Craig, Danny, Georgia
Today we stepped the SR3 heater down in 0.5 W steps, from 5W to 3.5W, according to plan. Here's what we found as we reduced the heating on SR3:
Buildups, cavity pole
- the DARM coupled cavity pole increased by a few Hz
- the optical gain (Kappa_c) decreased by ~2%
- POPAIR_B_RF18 decreased by ~5%
- The BNS range went up ~4 MPc, maybe this peaked at 4W on the SR3 heater
- The changes in arm power were only slight - maybe went up a little ar 4.5W, but decreased after that?
These values are shown in the first attachment, The kick in many signals just before we stepped down to 3.5 W is explained below.
DARM plant and laser noise couplings
We also monitored the DARM plant, frequency noise coupling, intensity noise coupling, and RF9 RIN coupling at each step.
- Craig will post the DARM plant and frequency noise measurements
- The intensity noise coupling at high frequency got worse as we reduced the heater power (second attachment shows darm during noise-buget-style ISS second loop intensity noise injections)
- The RF9 RIN coupling reduced slightly at 4.5W and 4W but was increased again when we measured at 3.5W (third attachment shows the 9MHz RIN excitation to DARM transfer function)
SRC ASC
While we were sitting at 4W on the SR3 heater we checked for detuning in the SRC ASC. This is the big peak in RF18 and RF90 in the first attachment; the 4th attachment is zoomed in during this time. We opened the loops and moved the sliders (top right plots), found nothing too interesting in pitch, but while aligning yaw we saw:
- some gains in RF18, but
- increase in RF90 (maybe we were aligning the SRC to compensate for a misalignment in the PRC?)
- a 2% increase in kappa_C (!)
- a 25 Hz drop in the cavity pole (!!)
- a corresponding dip in the range
We were surprised that the optical gain increased, that doesn't seem to hang together with the other pieces of information here. Maybe we should consider operating with 4W on the SR3 heater, and re-phasing the SRC ASC for this. [Edit: notes for attachment 4: at t = -4500s we turned the SRC ASC back on, which is why the alignment went back to its nominal level. The calibration lines were off before t = -4800s, and the calibration values before this time are not to be trusted.)
Other notes:
- The SR3 cage servo (really an oplev servo) was on for this measurement, this kept SR3 servo'd in pitch, but uncontrolled in yaw
- At 3.5 W on the SR3 heater we encountered a very slow (~2 minute period) oscillation in the corner DOFs. This settled when we returned the SR3 heater to 5W. We spent a bit of time tongiht with the ADS YAW DOFs off, to stop them from oscillating.
- We spent 2 hours and 34 mins out of observing today to do these tests, comfortably within our allotted 3 hours :-)
- While we were out of observing to make measurements we chopped the OMC ASC master gain for Sheila.
- To run our excitations we connected two LSC AO extra channels at the PSL racks to the common mode board excitation, and the 9MHz EOM excitation port, these need to be disconnected again.
We now understand that the increasing Kappa_C corresponded to a decrease in optical gain. So we were misaligning the SRC when we were aligning it by hand. The fact that RF18 was able to be improved with a misaligned SRC suggests there's room for improvement in the beamsplitter or PRC alignment.
We have made some DARM spectra from times during our SR3 heater test, and SRC alignment test.
First attachment shows DARM with the SR3 heater at 5W (black) compared to 4W (cyan), showing definite improvement in the bucket, which explains our range increase. A similar spectrum at 3.5 W on the SR3 heater sits somewhere in between these two.
Second attachment shows DARM with SR3 heater at 4W, with the normal alignment (blue), and when we opened the SRC ASC loops and "aligned" by hand, maximising POPAIR_B_RF18 (yellow). It seems like I can undo the SR3 heater improvement by misaligning the SRC...
The DARM plant did not change very much over the period of the SR3 heater move.
I measured the DARM plant 4 times, at SR3 power of 5W, 4.5W, 4W, and 3.5W. The DARM plant did not change very much this time.
According to some MCMC fits:
Optical Gain = 3.20 +- .28 × 106 cts/m (<0.8 % uncertainty)
DARM pole = 417.80 +- 15 Hz (4 % uncertainty)
Delay = 5.31 +- 1.7 × 10-5 s (33 % uncertainty)
Spring Freq = -5.12 +- 1.6 Hz (32 % uncertainty)
Spring Q = 37.11 +- 4.7 (13 % uncertainty)
This is a different result than the SR3 heater move at 30W input power. We moved the SR3 heater by less this time (from 5W to 3.5W rather than from 0 to 5 W before). We would also like to test the DARM plant for SRCL offset and DARM offset changes.
Also the frequency noise coupling to DARM did not change. Will post plots later once they are correct.
Posted DARM ASDs, calibrated frequency ASDs, and frequency coupling TF plots. DARM calibration: 6 zeros at 30 Hz, 6 poles at 0.3 Hz, gain of 1 Frequency Calibrations: Same as 46864. See attached freqCals.txt. Comparison to 30 W coupling: 45831. Before, we had a dip in the freq-to-DARM coupling at around 30 Hz where the radiation pressure and contrast defect effects destructively interfered. Now that effect seems to have flattened out. (Plot 3 in the PDF) Over the SR3 heater test cooling from 5W to 3.5W, it seems our freq-to-DARM coupling increases by a few percent, but does not change too radically.
Sheila was thinking the SR3 heater improvement could be attributed to the changing MICH and SRCL feed forward. I had a look at the coherence between DARM and MICH/PRCL/SRCL during the SR3 heater test, comparing a time at 5W (black) and a time at 4W (cyan), and am not convinced the coupling changed significantly. If anything the MICH coherence is worse at 4W than 5W in our frequency band of interest (20-60 Hz).
