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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.