Question about a laser that can Ionize air.

[aelshafiey]

Hello,

I'm trying to find any laser that can Ionize air/pressurized air. (something cheap)
So I saw some lasers that have 10 W (laser pointers) that are for 100$, They have a short wavelength 400 nm (and they are continuous ofc) I'm not sure if these can Ionize air or not, any ideas?

Also please if you have any suggestions please let me know.

Thanks.

 

[Sigurthr]

Still miles and miles away from ionizing air. You won't find a non-surplus system capable of air ionization short of tens of thousands of dollars. A SSY-1 NdYAG pulsed IR laser can do it in short bursts when paired with the correct lens.

 

[aelshafiey]

And the SSY-1 NDYAG is homemade correct (I just looked it up)
Also whats so special about it? (like whats its wavelength, Energy)

Thanks, I appreciate you help.

 

[aelshafiey]

Also what If I have 1500$, you dont think I will be able to find a laser that can Ionize air?

 

[Sigurthr]

Even with $1500 you'd still be looking at a Q-Switched NdYAG as your only option.

The SSY1 is a laser rangefinder that operates at 1064nm which was used in israeli military tanks. It had a passive Q switch, and only required the addition of a proper power supply (xenon flash lamp power supply), ignitor, and PFN (pulse formation network). They're exceptionally hard to find available since about 2012, and the newer versions (dubbed SSY2 here, but not officially) run at much less power. Even the SSY2 are hard to source.

There are only a couple of lasers which can ionize air that are not custom solutions:
1) SSY1
2) Q-Switched NdYAG (in all flavors of frequency multiplication)
3) Q-Switched CO2
4) Very High Power Ruby


Edit: to answer what's so special about SSY1; it was the cheapest and most common Q-switched NdYAG hobbyists could hope to own.


Lots of others can do it, but they're all custom solutions designed for air breakdown.

 

[aelshafiey]

I have the following 2 Q-Switched ND:YAG lasers, do you think they will work? (I dont think so because they have low energy)

 

[Sigurthr]

I cannot say for sure, because it really depends on energy density, and it could be possible if you had a (nearly) diffraction-limited focusing lens (extremely short FL). Those pulse energies do look quite low though.

There's an old declassified military document that shows the energy per pulse per volume of area of air per wavelength needed to cause breakdown, but I can't seem to find it anymore. It is the prime source of info on CO2 laser based air ionization though, if you wish to dredge the internet looking for it that would be the primary search query.

Wish I could provide more info but flashlamp lasers isn't my forte aside from basics and 1960s era info.

 

[Teej]

I did find this:

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


(The link is dead though)


I also found this:

Air Breakdown
You've all seen it - a flash and bang in the middle of nowhere from an invisible high power laser. A large enough electric field will strip electrons right out of the molecules of air (mostly N2 and O2). The result is a plasma which appears like a tiny (or not so tiny) spark, possibly along with a loud sonic effect. While the peak power required to do this is high, it can come from relatively small lasers.

(From: James Whitby (james.whitby@phim.unibe.ch).)

The main thing that matters is the irradiance of the beam (i.e. power per area). One way of making an estimate is to consider when the electric field strength in the laser beam is comparable to that experienced by an electron in a a molecule of 'air', or to reported values for the dielectric breakdown of air using DC fields. The presence of any particulate matter will have a big effect on the threshold.

For practical numbers with pulsed lasers see for example: "A numerical investigation of the dependence of the threshold irradiance on the wavelength in laser-induced breakdown in N2", Gamal YEED, Shafik MSED, Daoud JM JOURNAL OF PHYSICS D-APPLIED PHYSICS 32 (4): 423-429 Feb. 21, 1999.

For quick numbers, from another very short paper: Tambay et. al., Pramana 37(2) pp163-166, 1991. Using approximately 2 ns pulses in clean dry air at atmospheric pressure, the thresholds for breakdown were found to be about:

Wavelength Power Density
------------------------------
1064 nm 6 x 1011 W/cm^2
532 nm 3 x 1011 W/cm^2
355 nm 2 x 1012 W/cm^2

(Note the minimum value for the second harmonic, although this looks like an outlier in the data-set this behavior is highlighted in the text so was presumably reproducible.)

(From: Phil Hobbs.)

 

[Sigurthr]

I can reinforce the harmonic NdYAG efficiency; it was sited in that 1960s paper I talked about. For some reason 532nm is one of the most efficient at inducing breakdown. (I think ~180nm is the only one significantly more efficient and that's because O2 absorbs it VERY strongly and is the ionization wavelength of O2 to O3).

edit: typos

 

[LSRFAQ]

"And Lenin wrote a book on Marx,
And we sang Dirges in the dark,
Waiting for the spark to spark
The day the research died."

