How to direct lightning with a laser…

Climate change is a mess. It’s hotter in some places, colder in other, and in general, severe weather is intensifying with longer droughts, heavier rainfalls and stronger storms. This problem, caused mostly by our method of generation of energy relying on burning fuel and releasing carbon dioxide in the air has a few solutions, among them: the wind turbine to harvest wind energy and generate clean electricity.

But, here is a slight problem. A few days ago, I stumbled upon an article[1] explaining how wind turbines are actually at risk of severe damages when thunderstorms occur.

The issue here is the fact that wind turbines are acting like giant thunder rods, attracting lightning, especially when the turbine is placed on flat patches of land. Till now, nothing too surprising. But with an ever growing desire of producing more energy, wind turbines are getting bigger and bigger, sometimes taller than 200 m, including the length of the blades.

Small turbines and small blades are actually designed taking into account the lightning discharges: they include some conductive material under the glass fibre blades, so that the discharge can propagate along the blade without heating too much. But those bigger wind turbines with the 200 m tall structure absolutely need to be full carbon fibre to reduce weight. Carbon fibre, on the contrary to glass fibre, is somewhat conductive (resistive is probably a better way to say it), and so if a current passes through it, it heats and burn. Inclusion of lightning deflection system in the blades is difficult and the damages done on the blades are extremely costly…. What to do?

When I read this, something pops up in my mind, as usual: LASERS! After all, to a hammer, everything looks like a nail. But here, it’s actually a clever idea that has some weight to it, but may still require some work.

As a kid learning about physics, I was convinced that we could direct electricity and therefore lightning with a laser strong enough. I can’t say that I was totally right because I could not give a correct explanation. But I learned that I was not wrong either.

Femtosecond lasers are special in many ways. I will probably talk about them in another post. Today, I want talk about them quickly enough and just explain some basic facts. The key element is the pulsed nature of the laser is that allows to reach instantaneous powers really high, say hundreds of TW, (yes terawatts), while working at regular average powers of a few hundred Watts. Why is it good? Because at such high instantaneous powers, it is possible to create plasma in the air. A process called Laser Induced Air Breakdown. If done in the air with the correct optical setup, it is possible to even have what is called filamentation where a thread of plasma is created along the laser path when it propagates in the air. As it turns out, plasma is a state of matter where charged particles can move around freely. Plasma is therefore highly conductive. Femtosecond lasers can create long (say hundreds of meter long) plasma channels in the open air…

You might see where I go now: the idea is to shoot femtosecond lasers in the sky during a thunderstorm to generate a plasma filament from the ground to the cloud, trigger the lightning strike and guide it along the laser path / plasma filament.

Is it science fiction? Well, no! It has been demonstrated at small scale[2] in a lab and almost at large scale[3] with a Teramobile[4] outside, during an actual storm. The experiment was not done in a wind turbine farm but at the top of some hill (I know…), and it shows the effect of the lightning impacts with the laser activated. The result is simple: it was possible to trigger lightning with the laser. But, it was not possible to conduct the lightning from cloud to ground. As it turns out, the plasma filament generated by the ultra-fast pulse was too short-lived and the lightning was not able to propagate quickly enough along it. Indeed, the lightning goes almost at the speed of light, say 107 to 108 m/s, over a distance of 1 km. It takes lightning 10 to 1 micro seconds to travel that kilometre. But the plasma channel is “open” for just a few hundred of nano-seconds at most… Just a few orders of magnitude too short.

Timing is not the only issue here: the Teramobile is a special laser, heavy and cumbersome. It generates 350 mJ pulses at 10Hz, making 3,5 W of average power and pulse duration of 100 fs. To be fair, 350 mJ per pulse is relatively high, but the important figure here is the instantaneous power that we can compute quickly based on the previous figures, and it accounts to 3.5 TW.

To compare to a regular system like a 20 W, 100 kHz with pulses that are 250 fs, the instantaneous power is almost 1 GW… Not a lot compared to the Teramobile and maybe not enough to generate a filament long enough in the turbulent atmosphere.

Why so much power is required? Could we go for less? Laser Induced Air Breakdown occurs when the power density is roughly 10 to 100 TW/cm2. It does depend on several parameters such as air composition, air pressure, pulse duration, etc… A study[5] was performed for a Ti:Sapphire femtosecond laser and found the critical limit for filamentation with such a laser was actually 40 TW/cm2.

Obviously, if the laser already has an instantaneous power in the TW range, it will be easier to reach the critical limit for filamentation. But our regular GW system could also do the trick. It could actually be better than the Teramobile to create a longer lasting plasma channel, as the GW laser can be shot at a higher frequency, to a point where each pulses creates a channel that will last till the second pulse in the best case[67]. This could allow to create a plasma channel that would be open for a microsecond, long enough to carry the initial charge from the cloud to the ground and avoiding other conductive structures.

I think there is some potential in this… isn’t it?