Firing all switches repeatedly and reliably

Posted by Lerner on Oct 13, 2010 at 10:09 PM
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Repeatable firing on 8 capacitors

On September 29, we finally achieved repeatable firing of all attached capacitors.  That day, and on September 30 and October 1, we fired all eight capacitors that were attached in 11 successive triggered shots, with only three pre-fires in between.  This ability to fire all switches together and reliably gives us at last the ability to know exactly what current the machine will produce in each shot and therefore to optimize other conditions.

Shifting Trigger Effects

While last month we thought that we had the switch problem under control, having achieved the firing of all attached capacitors on a one-time basis, in fact we had to change track.  Our basic conclusion, that we needed a higher trigger voltage, was correct, but our method of achieving that—by increasing the voltage supplied to the trigger generator—did not work well.  First, the higher charging voltage, 30 kV instead of 20 kV, caused shorting in the trigger heads, destroying three more.  So we were forced to run with only eight capacitors attached.  Second, the trigger pins were still shorting out too rapidly to the adjacent electrodes, limiting the trigger voltage, which actually sets off the spark that fires the switch, to too low a value to do a reliable job.

Spark plug insulator effects

At the same time, the new Lexan insulator on the switch spark plugs continued to break.  Dr. Subramanian came up with a new insulator design that might increase their longevity, but required a larger hole in the electrode that the insulator and the trigger pin pass through.  Previously, we had been reluctant to increase the size of the electrode holes, since this might increase the difficulty the current had in jumping from the trigger pin to the electrode, once the trigger had fired.  However, to test the new design, we had a single new electrode made up. 

Mr. Lerner reasoned that, even with the old spark plug insulators, the greater distance from the pin to the electrode should hinder the shorting of the trigger and thus allow higher trigger voltages.  Testing the new electrode with the old sparkplugs proved that this reasoning was valid.  The trigger voltage leaped from 22 kV to 36 kV. So we immediately had all the electrodes modified to have large holes.  This then lead to our success in firing all eight attached capacitors within 20 ns of each other, and doing that 11 times in a row at 35 kV on the capacitors.  Even with this partial bank, we achieved over 1 MA current.

“Ruggedization”

We are now rebuilding to a new and more rugged design the four missing trigger heads, a task that should be completed in October.  We are confident that then we will be able to fire with all 12 capacitors together, reliably.  However, we do still have more work to do on the switches.  First, we need to test the new insulator design to see if we can increase the lifetime of their isolators beyond a few hundred shots.  Second, we have to move up in charging voltage to 40 kV and then to 45 kV without pre-firing, which may involve moving the electrodes further apart.