Bubba and I have stabilized the LVEA and VEA temperatures.
We still have known mechanical issues which are probably impacting the control loops so we should be able to improve things - especially XEND.
The one degree spike at XEND on the 19th was due to my attempt to outsmart the system.
Motorized variac heater controls are being ordered for the two end stations which should allow for better control and transient behavior.
We may be asking to raise the set point at XEND - so if anyone has thoughts on this please let us know.
On the evening of the 19th commissioners noticed large angular drifts on ETMX, see alog 15193. The attached plot shows temperature in end X over the past 3 days as well as the optical lever readouts. There is a clear correlation between temperature and optical lever readouts. The trace of the 0.8 C excursion on the 19th in the optical levers is obfuscated by several re-alignments of ETMX. One might wonder, if the optical lever temperature fluctuations correspond to actual optical alignment changes, or if they are just an optical lever feature. Since large alignment drifts were reported on the 19th, one might suspect the former.
Alignment changes of the order 0.5–1 µrad will require a re-alignment of the arm cavity.
Looking at a trend from 3 weeks ago shows that the temperature is now oscillating with a ~2.25 h period and an amplitude of ~0.15ºC.
Here is an LLO alog describing a measurement of the angular misalignment when their HVAC system was shut off. One can deduced a temperature dependency around 35 µrad/°C in pitch, see alog 13817. Yaw seems relatively stable. With a requirement of 0.5–1 µrad the temperature stability would need to be around 0.02ºC.
The other thing noteworthy is that it takes less than an hour for the vertical position to turn around after the HVAC system was engaged. This would indicate that the dominant thermal coupling is radiative through the chamber walls, and not conductively through the ISI. The reaction time of the ISI is of order 36 hours—which could be responsible for the long tail seen in restoring the position.
The calculation made by Brett in the above referenced link, as well as the measurements of pitch vs temperature that are reported by Stuart A in the LLO log, correspond to the pitch of the TOP mass of the quad suspension. According to the suspension model, the test mass pitch is 25% of the TOP mass pitch. So, the 30-40 urad/C for the TOP mass translates to 8-10 urad/C for the test mass. So 1 urad corresponds to 0.1 C. Still quite sensitive, but not so bad.
Not sure, if this still agrees with the observed optics drift. If I interpret the plot in alog 13639 correctly, the HVAC system was off for about 20 h, in which the temperature was rising by ~3.2 F. Assuming a linear trend we get 90 mK/h, or 15 mK in 10 minutes. The optics was drifting 0.5 µrad in 10 minutes. So, we get ~34 µrad/K at the optics.
On the other hand, looking at the temperature oscillations in EX we would expect to see 10 µrad-pp, but only see 1 µrad-pp. A time constant of ~1 hour would correspond to a pole frequency around 50 µHz which is abount 2.5 times slower than the observed oscillation. Assuming this leads to an attenuation of 8 dB, a reduction of 25% through the suspension chain would explain what we see in the optical lever.