Laser ionization of air: radius and required power

[machtzu]

Hi

I am trying to work out what ionization occurs passing laser light through air. I am trying to find out what the radius of ionized air would be, what distance it would cover (eg 1m, 2m etc...) and what the power of the laser would have to be. How would I go about working this out? Does anyone know the mathematical relationship that would describe this?

Thanks for your help,

 

[Claude Bile]

The power of a laser is actually of little importance in this case, what is important is the beam quality of the laser. Basically, the higher the beam quality, the tighter you can focus your laser.

The important factor is the irradiance of the beam (W/m^2), not the overall power. For a 20 W laser, you need to focus the beam into a 5 micron spot, an overall irradiance of about 8 TW/m^2.

The range of ionisation would roughly be the Rayleigh Length of the focused beam. Because you are focusing the beam very tightly in order to ionise it, the Rayleigh range would be very small (of the order of mm, rather than m).

The radius would depend on the spot size of the laser.

In summary, you need to know;

The ionisation threshold for air.
The Rayleigh range of your laser beam.
The spot size of your laser beam.
The overall power of your laser beam.

The calculations are simple for a rough estimate, a little more involved to get an answer with precision.

Claude.

 

[x_engineer]

I assume that all the questions/answers are assuming the laser radiation is less than about 14eV, above which N2 ionizes by photo-ionization.