It is not uncommon for the 2" amplifier heads to leak water at a very slow rate or to "discharge" a mist of water when the heads are fired. The primary cause for this condition has been the degradation of the Nd: Glass rod thrust washers and/or rod O rings used to seal the end of the Nd: Glass rods to the amplifier head. To date, this condition has only existed on the 2" diameter heads, but this condition could also apply to the 1" amplifier heads
Identifying type and location of a leak
Signs 5that a water leak exists:
There are several different signs and techniques that can be used to confirm if an amplifier rod is leaking. The most obvious initial sign that a small leak exists is when one of the LPL beam 527nm cameras indicates a slightly or severely distorted spatial profile around the outer circumference of the beam:
If a similar pattern (as shown above) is displayed from one of the two main 527nm profile cameras, then it can be assumed that the distortion is a result of a water mist or vapor is being discharged into the beam path after an amplifier-pair leg or legs when the amplifiers are fired. The next step is to determine which amplifier head(s) are leaking and causing the spatial distortion.
Obvious signs of a problem
There may be visible signs of water accumulation near either end of the amplifier rod, on optical surfaces that are near the end of the rod, or signs of damage on the Delrin thrust washer as shown below. These are all obvious signs of a problem that needs attention. Leaks can also exist in areas not as obvious as those shown here:
This optic used to reflect a small portion of the 1053nm beam had signs of prolonged leaks from the adjacent amplifier head as permanent water damage spots were spattered over a large portion of the optic.
Not-so-obvious signs of a problem-procedure to eliminate non-leaking ends/heads
If it is not obvious which amplifier head may be the source of the water leak, then the following steps will help in determining which end(s) of the the amplifier head(s) are leaking. Since the first sign that a problem exists may have been the camera image from one of the two 527nm cameras used to monitor the spatial profile of the four beams, it will be necessary to eliminate amplifier legs that are not the cause of beam distortion. Since each camera image displays two coincident beams simultaneously:
- block the input beam into one amplifier leg while allowing the input beam from the other parallel leg to pass through the respective amplifiers. Alternatively, the de-selecting of one amplifier pair from firing on the LPL GUI is another way of achieving the same thing.
- Since the energy into the screen that is imaged to the camera will be approx. 50% lower with only one leg firing, it will be necessary to remove or change the ND filters used with the camera.
- Fire the heads that are amplifying the non-blocked input beam and determine if the distortion in the camera image is present or not. If not, then reverse the input beam that is being blocked and fire the other parallel leg. If the beam distortion was present, then repeat the procedure but de-select one of the two amplifier-pair heads from the GUI, lower the ND level again and take another shot in effort to isolate the head with the problem.
If it remains difficult to isolate or determine which head or end is leaking, then try the following procedure:
Another not-so-obvious signs of a problem-procedure to identify leaking ends/heads
This is a procedure that can be used in the absence of an amplified or un-amplified light beam. This procedure will focus on determining if an individual head is leaking by using a technique that will indicate the presence of water discharging when the amplifier head is fired and a the laser seed beam is off. Since water droplets or water misting is quite easy to visibly see on an optical surface, this test will require that a large diameter (>3") optic be placed as close as possible to the end of each amplifier head being tested for water leaks. For the purpose of this test, an old, scratched optic was selected as the surface to be used to detect the water leak. It is not recommended to use an optic that has other practical uses than this test as water can be difficult to clean off of optical surfaces.
After the optic has been mounted in an optical mount and placed as close as possible to the amplifier end of interest, fire the head of interest at the normal operating pump voltage. After the head has been fired, remove the optic mount and look carefully for any signs of water on the optic surface that was facing the amplifier rod.
Test optic and mount placed as close a possible to the end of the amplifier head of interest.
With the oscillator/seed beam completely off, fire just the head of interest. After firing, remove optic mount and inspect:
Test optic clearly shows signs of water discharged after firing head.
Complete this procedure on each end of each amplifier head where there is suspicion of a leak, Or use several optics/mounts at different ends of amplifiers at one time to minimize the time required to identify leaks. This technique proved to be a simple and accurate way to determine the location of water leaks that only occur when the head(s) are fired.
Repair procedure for an amplifier head leak
The Delrin thrust washer compresses the silicone Oring around the amplifier rod when the retaining nut is tightened by using the appropriate spanner wrench (there are two spanner wrenches: one for tightening the Oring seal and one for clocking the rod). There is a chance that a water leak can be stopped by tightening the retaining nut, but it is more likely that a new thrust washer and/or silicone Oring need to be retrofitted.
