While LPPFusion’s research team expects to eliminate the major sources of electrode erosion, enough to get rid of significant impurities in the plasma, some erosion will still exist. It won’t be enough to bother us during the current experimental phase, but once we are engineering a generator that fires 200 times second, remaining erosion will limit the lifetime of the electrodes. But there may be a way to protect the electrodes better—with a coating of carbon nanotubes.
Carbon nanotubes (CNT) are a remarkable form of ordinary carbon, in which the atoms are arranged into atomic-scale tubes. They have very high strength and can conduct heat and electricity well. They have been widely studied since the early 1990’s.
Neil Farbstein of Vulvox Nanobiotechnology Corporation suggested to LPPFusion joint development of a coating of CNT to protect the future beryllium electrodes in the Focus Fusion generator. While more research is needed, the extraordinary qualities of CNTs may help to reduce two sources of erosion. The first is sputtering. In this process, high energy ions from the plasma hit against atoms in the electrode, knocking them out of the material one by one. Beryllium is only slowly eroded by sputtering, but CNTs may be still better. Due to their structure, with sheets of atoms surrounding tiny voids, CNTs can allow high energy ions to slow down gradually, dissipating their energy without knocking off so many atoms.
Second, between shots, a layer of boron may form on the electrodes after the molecules of the decaborane feed gas break apart. If the boron condenses fast enough to form a continuous layer, the current from the next shot will have to vaporize it off. (Boron does not conduct electricity at all well.) Since the vaporization temperature of boron is much higher than that of beryllium, some of the beryllium electrode will also vaporize, causing erosion.
CNTs could help this problem as they have high thermal stability. They don’t melt even at the melting point of tungsten, 3400 C. Whether they remain stable as high as boron’s boiling point of 3927 C not yet known. But if that is true, boron could evaporate from CNTs without damaging them.
LPPFusion and Vulvox Nanobiotechnology Corporation will be seeking government funding to investigate further CNT coatings of beryllium electrodes.
