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Lasers: promising applications for research and beyond

Lightning rods and antennas: applications of laser filamentation

Isabelle Dumé, Science journalist
On June 29th, 2022 |
4 mins reading time
3
Lightning rods and antennas: applications of laser filamentation
Aurélien Houard
Aurélien Houard
Researcher at LOA* at ENSTA Paris (IP Paris)
Key takeaways
  • The phenomenon of laser filamentation has several applications such as laser booms, lightning rods, and antennas.
  • The idea of the latter is to replace metallic conductors, which are quite large, with plasma conductors produced with these femtosecond filaments.
  • For the lightning rod it is a matter of making a very long filament capable of guiding the lightning, and possibly triggering it before the storm cloud arrives near a sensitive site, such as an airport.
  • This laser effect works well in the laboratory, and scientists are working to improve its effectiveness at greater distances in air at atmospheric pressure.

The phe­nom­e­non of laser fil­a­men­ta­tion occurs dur­ing the prop­a­ga­tion of intense laser puls­es of fem­tosec­ond dura­tion. It can be exploit­ed for many appli­ca­tions: rev­o­lu­tion­ary improve­ments in opti­cal remote sens­ing for atmos­pher­ic sci­ence; super­son­ic laser booms to reduce the pres­sure of the shock wave at the front of an air­craft trav­el­ling through the atmos­phere at super­son­ic speed; laser light­ning rods to pro­tect against light­ning; or vir­tu­al plas­ma anten­nas for radio wave emission.

High peak pow­er laser beams induce sev­er­al impor­tant non-lin­ear effects that occur as they prop­a­gate through the air. These effects cause some of the beam ener­gy to spon­ta­neous­ly self-focus so that it con­cen­trates to form intense chan­nels of light called fil­a­ments – bands of light a few microns wide and up to sev­er­al metres long. This self-focus­ing occurs at laser pow­ers above a cer­tain thresh­old and increas­es the inten­si­ty of the beam to the point where atoms in the atmos­phere are ionised, gen­er­at­ing a plas­ma. Fil­a­ments are typ­i­cal­ly pro­duced when the laser beam has a peak pow­er of more than 5 gigawatts (GW). In sim­ple terms, laser fil­a­men­ta­tion is a spe­cial laser prop­a­ga­tion regime obtained when very intense beams have a dura­tion of only a few hun­dred fem­tosec­onds (10-15 s).

If the beam con­tains only a few mil­li­joules of ener­gy, all its pow­er will be self-focused into a nar­row beam and pro­duce a sin­gle fil­a­ment. How­ev­er, small vari­a­tions in cross-sec­tion­al inten­si­ty, togeth­er with air tur­bu­lence, make beams of a few cen­time­tres in diam­e­ter and of ener­gies on the order of joules to self-focus into mul­ti­ple fil­a­ments. The result is many fil­a­ments – up to 1,000 – dis­trib­uted more or less ran­dom­ly over the cross-sec­tion of the beam.

These fil­a­ments can be used for a vari­ety of appli­ca­tions – to guide and con­trol elec­tri­cal dis­charges, for exam­ple, since they cre­ate a pref­er­en­tial path for these dis­charges. They can be chan­nelled over a dis­tance of up to five metres in a straight line.

Virtual plasma antennas and laser light bars

These dis­charges could be used to make a type of anten­na that exploits the con­duc­tive prop­er­ties of straight dis­charges for trans­mis­sion in the radiofre­quen­cy domain. The idea here is to replace metal­lic con­duc­tors, which are quite big, with plas­ma con­duc­tors pro­duced with these fem­tosec­ond filaments.

The oth­er impor­tant appli­ca­tion is the laser light­ning rod. This device is sim­i­lar to the vir­tu­al plas­ma anten­na but extends over sev­er­al hun­dred metres. The idea is to make a very long fil­a­ment capa­ble of guid­ing light­ning and pos­si­bly trig­ger­ing it before the storm cloud arrives near a sen­si­tive site, such as an air­port. This tech­nique could help pro­tect these sen­si­tive tar­gets by divert­ing the light­ning to a cap­ture point. We are work­ing on this sub­ject in our lab­o­ra­to­ry as part of the Euro­pean “Laser Light­ning Rod”.

