"We wanted to make a first demonstration that the laser can have an influence on lightning, and the easiest thing is to guide it", explains to AFP Aurélien Houard, from the Applied Optics Laboratory at ENSTA-Ecole. Polytechnique in the Paris region.

It is the culmination of a twenty-year collaboration with physicist Jean-Pierre Wolf, from the Applied Physics Group at the University of Geneva, and involving six institutes.

Lightning strikes, which occur 40 to 120 times per second around the globe, cause more than 4,000 deaths each year and economic damage amounting to billions of dollars, recalls the study published in Nature Photonics.

Lightning is a discharge of static electricity that has accumulated between storm clouds or between these clouds and the Earth.

Since the invention of the lightning rod attributed to Benjamin Franklin in the 18th century, science has only made progress in protecting itself from it by building ever higher masts to guide it.

The team of Messrs.

Houard and Wolf used a laser as a lightning rod.

Its beam creates a plasma, air charged with ions and electrons, which is also heated by this process.

The air crossed by the beam "then becomes partially conductive, and thus a preferential path for lightning", a bit like a cable, explains Mr. Houard.

Scientists had unsuccessfully tested this theory during a campaign in New Mexico in 2004. The fault was a poorly adapted laser, and terrain where it is difficult to predict where the lightning will fall.

Lightning Precursors

The solution?

They found it at the top of Mount Säntis, 2,500 m above sea level in the pre-Alps of northeastern Switzerland.

The icing on the cake, the place is equipped with a 124 m high telecommunications tower, struck with the quasi regularity of a clock at the rate of one hundred lightning strikes per year.

Lightning during a thunderstorm in Montevideo, August 27, 2022 © Mariana SUAREZ / AFP

After two years of construction of a very powerful laser, made by the German Trumpf, and several weeks to assemble it in pieces by cable car, the largest helicopter in Switzerland has deposited containers there to house a telescope.

The telescope is used to focus the laser beam to obtain the strongest intensity at 150 meters high.

The green beam of the laser goes from a diameter of 20 cm at the start to a few centimeters.

In the summer of 2021, the scientists tuned their laser to create a plasma above the tip of the tower.

And managed to photograph the guidance of a flash of lightning by the laser for about 50 meters.

Three other guidances were corroborated by interferometry measurements.

Lightning develops with precursors (similar to branches) that start from the clouds, and from the ground when the electric field is strong enough.

It is through the junction of these precursors that "the current and the power of a lightning bolt really appear, once the ground is connected with the cloud", explains Mr. Houard.

The laser guides one of these precursors.

Thanks to this, "he will go much faster than the others and straighter. He will then be the first to connect with the cloud before lighting up. In the end, this precursor becomes the lightning bolt".

Once the demonstration has been made that a flash of lightning can be guided, it remains to be confirmed by other experiments.

And then to try to trigger lightning, to better protect strategic installations, such as airports or rocket launch pads.

It would suffice in theory to initiate precursors, and in practice to have a fairly high conductivity in the plasma.

What researchers do not think they have yet mastered.

© 2023 AFP