Guidance of the beam by laser. It could be a superpower. However, this is not a science fiction scenario, but the objective of the European research project FET-OPEN Laser Lightning Rod, in which the Applied Optics Laboratory (* LOA) participates. The goal is to build a laser lightning rod to protect yourself from the rays. Scientists are preparing to test it in Switzerland, at the top of the Säntis mountain.
Lightning phenomena
During a storm, the lower part of the cloud becomes negatively charged. This results in positive charges on the surface of the ground, several hundred meters below. The result is a potential difference of up to several tens of millions of volts. Locally, at the level of the cloud or the ground, the electric field can exceed the threshold beyond which the electrons are freed from the air molecules: the air is ionized and a discharge is initiated. This discharge progresses upwards or downwards as the case may be. A downward discharge causes an increase in the electric field at ground level, until an upward discharge begins. When these upward and downward discharges meet, a very strong current flows from the cloud to the ground, causing lightning and thunder. It’s love at first sight.
The standard lightning rod, invented by Benjamin Franklin in 1752, initiates upward discharge by means of a tip which locally strengthens the electric field. But it has certain limitations. For example, it is not suitable for airports or space launch pads, where it is necessary to protect very large areas. The alternative, consisting of mini rockets towing a wire rope, has also been proven effective in activating lightning, but is difficult to reuse and poses risks when the rocket falls.
A European project and an experience in Switzerland
The Laser Lightning Rod project consists of guiding electric discharges by laser filamentation. This phenomenon, which has other promising applications, occurs when an intense laser beam passes through a medium such as air. The nonlinear effects make the beam very precise. This results in the ionization 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 in rays, is therefore facilitated in this channel, which makes it a preferred path for upward discharge.
Guided electric discharge in a laser beam in the laboratory. Credit: Pierre Walch A laser beam that guides an electric discharge in the laboratory. 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 develops laser pulses emitting picoseconds (of a duration of about second). This new laser is more energetic than those previously used for this type of experiment, which creates longer filaments ”, underlines Pierre Walch, doctoral student at LOA. In addition, it is triggered 1000 times per second, a speed which maintains the low density of the channel and even decreases the density of the air a little more after each pulse ”. Following laboratory tests, the team is challenged to deploy this system, which weighs one ton and is ten meters long, in the Swiss Alps at the top of Mount Säntis (2502 meters) to conduct an observation campaign. from June to September. , when storms are frequent. If the experiment is successful, it will be the first real demonstration that a laser can influence and guide a beam.
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