Apologies to Don Mclean..


My ophthalmic friends get air breakdown about every third shot at 7-10 mJ and less then 2 nanoseconds. It is part of their testing procedures for certain eye ND:YAG surgery lasers. They have a beam with controlled mode shape and very good focusing optics, which start with up-collimation and spatial filtering. I would not depend on such a small laser for a trigger. Air breakdown is used so the technician is not forced to carry an oscilloscope and very expensive photodiode. They are dependent on a piece of dust being near the focal point.

Having a small amount of particulate in the air greatly lowers the air breakdown threshold.

The lasers I used to service, used EO Q-Switches and a tiny Bullseye gradient on the middle of the output coupler to force a TEM00 mode buildup.

We easily obtained air breakdown at 125 mJ and 5 to 7 nanoseconds (1064 nm) with a tightly focused TEM:00 Beam. At 250-350 mJ, breakdown is almost certainly guaranteed with clean air at 1064 / 7 nS.. If you have a surface for the focused beam to interact with, the threshold is very low. I can attest to the fact that Green works better in general.

Generally a EO or Cr:YAG qswitch is used to get the shorter pulses with SHARP leading edges from lamp pumped ND:YAG.

So the usual scheme for a precise TEM:00 beam for a triggering system is thus:

1. The flashlamp(s) are continuously simmered at 100-200 mA DC to avoid the need for a trigger pulse. . Commercial simmering modules provide the starting pulse and simmer and are not expensive. These lasers are water cooled, so simmer heat is not a issue. Simmer in fact improves lamp life and increases pulse to pulse stability.

2. The storage capacitor is dumped into the simmering lamp using a IGBT or SCR and a blocking diode. A proper sized inductor in series with the capacitor shapes the current waveform for a critically damped, single cycle, pulse with no ringing.

3. After one upper state storage time, ie ~ 250 uSec for Nd:YAG, the Q-Switch is triggered by a precise triggering clock synced to the experiment.
So the first thing done by the experiment controller is dumping the charge into the lamp. The second thing it does is gate the driver for the EO Qswitch. AO Qswitches are generally not desirable for precision timing tasks nor for making a sharp edged optical pulse.

4. By edit/ redacted, never mind, probably should not have posted that. The upper state storage time also allows for some considerable slop in the external timing, as long as the laser is sized large enough.

5. A b-dot sensor halfway up the Marx chain is also often used to detect firing after the laser initiates the pulse.

So I'd start with a commercial Electro-Optically q-switched laser and focus it very close or onto the surface of the Marx electrode. Most of what is different from a standard industrial low rep rate, flashlamp pumped laser is the intracavity plate polarizer, the bulls-eye coating for mode control, and the EO Qswitch for precise timing.

Proper selection of the spot of sacrificial material for laser triggering is required.. Brass for low energies, Tungsten or certain copper alloy for the higher energies. Most users would work harder and obtain the air triggering so as to avoid having to replace sacrificial material.

Ekspla makes great EO cells and precision driver boards for this purpose. A 250-350 mJ Ekspla laser might just be idea.


10K would get you a flashlamp, used cavity, rod, optics, and the EO cell with driver. It would not get you a new power supply or a complete laser with the heat exchanger, DI based water cooling, safety features, warrenty etc. Four times that would be more typical for a new unit. Manufactures some times have a used or trade-in unit laying around that would go for less.

A Ebay rangefinding laser would get you the surface plasma breakdown, but you would NOT get precision timing from just firing a flashlamp with a passive Q-Switch. Its not likely to get you a consistent air breakdown with the typical long pulse, ringing lamp driver, and poor mode structure they emit.

Disclaimer, I used to work for the US Rep for Ekspla. My former competitor, Continuum, might also be of service.

The stats for the lasers you posted do not suggest power levels and pulse durations suitable for triggering a spark gap by direct air breakdown. You might get a surface breakdown, but those have lifetime issues, especially when your trigger material sputters all over the place and causes false triggers down the road.

I wish I could tell you more, but I'm limited by certain agreements.

Steve

 

[Sigurthr]

Thanks a ton, Steve! Really awesome in-depth info you've given.

 

[Polonium210]

Here is an air breakdown shot from my SSY1 laser.

-Rob

 

[LSRFAQ]

Here is an air breakdown shot from my SSY1 laser.

Rob, Amazingly beautiful beautiful image.

What what is implied in his post for a Marx generator triggered by a laser, is that he wants precision timing for less then 10 Grand. The SSY 1 has a timing drift measured in 10s of milliseconds, simply because of the trigger coil and passive Q-switch.. The OPs employer probably wants 10 microseconds.

Steve