LPP’s long-awaited new switch design was successfully tested in mid-May, after some improvements to the design. The switch consisted of solid copper plates, with a larger spark gap (to hold off higher voltages) and a much more robust spark plug
They tested the new switch three times at 40 kV and three times at 45 kV, getting no flashovers and getting perfect, on-time firing in one case at each voltage. While we may need some additional fine tuning, we are confident enough in this design to order the remaining 11 switches (plus some spares). To monitor all twelve switches and LPP’s instrument suite, we purchased two additional oscilloscopes which are already in operation, bringing the total number of available oscilloscope channels to 24.
To test the switch at high voltage, above the 35 kV that the existing switches pre-fire at, we had to fire with only one capacitor. But to make a realistic test, we wanted to charge all 12 capacitors, so that the bank would charge slowly and the switch would be remain at a high voltage for a longer time, as when we will fire the whole bank. We realized that firing one capacitor—an asymmetric current—would slightly tilt the main steel plate, possibly breaking the main hat insulator, but we thought that by limiting the test to six shots, we should avoid a breakage.
We did avoid a breakage of the hat insulator, but unfortunately on the last shot of the series, there was a breakdown of the Mylar sheets that keep electric current from shorting between the top and bottom plates of the device. Four separate breakdowns occurred close to the switches. The resulting small explosion damaged 6 of 8 aluminum plates, which will have to be replaced. LPP lead scientist Eric Lerner strongly suspects based on the evidence, that the tilting of the plate from firing one capacitor also tilted the aluminum plates, allowing air to get between the Mylar sheets. This could have caused a breakdown, as electrons can accelerate in air gaps between the sheets. If this is the case, such breakdowns can be avoided just by symmetrical operation of the device. We will be further examining this and other explanations as we complete our look at the breakdown.
We don’t think repair of the damaged aluminum and Mylar will significantly slow our schedule as we will also be waiting for the new switches. The re-assembly needed will also give us an opportunity to improve the Mylar design, especially the places where the Mylar is near the switches, so as to ensure reliable operation at 45 kV, even if pinch voltage matches or exceeds our expectation of 120 kV.
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