Tame lightning to better protect yourself from it

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Taming lightning for better protect it

During testing carried out at the summit of Säntis, the scientists found that the discharge could follow the laser beam for several tens of meters before reaching the tower (in red and white).

Guide the lightning and, why not, trigger it one day: this is the bet of scientists who bring the first experimental demonstration with a laser at the top of a Swiss mountain, as reported in a study published on Monday.

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

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

The laser was focused above a 124 m transmitter tower belonging to the operator Swisscom, equipped with a traditional lightning rod.

Lightning strikes, which occur 40 to 120 times per second around the globe, cause more than 4,000 deaths and billions of dollars in economic damage each year, says the study published in Nature Photonics< /em> (in English).

Lightning is a discharge of static electricity that has accumulated between thunderclouds or between thunderclouds 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 against it ;by building ever higher masts to guide it.

The LLR laser was installed at the top of Säntis (2500 m) and activated from June to September 2021.

< p class="e-p">The team of MM. 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 traversed by the beam then becomes partially conductive, and thus a preferential path for the lightning, a bit like a cable, explains Mr. Houard.

Scientists had tested without success this theory during a campaign in New Mexico in 2004. The fault of an ill-suited laser and a terrain where it is difficult to predict where the lightning will fall.

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The solution? They found it on top of Mount Säntis, 2500 m above sea level in the Prealps of northeastern Switzerland. The icing on the cake, the place has a 124 m high telecommunications tower, struck with almost clockwork regularity at the rate of 100 lightning strikes a year.

After two years of building a very powerful laser, made by the German Trumpf, and several weeks of mounting it in pieces by cable car, the largest helicopter in Switzerland 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 initially to a few centimeters.

In the summer of 2021, scientists tuned their laser to create a plasma above the tip of the tower. And they managed to photograph the guiding of a lightning flash 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 power of a lightning bolt really appear, once the ground is connected with the cloud, explains 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 illuminating. In the end, this precursor becomes the lightning bolt.

Once the demonstration has been made that a lightning bolt can be guided, it remains to be confirmed by x27;other experiments. And then to try to trigger lightning, to better protect strategic installations, such as airports or rocket launch pads.

In theory, it would suffice to initiate precursors , and in practice to have a fairly high conductivity in the plasma. What researchers do not think they have yet mastered.

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