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Version 8 Installs water
cooled amps.
Version 7 main improvements was on beam quality and
assured single mode. Also amplifiers pumped by helical lamps improved laser
beam amplification without distorting TEM00 mode like linear lamps
pumping arrangements of version 6.
A return to version 6 type amplifiers with water
cooling allowed the laser to be fired more often. With air-cooled helical
pumped amplifiers, immediately after firing, the laser rod rose in
temperature 7 to 8 degrees centigrade and required 30 to 45 minutes to
cool down. Water cooled rods cooled down within 5 minutes. Also noted was
an increase in efficiency due to the water-flooded cavity which improved index
matching efficiency from fresnel refraction of the lamps and laser rod.
The power supply was reworked for the dual linear
lamps. These lamps are 9.5mm bore 5.625 arc length lamps. They are ran both in
series and triggered by 25kv pulse used for the helical lamps. The 460 ufd cap
for each amp was rewired with 95uh 10guage inductors and 1 trigger transformer
saturated around 50uh and yielding 600 usec long pulse. This allowed the
damping factor to be 0.8. With such short pump time the oscillator was fired at
the same time and normally the single pulse arrived around 575 usecs
later.Oscillator used two 4 inch x 7mm bore lamps. Each lamp had it's own
trigger circuit and power supply of a 112uh inductor, 35uh saturated trigger
transformer, and 1300ufd cap.
The two Laser rods for amplifiers amp1 and
amp2 are 0.05 percent doping and are 3/8 inch x 6 inch long. Oscillator
used a 0.03 percent doping 7mm x 4.5 inch long rod.
Laser output: With a single 12 nanosec single
mode 17.55mj oscillator output at 1190v (1840j, 416j/cm3).
Amp1 set at 2300 volts(1300 joules, 119j/cm3) yeilded 116.8mj and
amp2 at 3000 volts (2100j, 193j/cm3) yeilded 1.47 joules. This is
very efficient oscillator amplification for the total electrical input of 3400
joules! Concern should be given to keeping the propagating beam power
density to less than 5 joule/cm2 to avoid rod end damage
due to the small amplifier rods as well as back reflection of the rods.
Maximum tested power was over 2J at amplifier voltage of 2800 volts for amp1
and amp2. Beam size was 7mm of the 3/8 inch rod. Calculated beam density was
6j/cm2.
Amplifier 1 was ran very soft inorder not to
increase divergence due to pump induced negative diopter thermal lensing.
Amplifier 2 was ran moderately due to rod end damage concerns but overall the
limiting amplifier rod sizes forced reduced power amplification just to avoid
the thermal lens issue creating too large of a propagating beam through the
amps. Such large beam output increases the diffraction ripple on the output due
to limited amplifier aperture. Output beam size was 1/4 inch. If
both amp1 and amp2 had been ran at the 4,000 joule level the output would have
been around 2.5 joules or more but the beam quality would have suffered
greatly. The increased output was also with increased divergence which
kept the beam power density from rising too great but the larger beam size
caused higher diffraction ripple due to the limiting aperture. Amp 2 could be a
1/2 inch or larger rod if higher output is needed and diffraction needed to be
kept low. For holography the 1 joule output was deemed sufficient.
Power supply was upgraded for safety by replacing
bleeder resistors with higher kilovolt resistant bleeder resistors 3meg at 64KV
rating.
3eta.jpg Both version
7 and 8 used this 4 plate etalon for OC. These large air spacers of 25mm allow
for narror transmission peaks and the thin 2mm plates allow for wide spectral
separation from the transmission peaks. Reflectivity of this plate is 86.5%
peak.
rw81.jpg Laser head
View 1
rw82.jpg Laser
head View 2
rw8b.gif Beam profile
expanded by 25mm negative lens and projected on wall from 108 inches
away.
