Madi, Camilla, TJ

Recently been taking measurements of the ALS beam at EX, initially to observe the beam as it returns from vacuum. This was done using a Pellicle beamsplitter placed in the path of the ALS beam between the PBS and the LPM, which picked off the return beam for observation. Images from this measurement are attached. It was observed that the return beam was elliptical in shape, with two intensity peaks.
The input beam was also re-measured (previous measurements recorded here). It was seen that even the input ALS beam is elliptical in shape, and as it gets closer to the PBS, a lobed intensity pattern begins to appear. Considering the return beam is also elliptical with two lobes in the intensity pattern, the effects of the in-vacuum system are unable to be separated from the input beam characteristics, and limits our ability to use the ALS beam as a diagnostic tool for the in-vacuum path.

Also concerned for the performance of the ALS, that that the ALS beam is elliptical before going into the vacuum system. Potential future task will be to find out where the beam gains this ellipticity and correct it.