Underwater Arcs: Different Switches

The current was destroying the mechanical relay I was using, but a friend gave me some SKT 55/06D Thyristors. After a small trigger charge (~V, ~100mA) thyristors act as diodes, so they can be used as switches which stay open until the current through them stops and they reset. These are rated up to 700V, which gives roughly half the stored energy that the capacitor is capable of. The steady state current limit is only ~100 but the surge current is rated to 1300A, so they should be fine for this application. I’ve been concerned about the difference in shot quality, especially with wasted energy by the arcing on the relay, so I thought I would compare the two.

To measure current in the main line, I made a Rogowski coil. Changing currents induce a voltage in the coil, which can be measured in the usual manner. I tried calibrating the readings to the output of a signal generator, which gave a reading of 1.5E-6V/AHz. Unfortunately I was only able to calibrate it at higher frequencies, greater than 1MHz, as otherwise the signal was too small with the test current I could produce. The output voltage should correspond to dI/dt, but in this case it also had a phase lag of ~pi/2 behind that. Also the output was nonlinear in frequency, increasing faster than linearly above 5MHz. Obviously, this could be done better, and there are proper ways to compensate for the inductance of the coil and so on. There is also an amusing tradeoff, which is that without too much more efficiency, the coil would reach voltages beyond what is safe to probe with my oscilloscope, while with the efficiency currently, the voltages are low enough it is hard to test.

Below are three measurements of voltage on the coil and on the main line, two for the relay and one for the thyristor. The thyristor has much more consistent shots.

Notice difference in time scales
These all have pretty similar decay constants, implying the power dissipated by the switch is similar.

In all cases, which the swtich closes there is a momentary current increase seen on the Rogowski coil (hard to see). Then comes a ~0.5 ms period of roughly constant voltage, before the saltwater breaks down. I assume that the water in the gap heats up during this period. After this, the current spikes as resistance of the spark gap drops to around zero, and the resistance of the system is set by the 1 Ohm current limiting resistor. Finally, at around 150V the current is extinguished.

In terms of diagnostics, it would be nice to get a peizoelectric pressure sensor, but I am also concerned about difficulties due to frequency response with something like that.

The color also appears to change depending on the shot power, so I believe it would be interesting to look at spectra. I got a diffraction grating but haven’t taken any data.

I have realized that a better way to deposit energy into the water would be to limit the current with an inductor instead of a resistor, and hook up my other thyristor across the capactitor reverse to the starting polarization. This would allow the current to flow in the loop longer, dissipated only by the spark gap and the voltage drop across the thyristors. I’m not sure this would be better for pressure waves/shock wave generation, as the initial breakdown might be the main contributor for that. The speed of sound in water is ~1km/s, so during the ms discharge of the capacitor sound has had the time to travel a full meter, which is much larger than the system. The time for the initial breakdown is 1us, giving 1mm, about the size of the spark gap. This is very rough logic-it may be that the slower timescale is fine if enough power is delivered.

I also aquired a real ignitron…

Author: Garth Whelan

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