New switch design, after tweaking, fired successfully all the way up to 45 kV
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, described in earlier reports. Initial tests in late April allowed for a series of improvements in the new design. First, testing at lower switch-gas pressure made it clear that better protection against flashover (shorting along the insulator) within the switch was needed. Protective Mylar and higher gas pressures were added, but the most decorative innovation was polishing the plates to a 2-micron mirror finish. (A finer finish smoothes away peaks in the electric field that can lead to flashover.)
Next, the research team found that the two-part plastic stabilizer that surrounds the insulator and provides pressure to seal an O-ring did not seal as well as the earlier design and allowed an air path for electricity to short between the top and bottom brass pieces. This flashover vaporized a small amount of the plastic, causing a small explosion.
A re-design added Mylar disks and insulating tape to prevent breakdown, as well as sealing the stabilizer with vacuum grease. Thanks to a suggestion by LPP’s engineer Fred Van Roessel, we were able to test the spark plug with the high-voltage power supply of the trigger without subjecting it to the full power of the capacitor. So breakdown occurred with a little spark that we could see rather than with a loud bang. This allowed us to fine-tune the Mylar protection and produce a spark plug that would not flash over up to 45 kV.
We 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.
The new switches will be a considerable expense, but we will be saving some money by using copper electrodes instead of much more costly tungsten-rhenium ones. Dr. Subramanian suggested the change, and we have tested the copper electrodes both in the old switches and the new one. Although the electrodes still melt somewhat in the new ruggedized design, the copper shows no more wear than the tungsten and costs a fraction as much.
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.