Re: A laser pointer killed my coil?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

At 02:22 PM 1/21/2003 -0700, you wrote:
>Original poster: "Chris Roberts by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <quezacotl_14000000000000-at-yahoo-dot-com>
>
>
>The main reason I thought something happened was because someone told me 
>that the laser ionizes the air and makes a path for the high voltage to 
>follow. I doubt this, it seems like a rumor. Just wanted to see if anyone 
>else had heard of this.

Only if the power of the laser is high enough to ionize the air.  That's a 
pretty high powered laser.. The E field required to ionize air is 3E6 
V/m.  Assuming the laser light is a plane wave, the corresponding power 
density is S=E^2/3770 (giving power density in mw/cm^2, for E in V/m)... 
Let's see..

That's the peak value for ionization, and the power density is in terms of 
RMS value, so we need to scale the 3 million down by .707 (1/sqrt(2)

(3E6*.707) ^2 = 4.5E12
So.. S = 4.5E12/3.77E3 = 1.19E9 mW/cm^2 or, about 1.19 MW/cm^2

Now, your laser beam is a lot smaller than a square cm.. say 1mm in 
diameter, or about 0.78 mm^2= .0078 cm^2

You'd need a mere 9.3 kW in the beam to get breakdown to occur.

In reality, at that kind of beam power, the losses in the air cause enough 
heating to result in "thermal blooming" due to the refractive changes. Also 
as the air gets hotter, it's density changes, reducing the breakdown field 
required.

I've seen photographs of a 20 kW CO2 laser with air breaking down along the 
beam, which was substantially larger than 1mm in diameter, so my 9 kW 
estimate, above, was pretty pessimistic.

However, 9 kW is a long, long way from, say 5-10 mW from your laser pointer.

Note also though that a UV laser with a very high powered pulse could knock 
a lot of electrons off the top load and provide a "virtual breakout point". 
1 MW pulses are easy to come by with pulsed N2 lasers, but the pulses are 
fairly short (few nanoseconds), and probably don't last long enough to get 
breakdown started.