LPPFusion’s lab team has completed the mechanical repair and reinforcement of the tungsten cathode. The first and most difficult step was to apply the fiber-epoxy composite around the broken rim of the cathode. (See April LPP Focus fusion Report for a description of how this damage occurred.) The purpose of applying the compo-site was to provide strong inward pressure on the cathode to close up micro-cracks that could impede the flow of current during FF-1 shots. The fiber, a thick nylon thread, was stretched to provide the inward force, while the epoxy adhesive fixed the fiber in place. The problem was the irregular broken surface that we were repair-ing produced forces that pushed the thread up or down as we were winding it around the rim. In a painstaking task, LPPFusion CIO Ivy Karamtisos guided the thread during many hours of winding to maximize the number of windings and to prevent the fiber form slipping off. (see video)
To maintain a constant tension but to avoid breaking the thread, LPPFusion Chief Scientist Eric Lerner monitored the tension with a torque meter (a mechanical device that measures the torque or twisting force on an axel or spool). We checked the torque meter by monitoring how much the fiber was stretching and by noting when the pull was enough to overcome the friction in the turntable that the cathode was resting on. Since we had to let the epoxy dry for a day between each layer of fiber, this critical step was quite time consuming.
As a result of this effort, we were able to stretch the fiber by an average of 18% in length so that with 34 windings round the cathode, in seven layers of fiber, we generated 350 psi of inward pressure. The micro cracks visibly closed up and 80% of the length of cracks ceased to be a significant obstacle to the current—something that we checked with a micro-ohmmeter, an instrument that can measure extremely small resistance to electric current.
The second step was to attach with more epoxy a steel brace to the outside of the cathode. The brace will serve to attach the cathode to a steel plate that will carry current from the rest of the machine. The brace replaces the function of the broken tungsten rim. With the help of our new temporary Research Associate, Mark Kalpheke ( see note below), we carefully machined and ground the brace to closely fit the surface of the fiber-epoxy composite. A closer fit minimizes outward tension produced as the epoxy shrinks and dries which would counter the inward tension generated by the fibers. (see Figure. 2)
The next step will be to apply indium to bridge over the remaining 3 cm of micro-cracks, thus providing a safe path for the current in this region. Then the brace and cathode will be bolted onto the steel connection plate. At that point we will be able to proceed to the final reassembly of FF-1 and continue our important set of experiments.
