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New laser weapon works in fog rain


Moontanman
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My guess is they send out a "probe" laser beam (or other EM wave?) at a wavelength that can transmit through fog, detect the return signal, examine it for how it's wavefront is distorted by the atmosphere, and then send the info to a deformable mirror which distorts to match the atmospheric error. The high-powered laser beam is reflected off this deformable mirror. This distorts the high-powered beam's wavefront so it cancels the errors in the atmosphere and arrives at the target as a concentrated beam.

 

The entire process must be fast enough to accommodate the rapidly changing atmospheric distortions and spatially fine enough match the atmosphere's spatial changes. The question is, as always, what is the range of such a device. I suspect that is classified.

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Didn't we have a thread a year or so ago when we worked out how much power a laser would need to get through even a small cloud of water droplets - it was immense

 

That is exactly the first thing I wondered. But can't you just evaporate all that water?

How much of the power gets scattered because of fog, and how much gets used to evaporate the water?

 

If your laser beam is 5 mm wide, and the linear velocity of the wind is 1 m/s (it's never very windy when there's fog), and your target is at 5 km, then per second you need to evaporate all the water in a total of 25 m3 of air. Since according to wikipedia, there is about 0.05 g/m3 of liquid water in fog, that is a grand total of 1.25 grams of liquid water per second. The power required to evaporate that is 2.8 kW.

 

The way I see it, is that the water that enters the side of the beam is evaporated, and never even gets to distort whatever light is in the center of the beam. The light at the center of the laser beam doesn't need much refocussing. (Even though the article seems to suggest something like that).

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That is exactly the first thing I wondered. But can't you just evaporate all that water?

How much of the power gets scattered because of fog, and how much gets used to evaporate the water?

 

If your laser beam is 5 mm wide, and the linear velocity of the wind is 1 m/s (it's never very windy when there's fog), and your target is at 5 km, then per second you need to evaporate all the water in a total of 25 m3 of air. Since according to wikipedia, there is about 0.05 g/m3 of liquid water in fog, that is a grand total of 1.25 grams of liquid water per second. The power required to evaporate that is 2.8 kW.

 

The way I see it, is that the water that enters the side of the beam is evaporated, and never even gets to distort whatever light is in the center of the beam. The light at the center of the laser beam doesn't need much refocussing. (Even though the article seems to suggest something like that).

 

That depends on 100pct of the power being used to heat up water - but in most ranges the majority of the light will be scattered

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That depends on 100pct of the power being used to heat up water - but in most ranges the majority of the light will be scattered

 

Correct, that was my assumption (and you are correct to challenge me on it).

 

Maybe they mixed in a little UV? Water is not very transparent for UV, as far as I know? Would it be possible that the trace-laser sort of clears the path in 1 wavelength, and the real laser uses a second wavelength? This is pure speculation, but I think it is interesting.

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Well, for a start, water is pretty transparent to UV more so than air at some wavelengths..

 

There's still a problem.

if you boil the water to steam you heat the air.

If you heat the air, you reduce it's refractive index.

If you do that, you are trying to get a laser beam to pass undiverted through what amounts to a negative lens.

 

This sort of thing happens

http://www.rp-photonics.com/thermal_lensing.html

 

There's also the problem of dust etc which might absorb some of the light and - at those power levels- promote ionisation of the air. ionised air is rather a good absorber.

 

So, as I said, I don't see how this is possible.

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Well, for a start, water is pretty transparent to UV more so than air at some wavelengths..

 

There's still a problem.

if you boil the water to steam you heat the air.

If you heat the air, you reduce it's refractive index.

If you do that, you are trying to get a laser beam to pass undiverted through what amounts to a negative lens.

 

This sort of thing happens

http://www.rp-photonics.com/thermal_lensing.html

 

There's also the problem of dust etc which might absorb some of the light and - at those power levels- promote ionisation of the air. ionised air is rather a good absorber.

 

So, as I said, I don't see how this is possible.

 

 

So you don't see how it could work so it can't work?

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So you don't see how it could work so it can't work?

 

 

With respect Moon the defence industry have a pretty strong record in promising miracles in order to remain at the front of the queue for govt money. I think the members of a science forum - when presented with a press release masquerading as a news story - have the right, and maybe even the duty, to express caution, debate the ways and means, and sort wheat from chaff; when the problems seem difficult and the declared solutions are hand-wavy I think scepticism such as John's is well-placed.

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With respect Moon the defence industry have a pretty strong record in promising miracles in order to remain at the front of the queue for govt money. I think the members of a science forum - when presented with a press release masquerading as a news story - have the right, and maybe even the duty, to express caution, debate the ways and means, and sort wheat from chaff; when the problems seem difficult and the declared solutions are hand-wavy I think scepticism such as John's is well-placed.

 

 

I was just poking the bear a little bit, I guess I should have used an emoticon..sorry John...

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So you don't see how it could work so it can't work?

No, what I said at the outset was that my guess was that it wouldn't work.

I explained that guess.

Perhaps, rather than making inapt snide comments, you might like to explain how the observed rules of physics are evaded in this case?

Poking the bear might be OK. Preaching isn't.

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Hey John, thanks for a good explanation of a couple of issues that I hadn't thought of (e.g. refractive index of steam / hot air).

Just a question through, you claim that water is transparent to UV. However, this graph show that the absorption coefficient for UV is several orders of magnitude larger than for visible light. I realize that water is also transparent to visible light, and I did find other sources claiming it is transparent to uv. I just want to find out how transparent it is. Would any significant portion of the energy be absorbed (and given the high power of the laser claimed in the article, I think anything over 1% can be called significant).

