember 8, 2016
New observations over the past month have lent support to the hypotheses LPPFusion Chief Scientist Eric Lerner has put forward to explain the erosion of the tungsten electrodes in the last experiment. In turn, the strengthening of these hypotheses has concretized plans for the next experiment. First, at the ICDMP conference in Poland, Dr. Monika Vilemova of the Institute of Plasma Physics in Prague had offered to analyze deposits on the windows of the FF-1 vacuum chamber. The analysis was performed using energy dispersive x-ray spectroscopy (EDS), a technique in which an electron beam causes atoms in the sample to emit x-rays, which allows identification of the emitting atoms.
Fig. 1 Scanning electron micrographs of deposits eroded from FF-1 tungsten electrodes. Upper left loose debris from bottom window—scale 200 microns (8 thousandths of an inch). Upper right: deposits on bottom window—scale 10 microns. Lower center: deposits from upper trigger window near insulator—scale 2 microns. Much smaller scale of particles from upper region indicates a different erosion process than in the lower region.
The new data confirmed that most of the deposits coming from the upper erosion region, near the insulator, were indeed tungsten oxide, as Lerner had hypothesized. This is good news, as the oxygen needed to make tungsten oxide can be excluded in the next experiment. Tungsten itself is much harder to melt and vaporize than tungsten oxide. Interestingly, most of the dust deposited on the bottom window was also tungsten oxide, with the rest tungsten metal. This dust came from the lower erosion region, within the hole at the end of the anode, and consisted of dust particles 10-20 microns in diameter (see fig. 1). This indicated that the tungsten oxide layer on the surface of the anode must have been around 10 microns or more in thickness to form similarly-sized droplets. Even the much larger loose debris from the lower erosion region, blobs of metal 100 microns across, were still about 28% oxides, again evidence of a 10- or 20-micron-deep oxide layer, at least in this region.
The depth of the deposits is important in understanding why the oxides were not cleaned off the anode during the hundred-odd shots of the last experiment. The new measurements indicate that about 60 mg of oxygen was in the oxides near the anode tip. This is the majority of the total oxygen that we estimated was accidentally in the chamber at the start of the experiment. If this depth of oxide was also present in the upper erosion region, then it would be able to provide enough oxides, and thus explain all the impurities, in the shots in that experiment. There was a deep reservoir of oxide available for hundreds of shots.
Why was the oxygen concentrated in just the regions that were being eroded? It was no coincidence. After the initial shot of the experiment, these erosion regions were the hottest, and the rate of oxidation of tungsten in the presence of steam is sharply dependent on temperature. (Steam formed after the shot from the reaction of oxygen with the deuterium fill gas.) The hot upper and lower erosion regions had oxidation rates hundreds of thousands times higher than the rest of the electrode or vacuum chamber walls, which were much cooler. So in a few tenths of a second, nearly all the oxygen was converted into tungsten oxide in these regions. This confirms Lerner’s concern that oxygen must be absolutely minimized before the first shot is taken.
Other observations allowed the team to compare the erosion of the tungsten anodes with the erosion of the previous silver-coated copper anode. Using a simple technique first published 20 years ago, Lerner recorded the hissing sounds made when running a finger over the roughened surfaces of the eroded regions of both copper and tungsten anodes. The recording was then analyzed using standard audio software that plots the intensity of the sound vs the frequency. Since higher-pitched sounds are made by roughness at a finer scale, the plots can be transformed into graphs of how rough the surfaces are at a given scale.
Fig. 2 Audio frequency measurements have been converted into relative measures of roughness, comparing the ratio of roughness at various scales between the tungsten and silver-coated copper anodes. The graphs show the ratio of tungsten/copper roughness for the upper erosion region near the insulator (top graph) and the lower erosion region at the tip of the anode (bottom graph). The top graph shows a considerable reduction of large-scale roughness for the tungsten electrode. The bottom graph shows increased roughness at all scales of the tungsten electrode, especially at around 20 microns. The big dip between the two central peaks is an artifact of the audio technique, produced because the anode has natural resonances at nearby frequencies. This is similar to the way a trombonist excites the natural resonances of the trombone by the vibration of his lips.
The measurements showed that erosion was substantially reduced in the upper region near the insulator, compared to the silver-coated copper, despite the fact that tungsten oxide is far easier to vaporize than silver (see Fig.2). This is again good news and indicated that the energy available in this region is too small to melt pure tungsten, so erosion can be greatly reduced or stopped if the oxygen is eliminated. The pre-ionization that reduced the energy available for erosion thus seems to be working. The much finer-scale erosion from this region is probably due to the low-temperature vaporization of the tungsten oxide, heated by its own resistance to the electric current.
At the same time, the erosion was much greater, producing larger-scale roughness, in the lower erosion region at the anode tip, confirming the 10-20-micron-scale erosion observed in the droplet sizes shown in Fig.1. This observation is evidence for the hypothesis that the anode tip erosion is due to recombination radiation from the hot plasma before the pinch. The energy, and thus the penetrating power, of the x-ray photons produced by this radiation is higher for tungsten than for silver. The tungsten-produced radiation penetrates about 6 microns, a reasonable depth for producing erosion pits that are 20 microns wide. The silver-produced radiation penetrates only 2 microns, creating erosion pits that are only about 4 microns wide. While recombination radiation will still be a problem in the next experiment, it will be eliminated as a concern in the following experiments with beryllium electrodes. The large scale of the erosion also rules out an earlier hypothesis that erosion was mainly due to particles from the plasmoid. These penetrate only 0.5 microns, so could not produce the large erosion pits and particles observed.