Progress in Switching – all capacitors firing, but more reliability needed

[From Eric Lerner’s LPP report of February 23 - sorry for the posting delay]

We have continued to make progress with the switches. 

This month we succeeded for the first time in getting all 12 capacitors to fire during the pulse, firing at 24 kV.  However, we cannot yet do this reliably and repeatably. 

At the same time, we have also created conditions in which most of the bank, 7-9 switches, will fire without pre-firing, allowing us to perform the experiments described in the first section. 

What we still need to do is to get repeatable and full-bank firing together.

The re-profiling done by the switch manufacturer, R.E. Beverly, did not resolve the pre-firing problem.  Neither did our attempt to go to higher charging voltages, up to 35 kV, which the literature shows should make such switches fire better.  We ran into significant problems at the higher voltages (see below).

We determined that part of the problem was that, in most spark gap designs, the trigger is located between the two electrodes, not within one electrode as with this design.  The existing design meant that it was easier for current from the trigger to short out to the adjacent electrode, rather than going across the gap to trigger the capacitor.  We modified the switches to allow the trigger head to stick out a small amount into the gap. In addition, we carefully measured and adjusted the trigger heads and top plates to reduce the variation in the spark gap (the distance between the trigger and the opposing electrode) from almost 10% to about 5%.

Together, these two steps greatly reduced pre-firing and allowed us to get one shot with all 12 switches firing.  However, the gas pressure in the switches that allowed all capacitors to fire also allowed too much pre-firing.  So we had to increase the pressure, which allowed reliable firing of 7-9 switches, giving us the 0.5-0.6 MA used in the angular momentum experiments.

We intend to further adjust and measure the bottom plates of the switches to reduce gap variability to about 2.5%, hopefully getting full bank firing on a reliable basis.