École polytechnique: A laser lightning rod

Guiding lightning by laser. This could be a superpower. However, this is not a science fiction scenario, but the aim the European research project FET-OPEN Laser lightning rod, in which the Laboratory for Applied Optics (*LOA) is involved. The aim is to build a laser lightning rod to protect against lightning. Scientists are preparing to test it in Switzerland, on the summit of the Säntis mountain.

Lightning phenomena

During a thunderstorm, the lower part of the cloud becomes negatively charged. This attracts positive charges to the ground surface, several hundred metres below. The result is a potential difference of up to several tens of millions of volts. Locally, at cloud or ground level, the electric field can exceed the threshold beyond which electrons are torn from the air molecules: the air becomes ionised and a discharge is initiated. This discharge progresses upwards or downwards depending on the case. A downward discharge, it causes an increase in the electric field at ground level, until an upward discharge is initiated. When these downward and upward discharges meet, a very strong current flows from the cloud to the ground, causing lightning and thunder. This is the lightning strike.

The standard lightning rod, invented by Benjamin Franklin in 1752, initiates the upward discharge by a spike which locally strengthens the electric field. But it has some limitations. For example, it is not suitable for airports or space launch pads, where very large areas need to be protected. The alternative consisting of mini rockets towing a metal cable, has also shown its effectiveness in triggering lightning, but is difficult to reuse and presents risks when the rocket falls.

A european project and an experiment in Switzerland

The Laser Lightning Rod project involves guiding electrical discharges through laser filamentation. This phenomenon, which has other promising applications, occurs when an intense laser beam propagates through a medium such as air. Non-linear effects cause the beam to focus very tightly. This results in the ionisation of air molecules and the formation of a plasma. “Above all, the suddenly heated air expands, creating a channel with a low density of air molecules,” explains Aurélien Houard, project coordinator and researcher at LOA. The movement of other particles, such as electrons from lightning, is therefore facilitated in this channel, making it a preferential path for upward discharge.

Décharge électrique guidée dans un faisceau laser en laboratoire. Crédit : Pierre WalchA laser beam guiding an electric discharge in the lab. Credit : Pierre Walch

To achieve this goal, the LOA team has joined forces with several partners, including the University of Geneva, the Ecole Polytechnique Fédérale de Lausanne and the German company TRUMPF Scientific, which is developing the laser emitting picosecond pulses (lasting only a few billionths of a second). This new laser is more energetic than those previously used for this type of experiment, which creates longer filaments,” stresses Pierre Walch, a doctoral student at LOA. Moreover, it fires 1000 times per second, a rate that maintains the low-density channel and even lowering the air density a little more after each pulse.” After laboratory tests, the team has the challenge of deploying this system, which weighs one tonne and is ten metres long, in the Swiss Alps at the top of Mount Säntis (2502 metres) in order to carry out an observation campaign from June to September, when storms are frequent. If the experiment is successful, it will be the first real-life demonstration that a laser can influence and guide lightning.

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