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# Controlling lightning with lasers

Aurélien Houard

► To cite this version:

Aurélien Houard. Controlling lightning with lasers. Photoniques, 2025, 131, pp.37-41. 10.1051/photon/202513137. hal-05073922

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Submitted on 19 May 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# CONTROLLING LIGHTNING WITH LASERS

## Aurélien HOUARD

Laboratoire d'Optique Appliquée, ENSTA, École polytechnique, CNRS, Institut Polytechnique de Paris, Palaiseau, France

*aurelien.houard@polytechnique.edu

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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 pulses 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
Uchida *et al*. in Fukui, Japan. The researchers twice observed a possible laser effect on lightning over a distance of 1 to 2 m, using a combination of several kilojoule nano-second lasers focused above a tall metallic tower [1].

A major breakthrough came with the development of amplified femtosecond lasers [2]. When these ultrashort pulsed lasers are focused in the air, they generate very long and uniform plasma filaments that can reach hundreds of meters with relatively moderate laser energies. This process known as filamentation is due to a combination of Kerr self-focusing and plasma defocusing that

saturate the laser intensity inside the filament. This discovery, at the end of the 90s, quickly triggered the launch of lightning control projects in Quebec with a project piloted by INRS and Hydroquebec, and in Europe with the Franco-German Teramobile project. Numerous laboratory studies were carried out with discharges of several meters produced by Marx generators (see example in Figure 1). In 2005, the first real-life campaign was organized in New Mexico by researchers from the Teramobile project [3]. The experiment consisted in producing plasma filaments at several hundred meters altitude

using a TW laser operating at a repetition rate of 10 Hz. In the presence of thunderclouds, the laser was expected to guide or trigger lightning. Unfortunately, very few thunderstorms occurred during the campaign, which nevertheless enabled them to observe the emission of RF signals due to the formation of filament induced corona discharges during the thunderstorms.

## The laser lightning Rod project

The Laser Lightning Rod (LLR) project was launched 10 years later, when new laser technologies based on amplification in Yterbium:Yag crystals allowed to increase the laser repetition rate by a factor of 100. These new ultrashort lasers, developed by Trumpf scientific in Germany, were capable of maintaining a permanent hot-air channel by generating plasma filaments at high repetition rate. This greatly increased the chances of intercepting and guiding a lightning bolt. Under the impetus of the Applied Physics group at the University of Geneva and the Laboratory of Applied Optics in Ecole polytechnique (which were already the main players in the Teramobile project) a consortium was then formed with the German company Trumpf scientific, Ariane group, interested as an end user, and lightning experts from EPFL and from the University of Applied Sciences and Arts Western Switzerland. The project officially kicks off in January 2017 with a funding of €4 million from the European commission.

The site chosen for the experiment was the other key to the project's success. Selected by EPFL, it is a 120 m high radio transmission tower on the summit of Mont Saentis (Appenzell, Switzerland, 2502 m), operated by Swisscom. The Saentis tower is one of the most frequently struck by lightning in Europe, but above all, due to the presence of the mountain

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Photograph of a 2 m-long electric discharge guided by laser filamentation (left) and without laser filament (right).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Photography of the laser developed by Trumpf scientific for the LLR project. (Copyright LLR project).</figcaption></figure>

and the tower, the lightning that strikes the tower develops into ascending lightning precursors that start at the top of the tower. Known as upward leader, these discharges are therefore easily intercepted by the laser. This configuration is similar to that used in the pioneering experiment by Uchida *et al*. in the 90s. The first challenge was to build the world's most powerful high-average high-peak power ultrafast laser, with the aim of delivering laser

pulses with an energy of 1 Joule, a duration < 1 ps at a repetition rate of 1 kHz. Trumpf's laser prototype is 1.5 meters wide, 8 meters long and weighs over 3 tons (see Figure 2). Its 5-block design means it can be transported by a cable car to the summit of Saentis, which is not accessible by car. The first two years of the project were devoted to the development of the laser by Trumpf, and to the study of lightning events at the Mont Saentis in the absence of laser.

Achieving an energy of one joule per pulse proved more difficult than expected, due to the strong thermal effects that occur at high power. In the end, including 5 amplification stages, the laser still achieved a record energy of 720 mJ at 1060 nm. After two years of development, the first tests with the laser were carried out at the former Orsay Linear accelerator laboratory in France, to characterize and optimize the filaments produced at 150 m using an enlarging telescope. Stopped for almost a year by the COVID pandemic, the project then resumed in 2021 for the final experiment, where the laser and its transport and focusing device were installed at the top of Mount Saentis.

