Laser to direct lightning

Summer 2021, a storm is brewing over the Alpstein massif in eastern Switzerland. At the Säntis meteorological station, the highest point in the region, Aurélien Houard, of the École Polytechnique, and his colleagues from the European Union prepare to materialize an idea worthy of a science novel. Fantasy: Controlling lightning by pointing a laser in the sky.

This idea, while surprising, is not new. In 1974, Leonard Ball of the American Optical Association first came up with the idea of ​​shooting and aiming lightning using lasers. However, it wasn’t until 2004 that the first experimental attempts at natural lightning finally saw the light of day, without success for nearly twenty years.

To understand the principle of a “laser lightning rod,” we must go back for a moment to the nature of lightning. During a thunderstorm, the lower part of cumulonimbus clouds is negatively charged, while the upper part is positively charged. This results in a voltage of several tens of millions of volts between the base of the cloud and the ground. Normally, air is not a conductor, but above a specific so-called “breakdown” voltage, electrons are torn from the molecules and current can then flow. Moving in tremors, a column of highly conductive air called a “tracer” separates away from the cloud (or from a high point on the ground) by means of cascading avalanches, tracing the least isolated path through the atmosphere. As it passes through, the tracer ionizes the air, turning it into a plasma (a state of matter in which electrons circulate freely). At the same time, the electric charges in the detector attract opposite charges to the ends of buildings and trees, from which new detectors appear that go to meet the first. When the tracers gather together, the ground and the cloud are connected by a conductive plasma channel, ready to receive a discharge of up to 200,000 amps: lightning.

Because of the nature of lightning, we understand the action of a lightning rod—essentially a metallic point connected to the ground by a conductive wire—a device first imagined by Benjamin Franklin in 1752. Another strategy is to create plasma in the air in an artificial, controlled way to drive a flash in formation: this is the path High energy laser.

To create such a plasma, Aurélien Houard and his colleagues used a technique based on the laser filament principle, discovered by French Nobel laureate, Gerard Moreau in 1994. An intense laser pulse changes the refractive index of the medium it passes through, causing it to self-focus in filaments capable of ionizing air and plasma formation there. Along the filament, air rapidly evolves into a hot ion-laden channel, providing a distinct path for lightning to propagate.

In previous attempts, researchers applied this principle, but to no avail. To produce enough power when a filament of ionized air emerges, laser energy must in fact be emitted for a very short time, typically on the order of a billionth of a second (picosecond). pulse time, a terawatt force (one thousand billion joules per second) can thus be reached. The difficulty is to emit enough pulses per second to keep the plasma filament stable, which only last about one millisecond. Previous experiments reached a rate of 10 beats per second, which was not enough.

The German partners of the Trumpf Scientific Lasers consortium, after three years of development, have succeeded in developing a suitable laser, with a frequency of 1 kHz (one thousand shots per second). By pointing this laser generated by a massive 14-cubic-meter device skyward, the team managed to keep 100-meter-long strands of ionized air atop the Säntis communications tower. This 124-meter-high tower is an exceptional site for observing lightning as it strikes an average of 100 times a year. In the summer of 2021, during the time the laser was on, the tower was struck four times by lightning. Each of these flashes follows the path traced by the laser, at a distance of up to 50 metres.

In Leonard Ball’s original idea, a laser could theoretically trigger a cloud discharge. So far, the team has not been able to reproduce this phenomenon outside of controlled laboratory conditions. But this would provide the possibility of causing lightning in a targeted manner and thus protecting vulnerable areas. Every year, in fact, in France 100 to 300 people are struck by lightning and the material damage amounts to hundreds of millions of euros. However, laser lightning rods are very energy-intensive and require particularly sophisticated bulky equipment. We have yet to settle for a while with the simplicity of the device inherited from Benjamin Franklin!

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