The long-awaited tungsten cathode is finally nearing completion in a Chinese factory. According to the latest reports by California-based Tungsten Heavy Powder(THP), which is performing the work in China, machining will be complete within a few weeks and delivery to LPPFusion should occur by mid-February. The 99.95% pure tungsten piece has been in the machining process for nearly two months. Because pure tungsten is as brittle as ceramic and liable to cracking, the part must be removed from machining frequently and heat-treated to relieve internal stress.
“We know that it seems the vital electrode is always two months in the future, sort of like fusion but with a shorter time line,” comments LPPFusion President and Chief Scientist Eric J. Lerner, “but this time we know the final stages of machining are actually underway.” (see Figure 1)
The long wait for the tungsten cathode comes from the inherent technical difficulty of the part, which is pressing close to the limits of present day tungsten technology. Since tungsten melts at a higher temperature than any other material, it can’t be poured and cast. Instead tungsten powder is pressed or sintered together at high pressure and temperatures to form a solid piece. This process makes it difficult to form thick pieces and the monolith tungsten cathode is over 15 cm tall. No factory outside of China is now equipped to make such a thick pure-tungsten piece.
This technical difficulty has led to long delays. While LPPFusion decided to make the monolithic tungsten cathode in May 2013, the initial supplier, also in China, fell through when it increased the estimated cost abruptly ten-fold to over $140,000. When no other supplier was willing to complete the part, LPPFusion contracted with THP to produce a cone-shaped blank and with a NJ-based company to machine the final shape, a process that was supposed to be completed by June, 2014. Unfortunately the NJ company first decided that a third firm was needed for initial machining of the part (see Cathode Gets More Machining) and then backed out of the deal entirely, citing too much technical risk.
Finally THP agreed to make the part start-to-finish and, after getting corrections on an aluminum model (see Tungsten Anode Goes Into FF-1; Aluminum Cathode Model Is Checked), began machining a new blank at the beginning of November. The entire machining process will take about two months.
Given the extreme difficult of making the tungsten cathode, why does LPPFusion not go directly to the final beryllium electrodes? The answer is that tungsten, with its extreme resistance to heat, is the lowest risk material for the next step of our experiment. We need to eliminate evaporation of the electrode and the resulting impurities to get a jump in the density of our plasmoid, and in the resulting fusion energy output or yield. We have firm experimental evidence that tungsten does not erode under the condition FF-1 is currently running. While we have developed and published well-founded theories of how pre-ionization can stop erosion of much less heat-resistant materials like beryllium, we still need to test those theories experimentally. That we can do safely with tungsten, with risking damage to the electrodes or disappointing fusion results. In this way we will confirm or refine the theories and technique, paving the way for the beryllium electrodes.
Those beryllium electrodes will be ready when we need them. We expect to order them in January, with delivery in the first half of 2015.