Switch Redesign—the 45 Degree Solution

Starting at the end of April, the LPPFusion team began testing our switches with the ceramic insulators replaced by Teflon ones. We had previously determined that Teflon is better at resisting the surface breakdown that had prevented the proper functioning of our switches. In surface breakdown, electric current travels from one conductor to another along the surface of an insulator.

We rapidly learned that the good news was that surface flashover within the sparkplug that fires the switches did indeed decrease and the switches became as hard to fire as we expected. The bad news was that when the switch gas mix was reduced enough to fire the switches (reducing the insulating SF6 gas relative to the argon) the switches prefired before charging to 40kV was complete. Visual inspection made clear that this was still due to surface breakdown, even with the Teflon parts.

Fortunately we were able to swiftly learn the cause and probable cure of this continued surface breakdown (also called “flashover”) . Since the beginning of the pandemic, our research efforts have been hampered by our lack of access to the resources of a major university library. For a long time, we had used Princeton University Library, which allowed aces to powerful technical search engines like INPSEC and thousands of technical journals to anyone physically present on campus. But with the pandemic, Princeton locked down and , to date, has not allowed non-University individuals back on campus. No other local university provided access.

But in May we hired a student intern, Alexandra Calabro, who is entering her sophomore year as a Mechanical Engineering student at University of Connecticut. Not only did Ms. Calabro have access to the University of Connecticut’s library resources, but she is a very efficient library researcher, with a great interest in all things fusion. She rapidly tracked down papers that showed a critical improvement was needed to stop surface breakdowns.

The papers from several research groups showed that a key parameter in surface breakdown is the triple-point angle—the angle between the insulator and the conductor at the point where the insulator, conductor and gas meet. If this angle is small, electrons released from the conductor can impact the insulator, breaking off a few electrons that travel back to the conductor in an amplification cycle that can lead to breakdown—the ionization of the gas and a large current flow (see Fig.1). But if the angle is large, more than 90 degrees measured through the gas, electrons moving from the conductor won’t bump into the insulator. The ideal angle turns out to be 135 degrees measured through the gas, or 45 degrees as measured thorough the insulator.

June20switchestop | lpp fusion
June20switchesbottom | lpp fusion

Figure 1. (Top) In the wrong angle between the conductor (red) and insulator (grey) electrons can bounce back and forth, amplifying a tiny number into a surface flashover current. (Bottom) In the correct angle between the conductor(red) and the insulator (light grey) no such multiplication can occur. In both diagrams the conductor is assumed to be negatively charged( cathode) and the field would be in the upward direction toward the positive anode. 

We have had to redesign the switch parts to achieve this triple point angle. We’ve taken the opportunity to remedy mechanical weakness in the parts as well. We expect our redesign to be complete by the end of June and new tests to start in early August.

To avoid having to rebuild and fire all 16 switches, Research Scientist Dr. Syed Hassan is preparing a test stand that will allow us to test just two switches, firing into an electrical dump, without affecting our plasma focus electrodes at all. This will greatly speed up testing the new switch design and arriving at the best conditions for firing the switches.

While waiting for the new parts, the LPPFusion research team is continuing preparation for remote operation during pB11 experiments planned for later this year and is writing up for publication our new plasma purity record.  

 

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