Hi,
Hopefully this answers your question
. Here goes my rather
long reply
When you use PPI, what essentially you are doing is to limit the
duration of the laser pulse(on top of the TTL/PWM, Analog DAC input
to the Power supply), so that it is spaced out. I do understand your
average "mA" drops significantly due to this, from that point it might
look,it should be fine to increase your power percentage(esp. If you
have overpowered supply-ie, one that could handle higher current,
also producing higher trigger/operating voltage), to reach an avg.
value less than 18mA max even with low PPI. But wait...
Now coming to failure mode of the tube, from what my understanding
is, it is a combination of one of thermal shock,increased leaking of
lasing gas due to high temperature, breakdown of the lasing gas
permanently into something that does not produce laser output(such
as CO2 breaking down to CO+O2). A catalyst gas such as Krypton in the
mix is supposed to help in converting back CO to CO2, but within limits.
The max. current determines the heat load(esp. with diminishing returns
of less laser output increase with more heat output increase beyond the
max. current-even continuos flow open CO2 medical lasers are hardly
20% efficient). There is also the possibility of contamination of tube with
vaporizing of the electrode material(further could reduce electrical insulation,
hot spots(coupled with an air bubble at the electrode end
), or even catastropical
rupture of the lasing window mirror,dislocation of the mirrors etc).
So, you are fine with an average current less than max in the above
respect. But the mechanism of lasing gas breaking down is not just
dependent on the max current alone, it is also the amount of voltage
that is being used. When you do not over power the tube (such as normal
operation within max. current), there is virtually no permanent
breakdown of CO2, because of the recombination due to Krypton.
But with higher voltages forcing more current while using low PPI's, it
is not going to be case. What you will find is a progressive reduction of
power after some time(how lucky is dependant on the quality of the
lasing mix in your tube, esp. Krypton being an expensive gas to fill).
So, make your own informed judgement on increasing the max current
with low PPI. The chances are that you cannot overpower your tube,
if your PS is producing just sufficient voltage for normal operation,
But nothing will stop you from using a 80W powersupply on a 40W tube.
This being the reason why the tubes are normally matched to the PS
during operation.
Regards
Raman