Reply To: Boron availability

[asymmetric_implosion]

andrewmdodson wrote: My understanding is that the switch only has to close extremely rapidly… the capacitor bank discharges completely into the reactor, expending functionally all the current and then the system resets for the next pulse…

Why does there need to be reverse conducting? Are you using the same electrical pathway to recharge the cap bank?! I thought that no power had yet been extracted from the DPF?

The switch needs to close rapidly but it doesn’t have to be 1 ns or so. The bank discharges completely but the PF is an underdamped ringing circuit. As a result some current flows back into the cap bank. Some solid state switches can’t handle reverse conduction from the ring. I know SCR type switches turn off when the voltage and current reverse. So what happens to the current that is trying to flow back into the bank? You have created an opening switch device. Opening switch devices are well known for pulse compression applications that need to generate high voltage in a fast pulse. The PF circuit is a charged inductor without an outlet when the switch opens after a forward going pulse. Inductors like to keep current moving so they generate voltage at the expense of stored magnetic energy. A fast closing switch can more than double the voltage left in the PF. The SRL group had the same problem so they implemented a massive collection of fast, high voltage diodes to channel the reverse current safely away from the switches to ground. Most gas switches don’t care about the direction the current flows so they happily ring until the resistance in the circuit converts the energy to heat or the system voltage drops below the sustaining voltage (<1 kV for most gas switches).

In an ideal PF, the reverse current would be small but it doesn’t always work out that way. In a good pinch att 0.25 MA I still see a negative ring of ~70 kA. That is a small fraction of the forward pulse but it could easily take out solid state components if the voltage rings up.

This assumes the voltage at pinch time doesn’t take out the solid state system. When a plasma focus ‘pinches’ the plasma to achieve fusion conditions, the impedance in the circuit increases dramatically. I can’t speak for FoFu-1 specifically, but a 1 MA machine typically generates ~250 kV at pinch time. The voltage is divided back to the bank by series impedance relationships. If the switches are a moderate to large impedance in the circuit which isn’t a bad assumption, they will see a substantial voltage drop. Suddenly you have a 40 kV circuit with a positive voltage discharging to near zero and a 250 kV spike appears at the the pinch. About 50% of the voltage will appear across the switches. Can a 40 kV hold off switch in forward bias deal with a 125 kV voltage spike in reverse bias? I don’t know for sure but I have my doubts.

Don’t hate the solid state guys, hate the universe. I’m pretty sure the solid state guys would love for the rules to be more favorable. 🙂