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April 2005 | IEEE Spectrum | NA 21

FILAMENT OF FIRE: The Teramobile laser [left]

shoots pulses from a shed to the space between an

electrode charged by a generating tower and a grounded

electrode [not shown]. The pulse ionizes the air,

creating a conductive filament. Artificial lightning

follows the filament’s straight path [below].

Stealing Celestial FireA laser has sparked artificial lightning in a laboratory, a first step toward controlling real thunderbolts

There has to be a better way oftapping a lightning bolt thanflying a kite in a storm, and agroup of French and Germanscientists just may have foundone. They have demonstratedin a laboratory that shiningpowerful laser pulses betweentwo electrodes elicits acontrollable form of lightning.They hope that their inventionwill eventually help to fend offlightning strikes on airportsand power stations.

They employed theirTeramobile laser, whose pulselasts for a mere 100 femtosec-onds and packs a peak power of5 terawatts. The pulse rips theelectrons from air molecules,creating a plasma; it alsochanges the refractive index ofthe air, a phenomenon calledthe Kerr effect. The effectfocuses the light just enough tobalance plasma-induceddiffraction, creating a straightand highly conductive channel,called a filament, which canstretch up to 3.8 meters be-tween the charged electrodes.

In the experiment, a 1- to2-megavolt electrode simulates

a thundercloud and an electri-cally grounded plate simulatesthe earth. The laser-inducedfilaments short-circuit theelectrodes, triggering an electricdischarge much like a lightningbolt. To complete the simula-tion, the researchers sprayedwater between the electrodes.“We expected that the ‘rain-water’ would scatter light andperturb the filaments, but thefilaments survived the interac-tion,” says Jérôme Kasparian, alead investigator and a memberof the group from the Univer-sity of Lyon, in France. Anotherteam hails from the ÉcolePolytechnique, in Palaiseau,France; two others come fromthe Free University of Berlinand the Friedrich SchillerUniversity, in Jena, Germany.

The main problem is theplasma’s milliseconds-longlife, which limits the distancebetween the two electrodes to10 meters. Kasparian says thatthe research groups hope tokeep the plasma going longerby heating it with a secondlaser. Still, they are far fromgetting filaments that could

span the 5 to 6 kilometersseparating a thundercloudfrom the earth.

Scientists have tried andfailed to manipulate lightningwith lasers for 20 years. Onereason, Kasparian says, is thatall experiments so far haveemployed lasers with relativelylong pulses of high energy,which create plasmas so densethat they scatter the laser light,preventing the formation of afilament. The Franco-Germanteam solved this problem withthe Teramobile, the first mo-bile laser ever to emit ultra-short pulses of moderate energy.The titanium-sapphire laserwas designed to track airpollution by analyzing lightthat bounces off atmosphericgases, but the researchersrealized that it could also pro-vide lightning “on demand.”

Matters don’t seem thatsimple to Martin Uman,director of the LightningResearch Laboratory at theUniversity of Florida, inGainesville. Every summer for11 years, he and his group havelaunched about 70 rockets

trailing grounded wire intothunderclouds, drawing light-ning half the time. “The cloudcharges itself back up in about10 seconds. You don’t know ifyou’re going to put the rocket[or the laser beam] in theright place,” he says. “Firinglasers or rockets is not goingto make any difference towhat’s going on up there. Youprobably have to go a stepfurther back and understandthe physics behind it.”

To understand lightning,however, you need to be ableto produce it reliably and fre-quently, and here lasers mighthave advantages over rockets.Typical rocket racks allow onlyup to 10 launches, Kasparianpoints out, while the lasertechnique could provide a“virtually unlimited number ofshots” at triggering lightning.

If lasers ever write glowinglines high in the sky, it shouldbe possible to control thecourse of lightning bolts. Thenpeople will not merely talkabout the weather, but dosomething about it.

—PRACHI PATEL PREDD