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| Image of the 124-m-high telecommunication tower of Säntis (Switzerland). Also shown is the path of the laser recorded with its second harmonic at 515 nm. Credit: Nature Photonics (2023). DOI: 10.1038/s41566-022-01139-z |
Forest fires, power cuts and damaged infrastructure…lightning fascinates and
destroys in equal measure, causing as many as 24,000 deaths a year worldwide
not to mention widespread destruction. Even today, the lightning rod
invented by Benjamin Franklin is the best form of protection. And yet, these
rods do not always provide optimal protection for sensitive sites.
A European consortium consisting of the University of Geneva (UNIGE), École
Polytechnique (Paris), EPFL, hes-so and TRUMPF scientific lasers (Munich)
has developed a promising alternative: the Laser Lightning Rod or LLR. After
testing the LLR on the summit of Säntis (in Switzerland), the researchers
now have proof of its feasibility. The rod can deflect lightning over
several dozen meters even in poor weather. The results of this research are
published in the journal Nature Photonics.
Lightning is one of the most extreme of natural phenomena. An abrupt
electrostatic discharge of millions of volts and hundreds of thousands of
amperes, lightning can be observed in a single cloud, between several
clouds, between a cloud and the ground and vice-versa. As fascinating as it
is destructive, lightning is responsible for up to 24,000 deaths a year.
From power cuts and forest fires to damaged infrastructure, it also causes
extensive devastation totaling several billion dollars.
Lightning-protection devices have changed little since 1752 when Benjamin
Franklin invented the lightning rod—a pointed, conductive mast made of metal
connected to the ground. The traditional rod is to this day still the most
effective form of external protection: it protects a surface with a radius
that is more or less equal to its height.
So, a 10 m-high rod will secure an area with a 10 m radius. However, since
the height of the masts is not limitlessly extendable, it is not an optimal
system for protecting sensitive sites over a wide area, such as an airport,
wind farm or nuclear power plant.
Making the air conductive
A European consortium led by UNIGE and École Polytechnique (Paris) has been
looking at how to solve this problem in close partnership with EPFL (EMC
Lab, Prof. Farhad Rachidi), TRUMPF scientific lasers, ArianeGroup, AMC
(Prof. A. Mysyrowicz) and the School of Engineering and Management (hes-so,
Prof. Marcos Rubinstein).
It has been working on a device known as the Laser Lightning Rod (LLR). By
generating channels of ionized air, the LLR was used to guide lightning
along its beam. Extending upwards from a traditional lightning rod, it could
increase its height virtually as well as the surface of the area it is
protecting.
"When very high power laser pulses are emitted into the atmosphere,
filaments of very intense light form inside the beam," begins Jean-Pierre
Wolf, full professor in the Department of Applied Physics in the Physics
Section of UNIGE's Faculty of Science, and the study's last author. "These
filaments ionize the nitrogen and oxygen molecules in the air, which then
release electrons that are free to move," continues Professor Wolf. "This
ionized air, called 'plasma,' becomes an electrical conductor."
Tests at an altitude of 2,500 m
The LLR project meant that a new laser had to be developed with an average
power of one kilowatt, one Joule per pulse and a duration per pulse of one
picosecond. The rod is 1.5 m wide, 8 m long and weighs more than 3 tons, and
was designed by TRUMPF scientific lasers. This terawatt laser was tested on
the summit of Säntis (in Appenzell, at a height of 2,502 m) already
instrumented by EPFL and HEIG-VD / HES-SO to observe lightning.
It was focused above a 124 m transmitter tower belonging to the
telecommunications provider Swisscom, which was equipped with a traditional
lightning rod. This is one of the structures most affected by lightning in
Europe. "The main difficulty was that it was a life-size campaign. We had to
prepare an environment in which we could install and protect the laser,"
says Pierre Walch, a Ph.D. student in the Laboratoire d'Optique Appliquée
(LOA), a joint research unit CNRS, École Polytechnique, ENSTA Paris,
Institut Polytechnique de Paris, Palaiseau, France.
The laser was activated every time storm activity was forecast between June
and September 2021. The area had to be closed to air traffic in advance.
"The aim was to see whether there was a difference with or without the
laser," explains Aurélien Houard, a research scientist in the Laboratoire
d'Optique Appliquée (LOA) and coordinator of the project. "We compared the
data collected when the laser filament was produced above the tower and when
the tower was struck naturally by lightning."
Effective even through cloud
It took almost a year to analyze the colossal amount of data collected. This
analysis now shows that the LLR laser can guide lightning effectively.
Professor Wolf further explains, "From the first lightning event using the
laser, we found that the discharge could follow the beam for nearly 60
meters before reaching the tower, meaning that it increased the radius of
the protection surface from 120 m to 180 m."
The data analysis also demonstrates that the LLR, unlike other lasers, works
even in difficult weather conditions—such as fog (often found at the summit
of Säntis), which can stop the beam—since it literally pierces the clouds.
This outcome had previously only been observed in the laboratory. The next
step for the consortium will be to increase the height of the laser's action
even further. The long-term objective includes using the LLR to extend a 10
m lightning rod by 500 m.
Reference:
Aurélien Houard, Laser-guided lightning, Nature Photonics (2023).
DOI: 10.1038/s41566-022-01139-z.