Ionization of Air with a Laser

[mecheng2011]

Hi guys,

I am currently doing a project which involves firing a class 4 laser into a small enclosure (200mmx200mm) and onto a target, which is to emit IR. I am trying to theoretically prove that there will be no ionization of the air (i.e. turn the air into plasma) inside the enclosure.

So far, I have an incredibily unreliable source and little else. From my searches I have found little information, but it may be because I am unsure of what exactly I am looking for (I am from a Mechanical Engineering background). The source is this: http://www.panoptesv.com/SciFi/LaserDeathRay/Ionization.html

Does anyone know if this information is valid/credible? If so, I could really do with any links to any kind of publication (book, journal, etc) from which I can reference properly, I don't really know where to find this kind of information.

Many thanks for help you can provide.

Alex

 

[nbo10]

I don't know about death rays but lasers can be focused with enough energy to break down air. Look into Laser-induced breakdown spectroscopy.

 

[Creator]

Hi guys,

I am currently doing a project which involves firing a class 4 laser into a small enclosure (200mmx200mm) and onto a target, which is to emit IR. I am trying to theoretically prove that there will be no ionization of the air (i.e. turn the air into plasma) inside the enclosure.


Alex

Hi Alex;

There are certain parameters and conditions necessary for laser ionization to occur.

So I hope you don't mind me asking:
1.Why are you trying to "prove" that ionization CANNOT occur?
2.Are you trying to "prove" that it cannot occur only under your particular conditions?
3.What is the laser frequency?

...

 

[mecheng2011]

Thanks for your help.

1.The reason is that there will be a camera looking inside the tool, and if there is ionisation this will flare the camera image.

2.Yes, I am only trying to prove that under my conditions it will not occur. If it will occur, then I will have to design the enclosure to be purged.

3.The laser details are:

Emission wavelength: 1064nm
Output Energy per Pulse: 160mJ
Pulse Duration: 17ns
Pulse Frequency: 1-20Hz (variable)

As this is for a unviersity project, I am pretty sure that it will not occur, but I want to be able to show I have considered that it could occur and can show references for any calculations I have used.

Thanks again

Alex

 

[ZapperZ]

Will the laser beam come to a focus in the enclosure?

I have a 10 mJ/pulse laser at 250 nm, with pulse length of 10 ns. It will only cause air ionization when it comes to a focus (which is why that section of the beam path is in vacuum). So here, the energy "density" in a unit volume will be a factor.

Are you able to test this out first?

Zz.

 

[mecheng2011]

Yes it is coming to a focus, with an intensity of around 5MW/cm^2. Looks like I have found a great journal on this topic, and I will post a link below for anyone who stumbles upon this in future!

http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA133211

Looks like anything below 1GW/cm^2 should be fine and not need to worry about ionisation of air.

Alex