# Laser to ionize air to run current, laser power per foot?

Hey guys I know that a laser can ionize air so that you could run current through air.

I was wondering if anyone could tell me just how powerful (watts) the laser would have to be If I wanted to connect 2 point that are about 100 feet apart? 2 points have very low current but do have high voltage, maybe 1000+volts

mfb
Mentor
I'm not sure if that is a practical idea at all, and I really hope this is a hypothetical question, but even then it will depend on the planned resistance and details of the air in between. Also, where is the point? Cables are so much more convenient.

The air would be normal outside air so humidity and how clean the air is would very hourly. I am hoping to get a general idea of about how powerful this laser would have to be. I don't know if we are talking about a military 10,000watt laser or something that could be build in my garage... And theoretical invention I have in my head has to use ionized air to conduct the current, thats one aspect of it that I would not like to change. The power of the laser is going to tell me if this idea is feasible

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davenn
Gold Member
2021 Award
this sounds incredibly dangerous for some one with little experience

Dave

Staff Emeritus
2021 Award
It's not feasible. To bridge a 100 foot gap, you're talking about what are essentially lightning bolts.

berkeman
Mentor
No, no, it's quite feasible. @supak111 -- please rent the movie "Real Genius". That will show you how it can be done. Just be careful while you place the target in front of the cinder blocks.

What current are we talking about?
There are different ways to ionize air. e.g. fire, microwaves, radioactivity, high voltage, lasers, UV light
You can use any of these methods to send a small current through the air. If the distance is large it will still work but the ions that leave at one end will not be the same ones that get received at the other. In that case the air just acts as a ground, i.e. ions would be moving between the emitter and the ground below it at both ends.
It would make this easier if we knew what exactly you want to use this for.
If you just want to transmit power, you could use a bright lamp and focus its light on some solar cells. Or use a pair of parabolic antennas and send strong radio waves.

A sufficiently energetic laser pulse can spark at a focal point, but I doubt this is being done in any "backyard" setting. Ionizing an entire 100 ft gap would require a lot more wattage than anything yet produced. There would be solid grounds to claim it is impossible.

This isn't dangerous at all, Im talking about very very small current high voltage. It wouldn't be any different then touching a taser or getting shocked by a cars spark plug coil... Hurts but not dangerous.

And yes the lighting ionizes the air thats how the current gets flowing but in a lighting there is a LOT of current and even more volts. I know there have been experiment where lasers were used to induce lighting, they would shine a laser onto a cloud and which instantly induced lighting to flow down the lasers beam/ionized air, I just can't find any info on how powerful this laser was which is what am hoping someone here has some idea on.

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ZapperZ
Staff Emeritus
There are a lot of things being overlooked here in this scheme.

1. It isn't just how powerful a laser is that is important, but also the energy density per unit volume. I have a 5 mJ laser that works fine in air, but when I focus it to a point the size of less than a mm, it causes the air to spark, i.e. it generated a plasma. So already here we have a situation of a laser that doesn't cause any ionization in air suddenly causing an ionization when it is focused. So it is not just power, but energy density!

2. Have you considered the "wall-plug efficiency" of the entire process? How much power can you generate from your current, considering that you have to (i) create the laser light and (ii) have the laser ionize the gas? The last process is actually quite inefficient, i.e. not every single photon will cause an ionization. In fact, I'd be surprised if you can have a 50% efficiency there. So your "current generator" here can easily be extremely inefficient when compared to just using all that power in a generator.

3. How are you expecting to generate a current in air that can be transported over a considerable distance? The mean-free-paths of both electrons and ions in air are extremely short (look it up). I don't see how a sustainable current can be maintained.

And this is all BEFORE we talk about the complexities of plasma dynamics.

Zz.

