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4hv.org :: Forums :: High Voltage
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Minimum resistance of plasma in the air

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Dr. Dark Current
Wed Nov 05 2008, 05:54PM Print
Dr. Dark Current Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Just wondering...
Lets say you have a spark gap of x cm in length and you connect an ideal voltage supply of y kilovolts across it. Now you ionize the gap (with flame, uv, etc.)

How much current is going to flow? Is there any limit?



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Dr. Slack
Wed Nov 05 2008, 07:40PM
Dr. Slack Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
There are two limits, the capacity of your power supply, and the one imposed by the physics of the fundamnental particles in the gap, which no-one if ever going to reach in a terrestrial experiment. The Sandia Z pinch dumps one million volts into a wire-primed gap from a one ohm transmission line, and 1 million amps flow. If you can source it, it will take it.
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...
Wed Nov 05 2008, 08:18PM
... Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Just be careful that the initial ionization gets the plasma hot enough that an appreciable amount of current will flow from your source. I have never tried, but I believe that a candle flame would have several megohms/mm of resistance so you would need a very high voltage to get any initial current flowing through the to heat things up and get the self-feeding effect going.

Keep in mind that for a plasma, the resistance is inversely proportional the to temperature. This is also called 'negative resistance' because the more current flowing the hotter it gets thus the resistance drops with increasing current. This lends itself to a self feeding effect where if you use a non-current limited supply after you get enough current flowing to overcome the heat lost in the gap due to conduction to the surrounding air the plasma will just keep getting hotter, which causes it to draw more current, making it hotter still, etc.
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Dr. Dark Current
Wed Nov 05 2008, 08:56PM
Dr. Dark Current Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
... wrote ...

Just be careful that the initial ionization gets the plasma hot enough that an appreciable amount of current will flow from your source. I have never tried, but I believe that a candle flame would have several megohms/mm of resistance so you would need a very high voltage to get any initial current flowing through the to heat things up and get the self-feeding effect going.

Keep in mind that for a plasma, the resistance is inversely proportional the to temperature. This is also called 'negative resistance' because the more current flowing the hotter it gets thus the resistance drops with increasing current. This lends itself to a self feeding effect where if you use a non-current limited supply after you get enough current flowing to overcome the heat lost in the gap due to conduction to the surrounding air the plasma will just keep getting hotter, which causes it to draw more current, making it hotter still, etc.

I know, but the resistance can never drop down to 0 ohms, so I was wondering how low it would get... probably very low.
I have ignited some arcs with candles easily, but this is not the main problem here smile

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...
Wed Nov 05 2008, 10:55PM
... Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
The resistance it will ultimately fall down to is governed by how much power your source and couple into the plasma, and the size of the electrodes (mainly the amount of power you can couple into the plasma). In short its limited by how much current your source can provide.
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Marko
Thu Nov 06 2008, 12:14PM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Dr. Kilovolt wrote ...

Just wondering...
Lets say you have a spark gap of x cm in length and you connect an ideal voltage supply of y kilovolts across it. Now you ionize the gap (with flame, uv, etc.)

How much current is going to flow? Is there any limit?

Not sure what would be the answer for your hypothetical question, but, resistance of plasma can get very low, lower than any known conductor providing enough temperature and pressure.

In JET Link2 plasma may drop only 1-2V for currents of 2MA.

The only limit is the amount of available power to keep the plasma hot. In ordinary arcs, it is pressurized by atmospheric pressure, inertial confinement and magnetic confinement.

Thus regarding your ideal voltage source, I can't really comment it. Would it destroy earth, or maybe the universe? I don't know, but any real source is surely going to be limited in power and energy output not to do so.
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Dr. Slack
Thu Nov 06 2008, 02:03PM
Dr. Slack Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
What all these posts are saying is that "resistance" is not a particularly useful concept for an arc, if you are using it to try to estimate how much current will flow. A spark gap is greedy, it will take whatever current you throw at it. You might as well ask what the resistance of a capacitor bank is when it is being charged. The load will have a voltage across it, and a current through it. Mathematically, these two quantities will have a ratio, and you can call that ratio a resistance, but it only tends to be useful when it's reasonably constant, or well defined by the material it's made of. So you will buy a 10ohm resistor, or your copper winding will change from 1.0 to 1.2ohms when it gets hot, and you can make useful predictions about current flow and voltage drop. But if you want to charge a cap bank, you supply it with a constant current source, and see how the voltage varies over time. The ratio of voltage and current will vary, the current accoring to the supply, the voltage according to the state of chage. If you want to drive an arc, you get the gap ionised, and then see what current it chooses to pull from your supply. The ratio of voltage and current will vary according to the temperature of the gap, which varies according to the dissipation. If you have a 10kV DC source through a 1 megohm resistor, it may sit there at 2kV across the gap with 8mA quietly hissing through it. If the source impedance is somewhat lower at your capacitor bank ESR + ESL, then life will be rather more dramatic. The rapid increase in temperature is what makes the bang.
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