Dan, Danny,

To test the SRM CO2 heater we first needed to get it aligned correctly. The method was similar to before when we tried this (42254, 41724):

• Lock IFO to ensure we align to the nominal SRM beam position (Previous attempts were done with single bounce beam)
• Perform an initial alignment of the beam with the red beam to visually see we are hitting the SRM HR baffle plate. Trace out the edge of the baffle hole and center ourselves in pitch on the hole
• Pulse the CO2 heater
• Position the beam on one side of the baffle hole then scan across in yaw
• Look at SRC2 pitch and yaw output to see how the point of the output beam has to change to keep it aligned on AS_C
• Adjust positions to see a sign change in SRC2 signals, i.e. the thermal lens has moved to the other side of the beam

To speed up the searching we hooked up the CO2 controller to SQZ LO SERVO EXC channel and drove it with a 30s square wave with a 20% duty cycle. We also connected the periscope picomotors to the ISTC6 picomotor driver using channels 5 and 6. This was to speed up the alignment by just being able to initially hand align the beam to one side of the baffle hole and then just continuously scan across to look for the pulses, occasionally adjusting pitch to keep it zeroed and pitch up/down to check we are still at a zero. We also connected the Thorlabs power meter to SQZ EXTRA AI 1 to monitor the pulse powers, there are two filters there to convert into Watts measured on transmission of the first beamsplitter (Most of the CO2 power gets dumped through this so only ~1.2W can ever go into the chamber, when the Thorlabs power meter reads 6W).

Last time we tried this the alignment with the single bounce beam it was awkward and took awhile but achievable. We have tried to align it several times over the last 5 days but have had no luck in finding a good yaw alignment. Pitch was relatively easy to find.

Yaw however always seemed to end up clipping. Here you can see us trying trying to sweep over the error signal, a slow increase is seen, then we peak, then we start decreasing in a linear way which looks like we are heading for a sign flip. That didn't happen. We then moved the bottom periscope mirror to try and shift the beam horizontally to get around whatever might be clipping but nothing. We then resorted to shifting the table around, but still no luck.

We were never able to align the beam to get a negative SRC2 yaw pulse, despite some pretty bold alignment changes with the table and picos. Given how hard it is we are wondering whether the hot IFO SRM spot position is positioned in such a way that we just can't get to the other side of the beam?

Next things to try here would be to look at the SRM IFO beam spot position and see if it moves much from a2l gains perhaps. Maybe we could also try aligning this in single bounce again and aim for a similar alignment as seen here with the illuminators on.