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Air ionization & lasers

DeathSimpl

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I'm having a little problem with this subject :thinking: so if you know sth about it, please help :)

To ionize air with a laser is a very difficult process, it requires a lot of energy (when I say ionization I mean plasma). How you will reach this energy? The most approachable way is by storing energy joule by joule, until you get to the level needed, then you fire the laser releasing that energy in an instance. When that beam hits the focus it will ionize the air in that area.

Now there's my problem,
I got informed and read that the ionization of the air highly depends on the different wavelengths of the beam too. About the short wavelengths was said that they are 'ionizing' and about the long wavelengths that they are 'not ionizing'. And my problems started since I am also surrounded with informations which say that air ionization in the practical science world is being achieved with Infrared lasers :thinking:

Does that mean that it is much easier to build up the power of an Infrared laser than to use an Ultraviolet laser instead?

..Actually this is the real thing I would like to know,

Which wavelength needs the least power to ionize air? And what is the reason for that?

My logics say for shorter wavelengths it needs less power to ionize air, mainly because those waves' photons are more energetic.

I would really appreciate if I get the answer sooner, thanks :yh:
 



Benm

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There are several mechanisms for 'breaking down air'.

First of all, there is dielectric breakdown, which causes a sort of spark in mid air. Wavenlength is not that important there, but power density is. The preferred laser for that would be a pulsed Nd:YAG or something similar.

Anohter mechanism is actually producing radiation that ionizes molecules like O2 in air. This is best achieved with ultraviolet lasers, notably 266 nm dpss.
 

Benm

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It seems so :)

Its interesting to see that in that particular test 532 did better than 1064 and 355, so you'd think there must be an optimum somewhere. I have no idea what the mechanism behind it is though.
 

DeathSimpl

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Actually that was my point, it's a good example how for different wavelengths the minimum power needed may vary.. Now what I think has specific amount of sense is one theory which said that some ions need to be driven by the wave for a longer distance (in one direction) before the electron/s they carry breaks out of its orbit..

This is the quote

"....if the electric field of the light changes direction faster, the ions will not have enough time to build up much energy before they start slowing down again. This means we want to use light of a higher frequency, which means we must use shorter wavelengths.

This can be quantified by defining a quiver energy

Eosc=9.3×10-6 I λ2
where I is the laser intensity (in watts per square meter), λ is the wavelength of the laser light (in meters), and the quiver energy is given in eV. If the quiver energy is greater than the energy needed to remove an electron, cascade ionization occurs. For air, the ionization energy is around 15 eV (15.6 eV for nitrogen molecules, 13.6 eV for oxygen molecules)..."
quoted from How to Build a Laser Death Ray: Ionization

By me there is some logic in that statement, higher wave frequencies are more energetic, but they are less able to start air breakdown, mainly because they're probably not in 'resonance' with the air..

Thanks for your attention guys :)
 

jesgi2003

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We ionize air with 150 mW of 800 nm at 1kHz focusing with a lens of F=20 cm, with more energy we obtain non linear effects.
 

Bluefan

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Ionisation of air is already a nonlinear effect. If you can ionise air you can definately produce other nonlinear effect.
You don't mention the peak power, only the 150uJ pulse energy, which makes me think it's a Ti:Sa laser, no other laser can produce short enough pulses to in order to have a high peak power.
 

DeathSimpl

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Do you guys think that it is doable to construct a laser able to ionize small amounts of air (lets say from 5mm^3 to 1cm^3) in very high rate, over 10000 times a second? Or may even be CW.. Do you think that monster would cost less than a 1/2 million..?
 

Bluefan

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you'll need a 30 gigawatt (30000000000 watt) green laser. Not gonna happen CW or at 10kHz, might happen if you have a few million.
 

jesgi2003

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It is a Ti:Za femtosecond amplifier laser, pumped by a diode láser and by a oscilator. We obtain a train of pulses centered at 800 nm an average power of 3.5 W at a repetition rate of 1 kHz and a duartion per pulse of 30 femtosecond (with a lens you can see and hear very well plasma in air...it is normal with a fluence in the range of TeraWatts/cm2). With 2 Optical parametric amplifiers we can obtain any wavelenght fron 200 nm to 3000 nm.
 

DeathSimpl

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Well that's a beautiful laser there, I guess 800 nm is very efficient in ionization tho.. If you didn't already, you should maybe make some tests to see which wavelength works best.. :rolleyes:

What about this Japanese device guys Three Dimensional Images in the Air ,
do you have any idea what that laser's actual characteristics would be?
And why is that project stalling and not going any further? Is the problem in the overall limits in laser technology or it's just the focusing mechanism? :thinking:
 

Bluefan

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A modelocked Ti:Sa has a huge peak power, even if it's not the most efficient wavelength is will work fine.

A display using very high power lasers won't be cheap and it wouldn't be safe.
 

DeathSimpl

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Why it wouldn't be safe, if the beam successfully creates plasma, it gets absorbed by it.. even in case if some beams do escape, the projector may have an absorption material above it hence the problem is solved :beer:
 

Bluefan

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The plasma won't absorb all of the laser,so you'd definately need some shielding material around the active volume that absorbs the laser radiation, which would make the display a lot less interesting.
 

Hemlock_Mike

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Using my SSY-1 NdYag head with Q switch installed, I observed the flash/bang several times. My wife, wearing my goggles, wasn't impressed :-(. It chipped my lens 5 times doing it but I've seen it happen !!!!!..
I guess few will ever "see" it happen. I wasn't expecting it because I was trying to """pop a balloon:( """

HMike
 

Trevor

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Using my SSY-1 NdYag head with Q switch installed, I observed the flash/bang several times. My wife, wearing my goggles, wasn't impressed :-(. It chipped my lens 5 times doing it but I've seen it happen !!!!!..
I guess few will ever "see" it happen. I wasn't expecting it because I was trying to """pop a balloon:( """

HMike
You accidentally ionize air while trying to pop a balloon?! I wish I could say that! :crackup:

...though my life feels empty now. All my accidents ever result in are black marks on furniture. ;)

Or pain.

-Trevor
 




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