Re: Quarter shrinker electrodes
Posted by Bert on 8/17/2018, 10:53 pm, in reply to "Re: Quarter shrinker electrodes"
Apparently I wasn't clear about my question. I was referring to the two electrodes which make up the firing switch. I see that you originally used a trigatron but then switched to a pure mechanical switch with two large brass electrodes. How is the erosion rate?
Oops - sorry for the misunderstanding.
Shrinking a large variety of coin sizes requires using a variety of cap bank energies (and voltages). The electrode spacing on the earlier trigatron switch was quite touchy - sometimes it wouldn't switch at lower voltages while self-triggering at higher voltages. And it would need to be torn down for cleaning and maintenance after 10-20 shots.
I was also looking at further increasing the voltage range and increasing the switch reliability, so I decided to use the same 2" diameter brass electrodes previously used in the trigatron with a long-throw solenoid. The result was a much more robust and reliable switch that has never once misfired. Although some tin evaporates from the brass during high-current switching, the cumulative electrode wear is minimal. Under a microscope, the electrodes look like a desert lakebed with small cracks where the tin has evaporated, and islands of remelted copper.
My existing solenoid switch has been used for over 4,000 shots with no maintenance and no misfires. A BIG difference from the old trigatron switch. In "Gas Discharge Closing Switches" (by Burkes, Schaefer, Kristiansen, and Guenther), the authors claim that brass provides consistently good results that are about the same, or better, than tungsten, copper, or tungsten-copper alloys for high-energy high-coulomb spark gap switches. The results we have seen seem to confirm that brass works extremely well. And, its inexpensive, readily available, and easy to machine - what's not to like?
I would recommend setting the switch so that it still has a small gap with fully closed by the solenoid to prevent contact welding. And enclosing it to reduce the incredible noise it makes when firing.