## The lightning experiment

Supervised by the University of Geneva, this 4-month installation on the top of Mount Saentis proved to be particularly complex. On the one hand, the laser had to be installed in an air-conditioned environment with filtered air, because at high average power, dust in the amplifiers can easily damage the entire chain. It

## OPTOMECHANICS FOR INTENSE LASER

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DEFORMABLE MIRRORS

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BEAM TRANSPORT

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TARGET HOLDER

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isp-system.fr

contact@fr-ispgroup.com
a

<figure><img src="image_11.png" /><figcaption>**Figure 3.** Images of the laser guided lightning flash recorded by two cameras over Mont Saentis on July 24 2021. Reproduced from [4]. CC BY 4.0</figcaption></figure>

b

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in Figure 3 were recorded. It can
be clearly seen that over 60 m, the
lightning bolt, which starts at the top

was also necessary to build a 3 m high telescope shelter to direct and focus the laser beam, and to cover the entire laser path between the building and the sending telescope. The installation of this shelter required the helicopter transport of two 10' high containers and 20 tons of concrete to stabilize the structure, which had to withstand winds of 80 km/hour. In addition, a special device was designed to protect the telescope's exit optics from the heavy rain that accompanied the storms, and the whole experiment had to be connected to a generator during each storm, to protect against frequent power cuts. The laser was then activated whenever thunderstorm activity was forecast, between July and September 2021. Beforehand, the area had to be closed to air traffic for a 5 km radius around the laser beam.

Once all these problems had been
resolved, the experiment could be-
gin in mid-July 2021. A week later,
the first bolt of lightning struck the
tower while the laser was in ope-
ration. Luckily, that evening, the
cloud ceiling was very high and the
lightning was perfectly visible from
the neighboring peaks. Using two
high-speed cameras positioned at
different angles, the images shown

of the tower, follows the path of the la-
ser beam. This is precisely the length
over which the laser beam produces
plasma filaments with this tele-
cope configuration. Subsequently, a
similar guiding effect was observed
during three other lightning events,
using a VHF detector capable of
tracking the movement of lightning
tracers in clouds, when the cloud pre-
vents the lightning from being seen
with a camera (see Figure 4).
This experiment therefore demons-
trated that an intense ultrashort la-
ser beam was capable of repeatedly
guiding lightning over a distance of
more than 60 m, even under difficult
weather conditions, as the mountain
was regularly shrouded in dense fog.
Many questions remain unanswered,
concerning the reproducibility of
this effect, or the ability of the laser
to stimulate the number of lightning
discharges. But these questions

The laser system was located in the radome building, sheltered in an
air-tight, air-conditioned and thermally isolated tent (see image).
After exiting the tent, the laser beam was directed downwards
by a conduit through the radome wall to the terrace, where a 4"
folding mirror directed the beam into a beam-expanding sending
telescope. The entire laser path toward the telescope, was
protected by an isolated aluminium housing to prevent any beam
leakage and to reduce the perturbation from the environment.

<figure><img src="image_13.png" /><figcaption>Layout of the experimental setup installed on Mont Saentis (Adapted with permission from [4].) CC BY 4.0</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure 4.** 2D maps of the VHF sources emitted during a lightning event with the laser on (a) and without laser (b). The laser path is shown in red. Reproduced from [4]. CC BY 4.0</figcaption></figure>

would need more statistics to be answe-
red, and installation difficulties meant
that we were unable to take advantage
of a large proportion of the season's
lightning events. Therefore, additional
experimental campaigns would be ne-
cessary to qualitatively assess the guiding
efficiency of the laser filaments.

Conclusion

The Laser lightning rod experiment de-
monstrated that a high-repetition rate
terawatt laser could guide lightning

over 60 m by forming plasma filaments
in the air. For lightning protection ap-
plications, the next step is to increase
the length and guiding efficiency of
the laser filaments, for example by
doubling the laser frequency to obtain
more energetic photons. Another im-
portant topic in the control of lightning
will consist in the demonstration of
lightning triggering with lasers. This
should require the formation of long-li-
ved conductive plasma channels, but
their formation over long distances
remains a real challenge [5]. ●

REFERENCES

[1] S. Uchida, Y. Shimada, H. Yasuda, et al., J. Opt. Technol. 66, 199 (1999)

[2] X. M. Zhao, J.C. Diels, C. Yi Wang and J.M. Elizondo, IEEE J. Quantum Electron. 31, 599 (1995)

[3] A. Mysyrowicz, J.P. Wolf, Photoniques **127**, 32 (2024)

[3] C. Herkommer, P. Krötz, R. Jung, *et al*. *Opt. Express* **28**, 30164 (2020)

[4] A. Houard, P. Walch, T. Produit, *et al*. *Nat. Phot.* **17**, 231 (2023)

[5] T. Produit, J. Kasparian, F. Rachidi *et al*. *Rep. Prog. Phys.* **87**, 116401 (2024)

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