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Scientists direct lightning with lasers for the first time | Physics

Scientists have shined lightning bolts with lasers for the first time in the field, according to a demonstration during heavy storms atop a Swiss mountain.

The feat, which involved firing powerful laser pulses at storm clouds for several months last year, paves the way for laser-based lightning protection systems at airports, launch pads and tall buildings.

“Metal rods are used almost everywhere to protect against lightning, but the area they can protect is limited to a few meters or tens of meters,” said Aurélien Houard, a physicist at the École polytechnique de Palaiseau. “The hope is to extend this protection to a few hundred meters if we have enough energy in the laser.”

Lightning bolts are huge electrical discharges that typically fire over two to three miles. The charge carried by a lightning bolt is so intense that it reaches 30,000°C, about five times hotter than the surface of the sun. More than a billion lightning strikes the Earth each year, causing thousands of dead10 times more injuries and damage amounting to tens of billions of dollars.

Traditional lightning rods date back to Benjamin Franklin who used to chase thunderstorms on horseback before his famous kite experience in 1752. But more recently, scientists have looked for other ways to protect buildings and objects from damaging impacts.

Write in the journal Nature PhotonicsHouard and his colleagues in Switzerland describe how they transported a powerful laser to the top of the Säntis mountain in northeastern Switzerland and parked it near a 124m-tall telecommunications tower which is hit by lightning about 100 times a year.

Scientists waited for thunderstorms to gather and between July and September last year fired fast laser pulses at storm clouds for more than six hours in total. Instruments set up to record the lightning strikes showed that the laser deflected the course of four lightning discharges upwards during the experiments.

A single strike, on July 21, occurred in conditions bright enough for researchers to film the path of lightning in two directions using high-speed cameras several miles away. The images show that the flash followed the path of the laser for about 50 meters, suggesting that the pulses helped direct the strike.

The laser deflects the lightning creating an easier path for the electrical discharge to flow. When laser pulses are sent into the sky, a change in the refractive index of the air causes them to shrink and become so intense that they ionize the air molecules around them. This leads to a long chain of what the researchers call filaments in the sky, where air molecules quickly heat up and move away at supersonic speeds, leaving a channel of low-density ionized air. These channels, which last a few milliseconds, are more electrically conductive than the ambient air and thus form an easier path for lightning to follow.

The laser is powerful enough to pose a hazard to the eyes of overflight pilots, and during the experiments air traffic was closed over the test site. But scientists believe the technology could still be useful, as launch pads and airports often have designated areas where no-fly restrictions apply. “It’s important to consider this aspect of security,” Houard said.

More powerful lasers that operate at different wavelengths could guide lightning over longer distances, he added, and even trigger lightning before it becomes a threat. “You avoid him going somewhere else where you can’t control him,” Houard said.

“The cost of the laser system is very high compared to that of a single rod,” said Professor Manu Haddad, director of the Morgan-Botti Lightning Laboratory at Cardiff University. “However, lasers could be a more reliable means of directing lightning discharge, and this may be important for lightning protection of critical ground facilities and equipment.”

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