The new tungsten monolithic cathode, key to LPPFusion’s next set of experiments, arrived at our laboratory in Middlesex, NJ on Feb. 27. Careful acceptance tests confirmed that it is 99.95% pure tungsten and is within the 150 micron tolerances that we specified, a significant accomplishment by the manufacturer Tungsten Heavy Powder and their colleagues in Beijing where the tungsten factory is located. To manipulate the 35-kg cathode, LPPF’s research team erected a gantry, which they used to lift the tungsten cathode, protected by a steel “hat”, out of its box.
An independent laboratory report, which arrived almost simultaneously with the cathode, showed that the tungsten failed under a tension of only 4,000 psi (pounds per square inch), far less than the anticipated tensile strength of around 40,000 psi. The test was based on testing samples cut from a tungsten cathode blank produced in the same way as the tungsten cathode. The reason for the low strength is that the height of the tungsten cathode made it impossible for the manufacturer to fully compress the powder into a seamless piece of metal. The density of the cathode is 18.3 gm/cc, while a fully compressed tungsten part will have a density of 19.3 gm/cc. LPPFusion was aware of this difference in density, but did not expect it would result in such a low tensile strength.
The tensile strength —the strength for slow, long acting forces—has a considerable effect on how the tungsten cathode is to be attached to the steel plate that hooks it into the circuit and to the vacuum chamber that hangs beneath it. Thus the original plan for using the thin tungsten rim to attach the cathode to the steel plate was no longer workable as the rim was simply too weak to withstand the stress applied once the assembly was completed. Chief Scientist Lerner made an error in not concluding from the laboratory tests that the assembly would fail, an error not caught by other team members. So after the cathode was attached to the steel plate in an assembly, a gradual fracture of the supporting tungsten rim took place overnight, starting with localized weak points on the rim.
However, the inner electrical current contact was on the innermost, unbroken part of the rim and remained functional. But cracks were identified that, if allowed to grow, could compromise that vital function.
Mechanical Engineer Tony Ellis succeeded in quickly proposing a replacement attachment method. In place of the tungsten rim, he designed a steel brace fit over the outer flange of the cathode to attach it to the steel plate. A high strength resin will fill the small space between the brace and the cathode flange. In this way, the force needed to press the tungsten and steel pieces together will be spread over the 1-inch-thick tungsten flange, not the 1/4-inch-thick rim. To stabilize the edge of the tungsten bottom of the cathode, and to prevent any further micro cracks, Rudy Frisch, an engineer and member of the LPPFusion Advisory Board, designed a fiber reinforced composite whose nylon fibers will be wound around the tungsten to compress it, while epoxy resin will be used to stabilize the fibers.
To minimize future errors when experimenting with the newly designed FF-1 components, the team has added new quality assurance procedures. In particular, a “checker” will be designated at all design development points and during assembly, to question decisions about device assembly before they are implemented.
The good news is, the low tensile strength will not affect the cathode’s ability to stand up to the shocks it receives when FF-1 fires.
Beryllium electrode have been ordered
LPPFusion is also preparing for the final set of electrodes, to be made from beryllium. The tungsten electrodes, with their extreme resistance to heat, will allow the tests needed to pave the way for the use of beryllium, which is a much more resilient material and much better suited to sustain the high x-ray fluxes expected in the later stages of the experimental program.
LPPFusion has now succeeded, after many months of effort, in negotiating agreements with three companies to supply and machine the beryllium electrodes we need. Rev Manufacturing of Palmdale California will machine two anodes, one 10 cm long and one 7 cm long and Hardric Laboratories of North Chelmsford, Mass. will machine a single cathode, 10 cm tall which can be used with both anodes. The cylindrical beryllium billets for the parts will be supplied by Ulba Metallurgical Plant, a state-owned company in Kazakhstan. The total cost of the parts will be about $135,000, although this may be somewhat reduced with the final design of the cathode.
The strength of the 97.8 % pure beryllium, 44,000 psi, is guaranteed by the manufacturer and will be confirmed by independent tests. The design considerations inspired by our recent experience with tungsten did however lead to a much cheaper way to attach the beryllium to the steel, saving about $12,000.
Since beryllium is export controlled, permission from the US Department of Commerce will be needed to import this material, and has been applied for. Assuming that permissions will not be long delayed, the beryllium electrodes should be available in the fall of this yea
SOURCE- LPP Fusion