Keita, Georgia

Today we took a profile of the ALS beam on ISCTEY, measuring the beam size for the input beam (going to the ETM) between the two beam expanding telescope lenses (first attachment). We took the same measurement at end X yesterday, and the beam has been measured in a couple of different positions at on ISCTEY previously (alog 52576 and links therein).

• The input beam is elliptical. If we assume the beam radius in the y direction is perfectly matched to the arm (recall that most of the power is in horizontal higher order modes, see nice cavity scan in Keita's comment here) we can calculate some minimum mode mismatch between the input beam and the arm cavity. Using a la mode to calculate the mode overlap between a circular beam with radius equal to the x-radius of our elliptical beam and a circular beam with radius equal to the y-radius of our elliptical beam we get the square of the mode overlap between an elliptical and circular beam. From this measurement we get an overlap of 88%. Note the mode overlap calculated through a la mode is
  • [(2 pi / lambda) × w1 × w2 × |q2* q1|^-1 ]² ,  which is an intensity overlap, rather than a field overlap.
• We can also propagate from our measured beam through the known lenses on ISCTEY, curved mirrors of the transmon and through the ETM, and then calculate the overlap between that mode and the arm cavity mode. the second attachment shows the beam propagated through the transmon to the ITM. I'm still working on this, but my current overlap calculation between our beam and the arm cavity beam is ~35%, which is obviously wrong.

We also took some photos through the illuminator viewport on BSC10, which is just above the camera viewport we took photos through on Friday. This allowed us to see more of the optics on top of the transmission monitor. The beams on the optics that we saw looked ok. Interestingly there is a beam hitting on a mechanical structure (visible in the third, fourth, fifth attachments). Adding a flashlight to the camera viewport, Keita identified this an obsolete HWS periscope. Maybe I'm missing it but I can't find the structure on the TMS assembly drawings. Based the location of the structure relative to the beam path (we think it is behind M6 in this layout diagram) we do not think this clipping beam is the main ALS beam, but it's not obvious which beam is hitting this structure.