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Message ID: 2064     Entry time: Mon Jan 29 20:22:06 2018
Author: Craig 
Type: DailyProgress 
Category: scatter 
Subject: Pre-Vent Vaccan Tranmission Scattering Drawing 

I drew the basic scatterers I see when viewing the inside of our vaccan from the transmission port with the IR viewer.  There are a couple of main points:

1) There are some definite ghost beams coming directly from reflections off the vaccan window.  They exist both on the REFL and TRANS side of the can, the TRANS ones are drawn below.  To make sure they were coming from the window, I took out our half wave plates and the lenses immediately behind the vaccan and blocked the main beam.  All points seen in the drawing remained steady.

2) The south cavity shield is in general brighter in IR than the north.  Both shields are bright relative to the rest of the vaccan interior.

3) It seems like the cavities are already angled slightly in the plane of the table.  Along the axis of the vaccan, if viewed from above, the cavities would be angled clockwise slightly, by maybe 3-4 degrees.  Perhaps Tara thought of scatterers when first assembling the can but was unable to completely mitigate them.

4) There is a distinct height difference in the main beam spots on the transmission window.  SCAV's beam is about 0.5 cm higher on the window than NCAV's.

5) In REFL, which is not shown in the drawing here, the south path has a visible scattered beam on the mirror itself, not on the cavity shield.  This is our most likely candidate for our main scattering noise.

 In short, we have to vent.  We want to start pumping up tomorrow.  Our first order of business will be to investigate the wedge angle of the windows and make a geometric model of the beam paths in our vaccan.  Then we can precisely place the cavities, place our cleaned beam dumps, lower the laser power and test our new alignments in air, mode match, then pump back down.  This will be a week of effort, plus another week for unforeseen circumstances.

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