Keita, Georgia

Today we went to End X to take a look at the ALS beam on ISCT EX as a comparison with end-y where the mode matching from the green beam to the arm is bad. At end X the input and reflected beams are much less clean than end Y.

• Keita took some photos of the beam position on the viewport. It is much closer to the center than the ALS beam at the Y end station which we checked on Friday.
• We took a beam profile of the beam input green beam, between the two beam expanding telescope lenses on ISCTEX - L6 and L7 on D1800270. Surprisingly, it was quite astigmatic (ellipticity of 0.8 at the points we measured).
  • First attachment shows the beam profile data plotted in a la mode, data is at the bottom of this log post
  • Second attachment shows an image of the beam profile, note the beam profiler axes are flipped 90 degrees - x is y and vice versa
  • A quick back of the envelope calculation says that the ellipticity of 0.8 would mean something on the order of 20% difference between the incoming beam radius and the arm cavity mode, so a 0.2^2 = 4% mode mismatch.
• We also put the beam profiler at a few positions along the reflected beam path
  • First we looked at the reflected light which leaks onto the HWS path. The beam here was junk light (see profile in third attachment). When we added a half wave plate between the periscope and ALS-M11 and rotated it to change the polarization going in we found power alternated between one of two lobes, with a period of 45 degrees between peaks (see profile in fourth attachment).
  • We also looked at the profile of the reflected beam after the isolator, between ALS-M17 and ALS-BS6 (see peanut-shaped beam in the fifth attachment)
  • We also looked at the reflected beam down stream and expanded, before ALS-WFS_B, which I'll attach tomorrow because I forgot to retrieve it from the wincam computer. Edit: attached the profile taken on the WFS path (sixth attachment), the beam is less lobey and more elliptical at this point.

• Tomorrow we'll take a similar profile in the beam-expanding telescope on ISCTEY.