CONTROLLING lightning with lasers
LARGE SCIENTIFIC PROJECT
CONTROLLING LIGHTNING WITH LASERS
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Aurélien HOUARD Laboratoire d’ Optique Appliquée, ENSTA, École polytechnique, CNRS, Institut Polytechnique de Paris, Palaiseau, France * aurelien. houard @ polytechnique. edu
Lightning is one of nature ' s most extreme phenomena. As fascinating as it is destructive, it causes billions of euros worth of damage every year. The idea of using lasers to control lightning dates back to the 1970s, but it wasn ' t until 50 years later, with the European Laser Lightning Rod project, that it was demonstrated for the first time that powerful lasers could guide lightning over long distances.
https:// doi. org / 10.1051 / photon / 202513137
This is an Open Access article distributed under the terms of the Creative Commons Attribution License( https:// creativecommons. org / licenses / by / 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Every year, lightning is responsible for billions of euros in damage and over 20,000 deaths worldwide, triggering forest fires, power surges and disrupting air traffic at airports, since aircraft are highly vulnerable during take-off and landing. Lightning occurs spontaneously in large cumulonimbus clouds when convection of water particles creates space charges, leading to an accumulation of electrons at the bottom of the cloud. This negative electric charge produces an electric field of the order of 50 kV / m, sufficient to initiate downward discharges towards the ground or, in some cases, even upward discharges from a tall tower or mountaintop. The main existing method of lightning protection is the lightning rod, invented by Benjamin Franklin in 1752. Consisting of a metal rod up to 100 m high, the lightning rod can protect a ground installation up to a radius of 30 m by attracting lightning discharges passing nearby. To increase the range of action of the classical lightning rod, the idea of using a powerful laser was proposed by Ball et al. as early as 1974. Controlling an electrical discharge using light is a not trivial idea, as the speed of photons limits their interaction with the free electrons that form the discharge. To control the lightning flash, the laser has to modify the medium in which it propagates, i. e. the atmosphere. It can either ionize the air to make it conductive, simulating the effect of a metal lightning conductor that can attract discharges by bending the field potential, or it can create a preferential path for the discharge by heating the air. Both solutions rely on the laser ' s ability to ionize the air on its path over very long distances. The first experiments of this type were carried out in USA and Russia in the 1970s, using nanosecond pulsed lasers with an energy of several kilojoules. These high-energy lasers are capable of producing very dense but discontinuous plasmas channels, and the light energy required is very high( of the order of 600 J / m), which limits the length of the plasma column to a few meters. A first lightning guiding experiment was nevertheless carried out using this method in 1997 by
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