Thomas, Dan

Tonight we tested the SRM heater in single bounce. Our aim was to repeat the same alignment procedure that was done at LLO during their SRM heater test. A course alignment was first achieved using the red alignment laser by scanning it around the first aperture of the SRM baffle using the top periscope mount. We counted how many 1/4 turns it took to traverse the whole thing and then went back by half the amount. The picture attached shows the initial alignment tried. After this we applied a 10mHz square wave to the TTL input of the CO2 laser to modulate the induced lens (later we increased this to 30mHz to get faster feedback). The total power we injected into the viewport was ~400mW which is about 1/3 of the max power we can send in.

A time varying lens should not cause a change in the alignment downstream at AS_A, AS_B, or AS_C it it is well aligned to the single bounce beam. If it is displaced from it we should see variation in the alignment with the same frequency of our square wave. When we go through an optimal alignment the induced yaw/pitch should be inverted relative to the lens applied. We first found this inversion roughly by hand and then tried to fine tune it with a picometer. We aren't capturing the pico positions or the CO2 output power data to plot at the moment. However we were able to find inversion points in pitch and yaw for AS_C. For pitch this happened around the 2400s mark and yaw 2100s. The final rough alignment of the red beam position on the SRM HR baffle and periscope screw positions are shown for reference.

There's still some alignment fine tuning we want to do tomorrow as well as doing some OMC scans to see how the mode matching actually varies. We might also try and put the Nanoscan at AS AIR to see how the beam shape is changing too.