Lawrenceville Plasma Physics are working towards commercial nuclear fusion using dense plasma focus fusion. They have their December, 2010 report. They have had problems with switches that has cost them a few months of time to resolve. They have repeatably fired the bank with 10 capacitors attached, giving reliable shots above 1 MA at 33-34 kV. They believe that they now clearly understand the previous pre-firing and insulator-breakage problems. This will enable them to continue gathering data and testing their theories while they prepare a thorough ruggedization of the switches that will enable them in a few months to reach full power with all 12 switches at 45 kV, and push on to their goal of 2 MA current. They are implementing a redesign which will take until March to complete because delays to get some components. So in April, 2011 they will be able to push ahead with 12 switches at 45 kV.
LPP has completed 3.5 of their 8 milestones. They are a year behind the original schedule, which can mostly be blamed on the switch problems, but they are making good progress. In 2011 their plan is to finish the trigger electrode revision, which will allow them to achieve 45kV and 2MA, optimize the gas pressure for that configuration with deuterium, switch to helium and nitrogen while continuing to optimize, then switch to hydrogen and boron. At that point they hope/expect to see a shot that generates 33,000 Joules of fusion energy, and all of their milestones will be complete. Unless they run in to more unexpected problems it is realistic that they might finish that by the end of the year.
We ran into a number of problems that had to be resolved in turn. Perhaps most importantly, we learned from our experiments that the pre-firing inevitably gets worse with more capacitors.
This is because the pre-firing is caused by a slow breakdown of the gas related to a phenomenon called “corona discharge”. With more capacitors attached, the power supply’s fixed output charges the bank more slowly, so the switches stay longer at high voltage. Because of this problem, the present configuration of the switches will not work with all 12 capacitors attached. Ten capacitors is our current maximum.
However we know the cure for this. We’ll move the electrodes in the switches further apart (see the attached diagram for further explanation). We will be doing this in our general redesign over the next few months. In addition, if the tungsten trigger rods get worn down so they are too thin or too rough, the field gets too concentrated, and this also leads to pre-firing. Temporarily, we have replaced the thinnest rods and are sanding the others carefully on a regular basis. In our re-design, we will use much thicker rods—probably one-quarter-inch diameter instead of one-eighth inch.
We also had trouble eliminating breakage. While our large stabilizer block prevented any cracking of the insulator above the plate, the rapid movement of the tungsten rods was still breaking the Lexan insulators near the tip. After several tries, we have used a solid cylinder insulator to provide maximum strength. So far, they have lasted 40 shots with only two cracking, so this is adequate for now, although too soon to tell their real lifetime. Again, we know the solution to the mechanical breakage: making the rods thicker so they bend less, and making the insulators thicker so they are stronger. These changes require replacing the top plates of all the switches and making the spark plug holes larger. This hole size has limited our past efforts with the spark plugs.
Finally, to prevent the electrical break-down of the insulators, we also have to make them thicker. All of this can be calculated, based on the experiments we have done, and we hope to complete design work very soon, probably early in January. However, there are considerable ordering delays on some items, such as the tungsten rods, and some additional testing will be needed, so realistically we will not complete the new switches until March. Until then, we will be running with the existing 10–capacitor bank.
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