 

 

That said, I am starting to see more problems than solutions with this laser to penetrate fog / rain. I just realized that if you evaporate the water, the resulting hot steam / air will also have a much lower density than the surrounding air, so you also get convection which increases the air flow through the beam, thereby increasing the number of water droplets that get in the way.

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Well, that data is presented "per metre" so, the amount of light absorbed is measured for the equivalent of a swimming pool rather than some fog.

 

You cited data saying that fog has something like 0.05 grams of water per m3

Bulk water is 1,000,000 grams per cubic metre so the fog has something like ten million times less water that what they measured in that graph.

Very roughly it will absorb ten million times less

So 1% absorbed corresponds to 100,000 on that graph (very roughly)

That's the hard UV below about 180 or 190 nm and bits of the IR near 10 microns

Oxygen in the air will absorb UV in that region and there's more oxygen present than water so it's likely to absorb more of the UV.

As I said, "water is pretty transparent to UV more so than air at some wavelengths."

Here is a fairly typical illustration of the absorbance as it would be normally seen in a lab (i.e with a 1 cm path-length measured WRT air)

https://us.vwr.com/store/catalog/product.jsp?catalog_number=BJAH365-4

 

It is transparent.

 

A much bigger effect would be due to scattering by water droplets- very little light is absorbed, but something like a tenth of it is reflected from the surface of water.

That's why is doesn't go especially dark on a foggy day- most of the light gets through- but you can't see properly- the light doesn't go where you expect it to.

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I would think, if you want to heat/vapourize the water droplets, you would use microwaves to match the molecular bond.

 

I still don't think this approach will work.

A better way may be to tailor the beam's wavelength such that the scattering is minimized by the average sized fog droplet, as John has implied above.

Edited by MigL
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Well, you could use microwaves to evaporate the water but, because of their large wavelength (compared to light) you can't focus them into such a narrow beam.

So, instead of trying to boil all the water in a 5 mm diameter beam you would be trying to boil the water in something like a 50 mm wide beam (probably more).

That's a hundred times more water so you need rather more power.

The 2.8 KW needed to clear the fog from a 5 mm beam was already a problem.

It just got a lot worse.

 

I still think this is a bluff.

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I would have to say I have a bit of a love affair with technology and tend to believe when I read in pop science articles. It stems from a mild childhood embarrassment when a teacher humiliated me in front of a science class because I had asserted that eventually lasers would be small enough to be hand held and that weapons grade laser would one day be a possibility as well. Star trek had just premiered and I was enthralled with it. He pointed that lasers required so much energy and enough equipment to fill the class room and how fragile they were and various things and of course the other kids laughed their asses off as kids will do.

 

I now carry a hand held laser pointer in my pocket every day to remind me that authority figures are not always correct.

 

Johns objections are difficult to counter but we've had lasers powerful enough to shoot down incoming artillery rounds for quite some time. I have to think that as technology marches on they will become more and more relevant to the military and punching through bad weather is close to the only stumbling block left.

 

John was correct to be sceptical and my link gave no details that would allow anyone of us to to dispute Johns Objections...

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Artillery Rounds? I heard mortars - slower and less hardened.

 

How do these laser weapons counter the idea of polishing the ordinance to a mirror finish - cover of Al polished up will reflect about ninety percent of all wavelengths from mid uv to mid ir. anything with higher frequency will be a devil to direct; if you can bounce it off mirrors then you can mirror a surface - and vice versa if you cannot use a mirrored surface to protect it is hard to use mirrored surface to direct

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Found one of their marketing videos:

 

http://www.boeing.com/defense-space/ic/des/hel_md_0514_vid.html

 

More than a little promotional, but all the imagery looks legit, particularly the laptop screens and power panel.

 

For UAV's they appear to be targeting the UAV's tail and for mortars causing them to explode. Makes sense, use their own strength against them. Sun Tzu would be proud.

 

I'm really not seeing anything official talking about specific weather testing results. There is one line in the official source on the subject, but no real details. My guess would be reduced operation in some fashion. At a short enough range it might not have been a major issue.

 

Found an excellent general resource here:

http://www.ausairpower.net/APA-DEW-HEL-Analysis.html

 

The HEL MD(HEL TD, in the document) really looks to be a spiritual successor of the THEL(later MTHEL) system.

Edited by Endy0816
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Artillery Rounds? I heard mortars - slower and less hardened.

 

How do these laser weapons counter the idea of polishing the ordinance to a mirror finish - cover of Al polished up will reflect about ninety percent of all wavelengths from mid uv to mid ir. anything with higher frequency will be a devil to direct; if you can bounce it off mirrors then you can mirror a surface - and vice versa if you cannot use a mirrored surface to protect it is hard to use mirrored surface to direct

 

 

Like most things in warfare, one side gains an advantage and the other side counters it... I have seen film of powerful lasers going through regular mirrors like hot butter though...

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There's no more information available than, "Under windy, rainy and foggy weather conditions in Florida, these engagements were the most challenging to date with a 10-kilowatt laser on HEL MD”.

 

How long did it rain? Were they actively firing the laser during these time periods? How far were the targets? Was it a Florida sun-shower or one of our rapidly moving downpours?

 

The next best thing is seeing it in normal operation and devising some of the components. Combining the two sources we know it is a Solid State Laser, that it has a "beam control subsystem", mounted on a HEMTT vehicle and that it destroys via burning/heating its target. Considering the probable relationship to the THEL/MTHEL we may also be able reason out how a souped up version might act based on the data for it.

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