Sigurthr
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- Dec 11, 2011
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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.
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:
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.
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.
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:
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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