Tightening the rod Oring compression without replacing the Delrin thrust washer/silicone Oring
- Remove key from LSS
2. Verify LCS water is not flowing
3. Insert spanner wrench designated for Orings into end of laser head and turn clockwise with low to medium pressure (do not over tighten). If the retaining ring does turn then there is a chance that tightening might be enough to stop leak.
4. Return to normal operation and test by first looking for any water seepage near end of rod that was tightened. If no, proceed to firing only the laser head that was tightened. Confirm that the head is either sealed or leaking by using the test optic technique described above in the "Not so obvious signs of a problem" section.
Delrin thrust washer/silicone Oring replacement procedure
Tools needed:
- Spanner wrench
- sharp dental type tool
- replacement Orings
- replacement Delrin thrust washer
It may be necessary to reposition the amplifier head differently in effort to gain better access to side of the head in need of repair. If so, use indexed plates on two sides of the laser head base before removing and repositioning head so that it can be re-positioned and aligned as it was before the service was required.
- Follow steps 1-2 mentioned above
- Place covers in front of all SHG crystals that could be negatively affected by potential water spray, as the following steps require disconnecting and draining water from the amplifier heads.
SHG crystal covered during process involving potential water spray
3. Drain water from head before removing brass retaining nut from head. To do this, disconnect both the input and output quick connect water fittings for the head needing repair:
Next, connect an adaptor piece and hose sufficiently long to reach a bucket placed on the floor to the hose connected to the low side of the head, thus extending that hose to allow water to be drained into the bucket.
Making sure drain hose is aimed into bucket, connect the adaptor to the high side quick connect so that the head can begin to drain.
After the head has drained as much as possible, connect an air-pressurized hose to the high side adaptor and blow the remaining water out of the head.
4. Using the Oring spanner wrench, remove the brass retaining nut from the amplifier head by turning wrench CCW.
5. Remove Delrin thrust washer. It may be useful to use a piece of tape to help in pulling the washer away from the head.
6. Remove the Oring using a sharp tool that can pierce the Oring so that the Oring can be pulled off of the rod and way from the head.
7. Install new Oring. The use of a small blunt-ended tool such as the end of a 5/64 ball driver will be necessary in order to press the Oring over and around the rod and between the rod and the stainless collar holding the rod. Try to press the oring so that it is seated as uniformly as possible, but using care to not press to hard, which could result in a puncture or cut into the new Oring.
8. Clean the end of the rod surface as best as possible. Since the diameter is 2", multiple swipes with a clean sheet of lens tissue before each swipe will be necessary. Take the time and patience required to do a thorough job. Acetone is best as an initial cleaning solvent to remove the bulk dirt and grime that may have accumulated followed by multiple swipes using methanol.
9. Clean the new replacement Delrin thrust washer and insert it into the amplifier head and carefully push it up against the Oring. Install the brass retaining nut and tighten snuggly using the spanner wrench to compress the Oring. Use care not to over tighten.
10. Reconnect water connections for input and output of amplifier head. This may require the head to be re-located back to the original position before dis-assembly. If so, be sure to secure the head as accurately as possible to the original position using the indexed plates that should have been installed.
11. Return the master key to the LSS and set the appropriate mode that will allow the amplifier head cooling units to turn on so that leak checks can begin. It's recommended that splash shields be used before enabling amplifier head cooling in the event that there is a significant leak that could spray on optical surfaces at or right after cooling unit turn on.
12. Enable the cooling unit. It may take 5-10 turn-on attempts before the amplifier cooling supply sufficiently primes the amplifier head and begins to flow without turning off. Observe both ends of the repaired head ASAP to see if there is any sign of leaks. If no leaks, continue to let the cooling unit run for an hour while checking for leaks periodically during that time.
13. Prepare to fire the amplifier head. Use the test technique of placing an old, damaged optic close to amplifier head ends prior to firing and verifying that there is not any water that has discharged from the head after firing. Operate only the amplifier leg that was repaired
14. If no water is detected on the optic, proceed as normal with operating the system at full energy. Operate only the amplifier leg that was repaired to confirm that the camera image of the 527nm is as expected with no sign of spatial distortion due to water discharge (shown below).
