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Femtosecond laser takes aim at thunderclouds

18 Apr 2008

Aiming high-power pulses of laser light into a thunderstorm has helped a French–German collaboration to deliberately trigger electrical activity in thunderclouds.

Researchers could soon have a better understanding of lightning strikes thanks to a mobile terawatt laser that probes thunderclouds with femtosecond pulses. The team has developed a mobile terawatt laser known as "Teramobile", which it claims has induced electrical activity in thunderclouds for the first time. (Optics Express 16 5757).

"The Teramobile laser emits 400 TW peak power pulses with a duration of 100 fs and pulse energy of 400 mJ and is the first mobile laser of its class," Jérôme Kasparian, a researcher from the University of Lyon, told optics.org. "The present results are very encouraging because they are the first demonstration of the effect femtosecond pulses can have on thunderclouds."

Conventional methods of triggering lightning use an electrically conductive channel to provide a path of least resistance to a potential discharge such as rockets pulling wires or classic lightning rods. The Teramobile team has instead used high intensity laser pulses to create an electrically conducting plasma channel.

"The plasma channel is a path of ionized air (known as a filament), which we found behaved like any conductor pointed towards thunderclouds," explained Kasparian. "We observed the preliminary step of a lightning strike (known as "corona discharge") at its tip and by adjusting the laser's energy, power, focusing and pulse sequence we may trigger a fully developed lightning discharge."

Ionized filaments are uniquely observed in the non-linear propagation of ultrashort, high-intensity laser pulses. "A dynamic balance is established between the self-focusing of the beam, which is due to the higher intensity in its centre increasing the refractive index," explained Kasparian. "This is balanced with the defocusing by the plasma generated by multiphoton ionization."

The group believes that the key to its success is the combination of a mobile laser that combined high-power pulses at moderate energy. Until then, previous attempts used lasers with longer pulses.

This femtosecond laser emits with enough pulse power to allow the laser light to self-guide and create a plasma channel that ionizes the air over large distances. "The ultrashort laser pulses concentrate their energy into a huge power, which leads to 100 µm thin channels of plasma that propagate over distances greater than 100 m," commented Kasparian.

The team plans to use the observable effect to establish which laser parameters improve the effect. It also intends to increase the plasma lifetime and density by building a new laser that will be 10 times more powerful and able to send out bursts of pulses rather than single pulses.

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