Pho­to­graph of the Mount Saen­tis in Switzer­land where the Laser Light­ning Rod project exper­i­ments are being car­ried out

We are try­ing to demon­strate this “light­ning guid­ance” under real con­di­tions – in the Swiss moun­tains. We have iden­ti­fied a site here where light­ning is very fre­quent and always occurs in the same place, which is ide­al for exper­i­ments. This would allow us to have more pos­si­ble light­ning events near our laser and to see if our tech­nique works with real lightning.

A supersonic laser boom

When we make the fil­a­ments – and our plas­ma – we heat the air, ionise the atoms and deliv­er laser ener­gy local­ly. This heat­ing occurs in a very uni­form line thanks to the fil­a­men­ta­tion process. We have recent­ly demon­strat­ed, using a scale mod­el air­craft in a wind tun­nel, that such fil­a­men­ta­tion heat­ing at the front of an air­craft trav­el­ling at super­son­ic speeds (Mach‑3) forms a bub­ble that deforms the shock wave on the nose of the air­craft. This reduces its drag by 20–50%. This heat­ing also reduces the ener­gy need­ed to move the air­craft for­ward, thus improv­ing its fuel consumption.

We are work­ing to devel­op this con­cept to find out if a long laser deposit gen­er­at­ed at a high­er rate can reduce the drag of the air­craft con­tin­u­ous­ly and if it can be used to con­trol its direction.

Pho­to­graph of the plas­ma fil­a­ment pro­duced upstream of the nose of a super­son­ic craft, in the wind tun­nels at ONERA Meudon

There are also sev­er­al appli­ca­tions relat­ed to the fact that fil­a­ments can pro­duce intense light with a very broad spec­trum at dis­tances of up to one kilo­me­tre. This can be inter­est­ing, for exam­ple, for a mul­ti-fre­quen­cy LiDAR (light detec­tion and rang­ing) sys­tem because many light fre­quen­cies are gen­er­at­ed in the fil­a­ment. These fre­quen­cies are scat­tered by the air par­ti­cles over a dis­tance of 1km and can be used to analyse the com­po­si­tion of the air.

In the con­text of LiDAR and opti­cal remote sens­ing, the plas­ma we cre­ate in the fil­a­ment can emit coher­ent radi­a­tion and act nat­u­ral­ly as a UV laser source. Mol­e­cules that are ionised or excit­ed in the plas­ma can ampli­fy a pho­ton emis­sion very strong­ly and the elon­gat­ed shape of the fil­a­ment makes this gain very direc­tion­al. If the inten­si­ty is high enough, one can excite the nitro­gen mol­e­cules in the air and so obtain a rel­a­tive­ly intense direc­tion­al and coher­ent ultra­vi­o­let emis­sion. So far, we have demon­strat­ed that this effect works well in the lab­o­ra­to­ry and we are work­ing to improve its effec­tive­ness at greater dis­tances in air at atmos­pher­ic pressure.

Références :

  • T. Pro­duit, P. Walch, C. Herkom­mer, A. Mosta­jabi, M. Moret, U. Andral, A. Sun­jer­ga, M. Azad­i­far, Y.-B. André, B. Mahieu, W. Haas, B. Esmiller, G. Fournier, P. Krötz, T. Met­zger, K. Michel, A. Mysy­row­icz, M. Rubin­stein, F. Rachi­di, J. Kas­par­i­an, J.-P. Wolf, A. Houard, The Laser Light­ning Rod project, The Euro­pean Phys­i­cal Jour­nal Applied Physics 93, 10504 (2021)
  • P.-Q. Elias, N. Sev­er­ac, J.-M. Luyssen, Y.-B. André, I. Doudet, B. Wat­tel­li­er, J.-P. Tobeli, S. Albert, B. Mahieu, R. Bur, A. Mysy­row­icz and A. Houard, Improv­ing super­son­ic flights with fem­tosec­ond laser fil­a­men­ta­tion, Sci­ence Advances 4, eaau5239 (2018)
  • A. Houard and A. Mysy­row­icz, Fem­tosec­ond laser fil­a­men­ta­tion and appli­ca­tions, Light Fil­a­ments: Struc­tures, chal­lenges and appli­ca­tions, Insti­tu­tion of Engi­neer­ing and Tech­nol­o­gy, pp.11–30 (2021)