Would a very powerful laser shot through air create a flame? If so, what might it look like?
Asked
Active
Viewed 100 times
0
-
3Just pure air? No fuel? It won't be a "flame," but it can ionize air and make a plasma. No different than lightning. – tpg2114 Dec 13 '16 at 03:28
-
1The US air force tried this a while back, as a missile interception system. Either they lacked power or the beam dispersed. It should be on Wikipedia – Dec 13 '16 at 03:40
-
1@CountTo10 Ronald Regan did a wee bit of dabbling in this field. I seem to recall 30 billion 1980s US dollars worth of dabbling. – Selene Routley Dec 13 '16 at 05:55
-
@tpg2114 What might that plasma look like in the seconds around the laser blast? – B T Dec 13 '16 at 08:18
1 Answers
2
Not really a "flame" per se, but a plasma, yes.
This is easily accomplished in a small lab, and it is part of experimental education at bachelor or master degree in many universities. With an intense-enough laser (which requires wearing eye-protecting goggles), air will be ionized. Whereas some answers like this seem to calculate that a laser cannot ionize air, the reasoning is incomplete. It is true that the energy of individual photons cannot overcome the binding energy of the electrons linked to atoms. However, putting many photons together can do that.
To achieve that, you must have a pulsed laser that concentrates its power both in space and time. It must be compressed in time and focused in space. Typically, a laser of wavelength $1 \mu$m with a pulse duration of 1ns, focused on a spot of $20 \mu$m, and an energy-per-pulse of 1mJ should suffice.
When ionization happens, air becomes conductive like a metal because there are electrons moving around freely. These get high velocities and create currents which, in turn, produce transitions in the bound electrons, thus emitting light (just like in a neon tube). In the end, it looks like little sparks.
