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Old 04-24-2013, 03:52 AM #1
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Lightbulb Laser Induced Breakdown of Air - math help requested

I was doing some searching to see if I could find the intensity threshold for laser induced air plasma formation and I found a neat document that looks to be of military origin detailing air breakdown as a means for communication channel formation amongst other things.

Here is a link to the file: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA133211

What interests me most is that it seems to confirm some info I found earlier about the breakdown threshold for a 10.6um CO2 laser being in the ballpark of 10^8 W/cm^2. That is 100 million watts per square centimeter.

Quote:
Chan et al^20 using a CO2 laser with a pulse length of 160 nsec obtained
breakdown threshold powers in air at 1000 Torr which vary from 5.5 x 10^9 W/cm^2
to 1.3 x 10^9 Watt/cm^2 .
Now, we know that a 10.6um wavelength can only theoretically be focused down to 10.6um, with this we should be able to determine the minimum power of CO2 laser needed to accomplish air ionization assuming perfect optics. From there we can derate the optics and make an educated guess as to what would be realistically achievable.

I know we have had some members achieve this with Q switched Nd:YAG lasers with high pulse powers, but I've yet to see anything with a CO2 unit. AFAIK CO2 lasers do not gain any increased output from short pulse lengths at higher input power. At least, I seem to remember reading something on REPAIRFAQ about that. Correct me if I am wrong, please.

It seems that dirty (particulate ridden) air is easier to break down as well:
Quote:
For A = 10.6um and particle diameters^18 of 2.2um the dirty air
breakdown is reduced to 2 x 10^8 W/cm^2 compared to the case of clean air of
2 x 10^10 W/cm^2 .
So, math is my weakness, and doing complex unit conversion makes my head hurt. If we assume a 100um^2 best possible focused spot size, given the reported intensity needed of 10^8 W/cm^2 and 5.5 x 10^9 W/cm^2, what CO2 laser beam output power would be needed? I would also like to use 500um^2 spot size as a worst case scenario, I'm pretty sure a half mm^2 is achievable with the cheap ebay optics.


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Last edited by Sigurthr; 04-24-2013 at 03:57 AM.
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