TEA Laser Question

[amdx]

I have a question about the laser design on the url below.

http://technology.niagarac.on.ca/staff/mcsele/lasers/LasersTEA.htm

If you scroll down to the Prototype Design section and note the drawing.
It shows a large plate capacitor with one bottom plate and
a split (top) plate. In effect two plates.
I'll describe the undrawn components: A hv dc power supply is connected
across the spark gap through a 100k ohm resistor.
An inductor is placed across the (top) split plates giving them a dc path.
As I see it the the hv supply charges the cap ( both halves of the split),
when the voltage gets high enough, the spark gap arcs. This shorts the top
left plate to the bottom plate. The left plate goes to zero potential and so
the right plate arcs across the lasing gap.

My question; why not eliminate the left plate and just short the left
lasing electrode to the bottom plate through the spark gap?
Thanks, Mike

 

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quote:
My question; why not eliminate the left plate and just short the left
lasing electrode to the bottom plate through the spark gap?
Thanks, Mike

beginners explanations simplifing a lot of the physics:

look up "traveling wave laser" or "Blumlein circuit", although that is
not a true "Blumlein"

A hypothetical quickie for beginners explanation (but very wrong) is
by collapsing the voltage on the smaller capacitor (left plate)very
fast, you can a even shorter discharge surging back across the main
gap.

If the spark gap is at one corner of the laser, and the capacitor
electrode is properly shaped, you can get the discharge energy to
"walk" down the electrodes nearly in phase with the lasing action, but
that is not the case here.


google "Sam's Laser FAQ," and look up nitrogen lasers.
Steve Roberts

 

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A little bit better explanation, as the left plate discharges its
voltage swings below zero, as the main gap starts to conduct, so you
now have a faster discharge with a much greater voltage across the
gap, The nitrogen's laser upper state is just a few nanoseconds
long, you need maximal energy transfer in the shortest possible arc
time to excite it.
The long plates drasticly lower the inductance of the circuit.

Steve Roberts

 

[amdx]

A little bit better explanation, as the left plate discharges its
voltage swings below zero, as the main gap starts to conduct, so you
now have a faster discharge with a much greater voltage across the
gap, The nitrogen's laser upper state is just a few nanoseconds
long, you need maximal energy transfer in the shortest possible arc
time to excite it.
The long plates drasticly lower the inductance of the circuit.

Steve Roberts

Thanks for the info, it sounds like you get ringing
and the second swing is negative with respect to the bottom plate so the
lasing gap potential is increased.
I don't see how the second (left plate) helps cause that
action to happen.
Mike

 

[Carl Ijames]

The spark gap acts like a small inductor (tens of nH) in series with a
small resistor (maybe 50 ohms?). The left cap and this inductance form
a series LC circuit that in the absence of the main laser gap and right
cap will ring down as an exponentially damped sine wave with peak
positive value with respect to ground equal to the inital charge
voltage, and peak negative value slightly less (due to loss in the
"resistor") than minus the initial charge value, so the sine wave is
basically centered on zero. The inductance connecting the left and
right caps for charging is much larger than this so on this ringing
timescale the tops of the left and right caps are only connected by the
main laser discharge gap. The maximum voltage across the laser gap is
slightly less than twice the charging voltage, but usually the gap
voltage will never reach this value. Instead, once the gap voltage
exceeeds the breakdown voltage (determined by the gas composition and
pressure, electrode spacing and shape, and any predischarge ionization)
the gap voltage will fall to some low value (hundreds of volts, maybe)
and the discharge gap current will quickly rise as both caps discharge
across the gap. This current risetime must be faster than the lifetime
of the upper lasing level to get efficient excitation and a population
inversion, and the ristime is controlled by the gap inductance, the
capacitance, and the self inductance of the capacitor leads (which is
partly why those leads are big sheets of material). You could actually
leave out the right capacitor but if the caps are equal in size you
would lose half the peak voltage, excitation energy and laser output.