Problem generating spark under water for cavitation bubbles

[Beekeeper]

TL;DR Summary

I cannot consistently generate a spark under water using a High Voltage Generator coil. Can you help explain why?

Hi,

I am trying to do some experiments with cavitation bubbles by generating a spark under water however I am having some trouble and was wondering if anyone would be able to help or offer some advice.

The problem I am having is I cannot generate the spark consistently. I place the electrodes in (distilled) water, apply the high voltage across them, they will usually spark a few times and then stop and fail to spark again after that.

The setup I am using is described in the following (attached) diagram:

The electrodes are just copper enameled wire with a gap < 1mm. I am using a small "High Voltage Generator Pulse Generator Coil" (goes by other names see this link) of around 1000kV to generate the high voltage across the electrodes. I am controlling a "High Voltage Pulse Generator Coil" with a trigger switch and an Arduino to precisely set the time for the spark. The electrodes themselves are placed in distilled water rather than tap water to avoid potential electrolysis.

Things I have tried:
- different spark durations from 50ms to 2000ms most have similar results
- different thickness of electrode wire from 0.1 to 0.85mm
- multiple strands of electrode wire 3 x 0.25mm and 2 x 0.63mm
- different quality distilled water (currently settled on high grade conductivity <= 1uS/cm)
- different coils, I also tried a 400kV coil with similar results

Although I have not read or seen anyone generate a spark with this "High Voltage Pulse Generator Coil" I thought it would be possible based on this paper which generated a spark using a small piezo electric igniter. But even so I am not sure if using the "High Voltage Pulse Generator Coil" is the problem so curious to hear thoughts on that.

I could try another circuit or setup, but was hoping to first get some advice and try to understand the problem first.

Thanks for taking the time to read my post.

 

[.Scott]

I would guess that after the first few sparks, your "distilled water" will be clouded by copper salts from your electrode. I would also guess that those copper salts are shorting your voltage.

You'll likely have this same problem with almost any electrode material. But I would try carbon rods.

 

[Beekeeper]

Ok thanks! Sounds plauseable and thanks for the suggestion to use carbon rods instead. I will give that a try.

 

[sophiecentaur]

I would try carbon rods.

Or platinum tips?

 

[sophiecentaur]

TL;DR Summary: I cannot consistently generate a spark under water using a High Voltage Generator coil. Can you help explain why?

Although I have not read or seen anyone generate a spark with this "High Voltage Pulse Generator Coil" I

Is this a joke or a spoof? I looked at the spec on Amazon and it seems to claim hundreds of kV and 5A output (nearly a Megawatt!!). It's reasonable to suppose this gizmo can make sparks for gas cooker ignition etc but that little box with thin wires coming out of it really doesn't look the part. Look at the care with which an old CRT in a colour tv manages to cope with very high volts.

Have you any way of measuring what your box actually produces? Good fun if you are careful of secondary accidents when the spark hits you and makes you jump.

 

[.Scott]

Teflon insulating film has a breakdown voltage of 60 to 173 Mv/m. So 700Kv would likely damage that insulation. But the product comes with warnings that it will be damaged if the maximum air gap is exceeded.

The specified maximum air gap is 1.5cm which corresponds to 45Kv. So, you're right, that's a very theoretical 700Kv.

As far as the megawatt, it's only a megawatt during the pulse - and they do not specify the pulse width.

I also noticed that they show 9 views of this device. And 9 out of 9 are side views. We never get to see what either end looks like. A good look at the high voltage end would have provided more clues about the practical maximum output voltage.

 

[Rive]

That thing is a high voltage coil driven by a pulse generator. The output of the coil is charging a capacitor through some diodes. When the voltage goes high enough and breaks down the insulation a spark is generated, discharging the capacitor. Then, all over again, few dozen times a second or more.
Weaker insulation means more frequent sparks at lower voltage.
If there is a constant load it may counter the charging pulses and prevent sparks to happen.
So actually the change in the water is a good explanation.
You may try to introduce some currents to the pool to disperse the salts.

different spark durations from 50ms to 2000ms

What you set there is the number of sparks in a batch. If you want single spark wit adjustable duration/energy then you need an entirely different setup.