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Frequency help needed. Answered

Does anyone know what frequencies of laser light can effectively and efficiently ionize normal air into conductive plasma?

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Masterdude
Masterdude

Reply 11 years ago

Same principle but I want to use it for a different purpose.

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Jack A Lopez
Jack A Lopez

Reply 11 years ago

I've got some more links for you:
http://panoptesv.com/SciFi/Ionization.html
http://www.patentstorm.us/patents/5675103/description.html

The first link is sort of a simplified explanation of the story of photo-ionization of air.  As explained in this link,  the sort of threshold, minimum amount of energy required to wrench a single electron off from an air molecule is approximately:

9.9e-19 joules = 6.2 eV

A single photon carrying this much energy would have a frequency of around 1e15 hertz, and wavelength of around 200 nanometers (200e-9 m = 2e-7 m)

The confusing thing is it seems like I've read other places that the ionization energy was more like 12 or 13 eV.  So that quote of 200 nm might be for a 2-step ionization process.  Maybe the author of that first link is confused. ( I know I sure as hell am!)

As I was saying, the work done in removing the electron doesn't have to be done all in one step (with one photon).  There are 2-photon, 3-photon, and even n-photon processes.  The trade-off being that you need squared, cubed, or n-powered, increases in intensity to make these happen.

The second link, which I found in the first one, is a patent for some sort of "stun-gun" based on this air-ionizing magick, and it goes into a lot more detail, even telling you what kind of lasers they're using, plus the pulse widths (in time), peak intensity, etc.

In all these myriad equations, the one parameter that really stands out for me is the amount of light intensity, or light power density, in watts per square meter (W/m2) .  The smallest number I saw was 5e10 W/m2.  Compare this to the intensity of say sunlight, which is about 1e3 W/m2.  So the peak intensity of their beam is like 50 million times brighter than the sun!   

In the patent it says the way they're doing this is by using very short, very intense pulses, specifically 50 millijoules of energy in a 10 nanosecond pulse for a peak power of 5 megawatts.   (All this with an aperature area of 1 cm2. )

Anyway, my point is that this looks tricky.

But don't let me discourage you.  If you do succeed in building something like this in your garage, please take lots of pictures, and write an Instructable about how to do it.


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kelseymh
kelseymh

Reply 11 years ago

Good writeup!  You made one comment, "The confusing thing is it seems like I've read other places that the ionization energy was more like 12 or 13 eV."  The ionization energy for atomic hydrogen is 13.6 eV.  Larger atoms (and molecules) have lower single-ionization potentials because of screening effects.

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Jack A Lopez
Jack A Lopez

Reply 11 years ago

Yeah.  13.6 eV is the ionization energy for the good ol' hydrogen atom.  That's probably what I was thinking of.  Anyway, I am glad to hear that the 6.2 eV sounds like a reasonable number for singly-ionizing a single air, N2 or O2, molecule.  Thanks for the comment.

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Masterdude
Masterdude

Reply 11 years ago

Thanks for the help. I doubt I will be able to use the information for anything (at least not now). Thanks for the links. They have reinforced the information that I got from different sources:
www.hsvti.com/
ionatron.com/

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guyfrom7up
guyfrom7up

11 years ago

uhhh, you mean turn air into plasma via only light?

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lemonie
lemonie

11 years ago

You can't do this. Someone else with a lot of money and resources perhaps.

L