Lightning rods and antennas : applications of laser filamentation

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.

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”.

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.

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.

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)