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Message ID: 2036     Entry time: Mon Jan 8 14:46:09 2018
Author: Craig 
Type: DailyProgress 
Category: scatter 
Subject: North Cavity Transmission Backreflection 

Yesterday I optimized the North cavity visibility for a long time, and achieved around 60% visibility, which is pretty good for us.  This was all to realign the North cavity to eliminate ghost beams which are visibly interfering with our main beam.  After all of the optimization, however, the ghost beam is still visible when the cavity is unlocked, and fringing with the main beam when the cavity is locked.

I figured I would take out the post-cavity lens (PLCX-25.4-154.5-UV) to see if we could eliminate the ghost beam that way (Photo 1).  However, something very unusual happens when I take out the post-cavity lens.  North REFL starts oscillating in a square wave fashion (yellow oscilloscope trace in Photo 2).  These REFL oscillations also occurs if I put the lens back in, but adjust the alignment to be normal with the vaccan window.

This leads me to believe that there must be some direct backreflection from the transmission table into the north cavity, which affects the levels of reflected light on the North FSS RFPD.  This could explain why our scatterer is so robust, but it does not explain why we see the scatterer on transmission even when the cavity is unlocked.

I am now going to try to track down the source of the north cavity backscatterer.  We have looked for this type of scattering before, and no obvious candidates stood out.

Edit: The backscattering is coming from the NCAV Transmission DCPD.  Work is underway to eliminate this backscatterer.

Edit 2: The North cavity REFL oscillations were caused by the ISS still being on, not backscatter.  The post-cavity lens position only mattered because some positions allowed light on the North Trans DCPD and some did not, and if any light was on the DCPD, the ISS tried to stabilize it's intensity.  The ISS is now off.  The true North path scatterer is still present.  Work still focused on eliminating this ghost beam.

Attachment 1: CTNTransTable.jpg  2.362 MB  | Hide | Hide all
Attachment 2: CTN_NCAV_REFL_Oscillations_without_postcavity_lens.jpg  2.056 MB  | Hide | Hide all
Attachment 3: CTNTransTablePart2.jpg  2.454 MB  | Hide | Hide all
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