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The laser in action. (TRUMPF/Martin Stollberg)
Don't try this at home, but beaming a laser into the
sky could avert lightning strikes, according to a new study from a team of
scientists who experimented with the lasers atop a Swiss mountain where a great
big metal telecommunications tower stands.
Physicist Aurélien Houard, from the French National
Center for Scientific Research's Applied Optics Laboratory in Paris, and
colleagues weathered hours of thunderstorm activity to test whether a laser
could guide lightning strikes away from critical infrastructure. The
telecommunications tower is struck by lightning about 100 times a year.
That's similar to the number of lightning flashes
that strike planet Earth or crackle between clouds every second. Collectively,
those strikes can cause billions of dollars of damage to airports and
launchpads, not to mention people.
Our best protection against lightning strikes is a
Franklin rod, nothing more than a metal spire invented in the 18th century by
Benjamin Franklin, who discovered lighting strikes are zig-zagging bolts of
electricity. Those rods connect to metal cables that run down buildings and
anchor into Earth, working to dissipate lightning's energy.
Houard and colleagues wanted to devise a better way
to protect against lightning strikes, fighting electricity with light.
"Although this research field has been very
active for more than 20 years, this is the first field result that
experimentally demonstrates lightning guided by lasers," they write in
their published paper.
With an increase in extreme weather events driven by
climate change on the radar, lightning protection is becoming increasingly
important.
The experimental campaign ran over the summer of
2021 from Säntis mountain in northeastern Switzerland. Short, intense laser
pulses were cast into the clouds during a series of thunderstorms and
successfully diverted four upward lightning discharges away from the tower's
tip.
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The laser and telecommunications tower on the summit
of Säntis mountain in Switzerland. (TRUMPF/Martin Stollberg)
Another 12 lightning strikes hit the tower during
those thunderstorm periods when the laser was inactive.
On one occasion, when the sky was clear enough to
capture the action on two separate high-speed cameras, a lightning strike was recorded
following the path of the laser for 50 meters (164 feet).
Sensors on the telecommunications tower also
recorded the electric fields, and X-rays generated to detect lightning activity
and corroborate its path, which you can see reconstructed in the video below.
For an idea first put forward in 1974 and tested
extensively in the lab, it's thrilling to see it finally work as designed in
the real world. Several earlier field trials, one in Mexico and another in
Singapore, had failed to find any evidence that lasers could deflect lightning
strikes.
"These preliminary results should be confirmed
by additional campaigns with new configurations," write Houard and
colleagues.
While the researchers are still figuring out why the
lasers worked in their trials but not in earlier experiments, they have a few
ideas. The laser Houard and colleagues used fires up to a thousand pulses per
second, much faster than other lasers used, allowing the green beam to
intercept all the lightning precursors forming above the tower.
But the recorded laser events only appeared to
divert positive lightning flashes, which are produced by a positively charged
cloud and generate negatively charged upward 'leaders'.
So how does it work?
As Houard and colleagues explain in their paper, the
laser sent skyward changes the light-bending properties of the air, causing the
laser pulse to shrink and intensify until it begins to ionize air molecules.
This process is called filamentation.
The air molecules are rapidly heated along the path
of the laser, absorbing its energy, then expelled at supersonic speed. This
leaves behind 'long-lived' channels of less dense air that offer a pathway for
electric discharges.
"At high laser repetition rates, these
long-lived charged oxygen molecules accumulate, keeping a memory of the laser
path" for lightning to follow, the researchers write.
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The experimental setup (left) and an image (right)
showing the filamentation zone above the tower. (Houard et al., Nature
Photonics, 2023)
Meters-long electric discharges had been steered by
lasers in the lab, but this is the first time the technique has worked in a
thunderstorm. Laser conditions were adjusted so that the initiation of
filamentary behavior started just above the tip of the tower.
"This work paves the way for new atmospheric
applications of ultrashort lasers and represents an important step forward in
the development of a laser-based lightning protection for airports, launchpads
or large infrastructures," Houard and colleagues conclude.
Reference: Nature Photonics.
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