How to direct lightning with a laser…

Le changement climatique est un gros problème. Il fait plus chaud à certains endroits, plus froid à d'autres, les catastrophes naturelles sont plus nombreuses, avec des sécheresses plus longues, des pluies plus drues et des tempêtes plus fortes. Ce problème est majoritairement causé par notre façon de générer de l'énergie qui vient principalement de la combustion de pétrole ou de charbon, ce qui relâche du dioxyde de carbone dans l'air. Quelques solutions existent, dont l'éolien qui capture le vent pour produire de l’électricité.

Mais il y a un petit problème de taille. Il y a quelques jours, je suis tombé sur un article[1] qui explique comment les éoliennes sont en danger à cause des tempêtes, et en particulier de la foudre.

Le problème vient du fait que les éoliennes agissent comme des paratonnerres, attirant la foudre, en particulier lorsque ces éoliennes sont placées dans des zones plates. Jusqu'à présent, rien de trop surprenant. Mais depuis quelques années, les éoliennes ont pris de la taille pour générer toujours plus d'énergie. Elles sont devenues de plus en plus grandes, dépassant parfois les 200 m de haut, en incluant les pales.

En fait, les petites éoliennes sont pensées en tenant en compte de la foudre. Les pales sont en fibre de verre, avec un matériau conducteur en dessous pour permettre à la foudre de partir vers la terre, sans faire trop de dégâts. Mais les éoliennes de plus de 200 m sont fabriquées en fibre de carbone pour des raisons de poids. La fibre de carbone, contrairement à la fibre de verre, est un peu conducteur (autant dire résistif). Ainsi, si un courant élevé passe à travers, ça chauffe et finit par brûler. L'inclusion de mécanismes de déflexion de la foudre dans les pales est difficile et les dégâts coutent cher... Que faire?

Quand j'ai lu ça, une idée m'est apparue: utiliser un LASER! Après tout, pour un marteau, tout ressemble à un clou. Mais ici, c'est plutôt une bonne idée qui demande encore un peu de travail.

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[6–7]. 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?