Pulsed terawatt lasers have surprising effects when shone through the air (2006) | Hacker News Reader

Pulsed terawatt lasers have surprising effects when shone through the air (2006)

americanscientist.org

73 pointskungfudoi30 comments

I spent countless hours of my PhD trying to get the White light continuum stable enough to do measurements with.

Our laser wasn't powerful enough to use air as medium, so we would use sapphire (very good, stable and predictable), Calcium Fluoride (absolutely bloody terrible, but it did give out a lot more UV light) and water (never got any measurements out of that, so unstable)

> using pulsed lasers to trigger and guide high-voltage discharges over several meters in the laboratory.

There is no way the military have seen that and haven't thought "I bet we can use that to fry our enemies"

I love the idea of generating two parallel beams and leveraging the conductivity of plasma as a sort of "wireless" taser. It's likely not feasible (I'm guessing someone has tried it), and it would be interesting to know why.

I suspect being hit by the beams might harm the victim, with the electricity just being a bonus. I doubt the laser generators are cheap and portable as well.

It has an EMP potential to fry electronics probably.

It's called an "electrolaser": https://en.wikipedia.org/wiki/Electrolaser. There are reports that they work in practice, but not yet well enough for use. I don't know why.

The laser power might kill the victim, but using this for actually sending electricity might work. Might be a good replacement for high power lines in some cases.

Last year, I attended a colloquium from a scientist from the army weapons' division specifically about this (filaments). He kept extolling the career opportunities available at his lab...doing weapons research.

I will be (if the funding comes in, fingers crossed) funded by the DoD too...and what I am doing is laser plasma related but it isn't directly weapons research.

"... back-directed light is the result of partial self-reflection of the forward-traveling beam..."

"the idea of using a high-intensity laser to ionize air along the beam, thus forming a conducting channel of plasma"

Real life light sabers are within the realm of possibility with existing technologies!

The kerr effect sounds interesting, even if it is air-only... what power density would it take to get self-focusing in a vacuum by gravitational lensing?

> what power density would it take to get self-focusing in a vacuum by gravitational lensing?

Not this is a question I'd look forward to be answered by Randall Munroe in his http://what-if.xkcd.com/

Yes. Send it!

Sent, I would love to see Randall Munroe's take on it.

The Kerr effect is present in many different transparent media, but not in vacuum. You're probably right that sufficient power density would give rise to self-focusing by gravitational lensing, but it seems like the necessary power density would be very high — the gravitational acceleration needs to be a substantial fraction of c very frequently, like 0.001c over every interval where diffraction gives you a divergence of one milliradian. A narrower beam moves the escaping light you need to capture closer to the center of mass of the beam, but also increases that divergence.

It isn't obvious to me how to calculate this. Can anyone help?

You likely couldn't get gravitational lensing, you would probably get pair production first, which would screw with your energy density.

Yes, I'd think the Schwinger limit[0] would be reached first, which is currently the dream of most of us in this field. The Irradiance for the Schwinger limit is 10^29 W/cm^2.

By contrast, the planned ELI[1] experiment will have pulse with a intensity of 10^24 W/cm^2. The current state of the art lasers of high peak intensity is ~10^21 W/cm^2 (current record I believe is 10^22 W/cm^2). There are proposals to getting to the Schwinger limit but they are still in the hypothetical stage.

[0] https://en.wikipedia.org/wiki/Schwinger_limit

[1] https://en.wikipedia.org/wiki/Extreme_Light_Infrastructure

You likely couldn't get self-focusing in a vacuum by gravitational lensing. Pair production would likely dominate the process first and kill your energy density.

This, while quite interesting, also feels like a member of a nascent genre: "XKCD What-If? citation trolling."

That is, hiking the peak power of the laser pulses that are applied increases the number of filaments that result without notably influencing the individual intensity or the energy each filament carries.

So these filaments are basically quantized particles?

...and their pulse length of 70 femtoseconds would be about 0.02 mm, or not too far off how wide the filaments are. I guess the proportions would be similar to a pair of saucers with the rims stuck together.

In case you missed it: Take a close look at the last photo, the one near the text "control of lightning strikes".

Most interesting thing in the article: they can likely use pulsed lasers to guide lightning!

Now the next question: does the laser consume more energy than the lightning bring ? Because maybe we can use that to harness energy in storms.

The big problem is building batteries that could absorb some buttload-Joules of energy in a fraction of a second. IIRC some researchers tried (just with metal rods instead of lasers) and all they got was expensive fireworks.

Well, maybe you could redirect the current directly into the grid and spread it accross it so that the power is divided accross the entire network and save fuel from power factories in the process. If you can predict the hits, you can balance the production accordingly.

You would certainly want capacitors rather than batteries to do the initial energy capture. That doesn't really help much though; the amount of energy in lighting is of a completely different scale from what we can handle with current technology.

How about using the energy to heat/boil water?

Two words: Graphene Capacitors :-)

Graphene, the only hot new thing more versatile than USB-C.

But you certainly can use USB-C to store energy from lightnings!

Isn't this known / due to ionized air?

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