douglas.cook@LIGO.ORG - posted 15:27, Thursday 03 October 2013 - last comment - 09:10, Friday 04 October 2013(7976)
SI Fibers in Dry Storage Cabinet Damaged from Soot and from Pulling Process
While investigating the smoke/fire damage to the dry storage cabinet used to store the finished SI fibers I also noticed an excessive amount of soot on most of the fibers caused by overheating the fibers at the end of the pulling process. The amount of soot would likely have render the fibers unusable due the the soot coating the fibers. During the pulling process it is necessary to shut the laser down within 1 second of completing the pulling cycle or a concentrated heat load occurs that emits a white soot that covers the end of the fiber which also rains down the fiber and under cuts the fiber. We use a vacuum system to pull the fumes away during this process that also showed excessive amount of soot in the clear vacuum hose. We need to be more aware of this during the next run. I will reiterate this on the next run and possible automate a laser kill switched. Image 2799 shows the soot from both sources, white from over heated fiber and black from dry cabinet smoke. Image 2806 fiber coated with white soot and ring where the fiber was under cut. Image 2810 ring groove Image 2811 assembly with clamp block, fibers and angle stiffeners. Image 2820 is the dry storage cabinet showing the soot from the fried power supply in the desiccant regenerating box. Cause is under investigation, possible power surge(?)
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I have sought comments on the white "soot" on the fibre ends. I quote Angus Bell's response. "I see nothing here that would require any change in procedure. The fibre ends showing the white soot also show the clear line between "soot" and "no soot" as the polish "up" stage removes the soot that accumulates on any part of the fibre that will be used. These white bits are scribed off. The polish stage lasts about 20 mins whereas the pull is 20 seconds and the time when the laser is on after the pull is only a couple of seconds. So any vapour production at each stage will be proportional to that time. The tube that goes to the vacuum will slowly fill with soot - that's what it does." With regard to the time between switching the laser off, Alastair Heptonstall informs me "I would say that the laser is shut down time after the pull is less than one second. There's an audible click when the solenoid brakes are applied to the motors, and I use that as the cue to turn off the laser." In conclusion these fibres would have been usable. It is very unfortunate that we had a problem with the cabinet. This is under investigation.