What current are we talking about?
There are different ways to ionize air. e.g. fire, microwaves, radioactivity, high voltage, lasers, UV light
You can use any of these methods to send a small current through the air. If the distance is large it will still work but the ions that leave at one end will not be the same ones that get received at the other. In that case the air just acts as a ground, i.e. ions would be moving between the emitter and the ground below it at both ends.
It would make this easier if we knew what exactly you want to use this for.
If you just want to transmit power, you could use a bright lamp and focus its light on some solar cells. Or use a pair of parabolic antennas and send strong radio waves.

I didn't realize microwaves can ionize air, that method would probably work for my purpose. Only thing is I would need to focus the microwave into a skinny beam so that the 100ft gap can be covered

Hey guys I know that a laser can ionize air so that you could run current through air.

I was wondering if anyone could tell me just how powerful (watts) the laser would have to be If I wanted to connect 2 point that are about 100 feet apart? 2 points have very low current but do have high voltage, maybe 1000+volts

Maybe 1000 000 V and very high peak power laser to break a 100 feet long air gap at normal pressure...

You still haven't answered any of my questions. There probably is a much easier way to achieve what you are trying to do but without some more information it's impossible to figure this out. Could you at least tell us how small exactly the current is supposed to be?

If you really need to send a stream of ionized air across a gap you could try to create an ion wind. http://en.wikipedia.org/wiki/Ion_wind
This video here seems to be showing a device that does just that.

It blows electrically charged air in one direction. If that was built big enough it might just be able to send a current over 100 feet.
You can even buy a tower fan with a built in air ionizer which will also send a (very small) current though the air.

The problem with this however is that a fan can't blow wind over a large distance.
You could send charged air much further with a vortex cannon like in this video, if instead of smoke you fill the "cannon" with ionized charged air.

Actually the voltage would probably be closer to 10+kv, and current I'm not positive on but it probably won't be more then 2 or 3ma. Your air cannon idea is very good, I wonder if I built a mini one thats only maybe 1/2" wide and made it automatic to shoot rapid fire if this would do the trick. It would actually be cool since it would not use much power to do the work. I wonder what would be the best way to ionize a lager amount of air... They have these tiny air ionizers on eBay but they are small so probably not fast enough.

Actually the voltage would probably be closer to 10+kv, and current I'm not positive on but it probably won't be more then 2 or 3ma.
If you think about conduction current of this magnitude through ionized channel 100 ft long it won't work. Not at sea level air pressure with 10 kV. Even with channel currents stronger by several orders of magnitude, the self-sustained discharge required plasma gradients are like 1000 V/m.

blue_leaf77
Homework Helper
You are talking about photoionization There are a couple of mechanisms governing photoionziation:
1. Multiphoton ionization, where atoms absorb multiple photons whose total energy exceeds the ionization energy of the ionized atoms. This kind of photoionization takes place in the intensity region of ## 10^{13} ## to ## 10^{15} W/cm^2##.
2. Tunnel ionization which takes place at intensities above ## 10^{15} W/cm^2##, we are in strong-field regime.
Note that both ionization methods demand high intensity, not high power. So you have got to either focus the beam tightly or have ultrashort pulse.
Indeed there have been the so-called self-guided laser beam, this effect is used in LIDAR application, where the light pulse is short enough to induce nonlinear effect on air molecules. This nonlinear effect causes self-focusing where the focusing spot can provide a high intensity-exposed volume to ionize atoms and forms free electrons. Plasma dispersion from free electrons leads to beam defocusing (diverging lens). This chain of self-focusing and self-defocusing repeats itself along the beam propagation. However, note that even if the beam manages to produce plasma at some point, the plasma itself causes the beam to diverge preventing adjacent volume in space to get ionized. This will form alternating pattern between plasma and neutral molecules distribution along the beam propagation, and electricity cannot easily break trhough between successive plasma lumps.

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berkeman
Mentor

@supak111 -- check for a personal conversation from me...

davenn
berkeman
Mentor
OP says that the project is proprietary, so he cannot release any more details. Thread will remain closed as a dangerous discussion, given the level of experience of the OP.