Dan, Kiwamu,
We locked the PRMI on the sidebands to assess the current recyclying gains. The result will be posted later.
We did the initial alignment sequence to get back to a good global interferomter alignment. One thing I have to note is that I had to touch PR3 in yaw by 2 urads in order to recover a high RF power in ALS COMM. It is now back to 3 dBm in the monitor. Also this gave a good spot position on the ALS X camera as it was clipping before I moved PR3. The clipping seems to be fixed now on the camera. I aligned TMSY, ETMY and ITMY using the green light with a hope that they still represent a good IR alignment. After going through all the alignment sequence, the ALS DIFF beatnote came back to a high RF power of about 0 dBm. So I think the global alignment came back to as good as before.
The PRMI was locked very easily by setting LSC_CONFIGS to PRMI_sb_OFFLOADED. Then we aligned the OMs and did OMC scans in order to evaluate the recyclying gains. The data is now under some analysis. After the OMC scan, we attempted to lock the PRMI on the carrier, by simply flipping the sign of the PRCL control sign. We tried different gain settings MICH which uses REFL45Q, but did not get good lock tonight. So, we still don't know the carrier recycling gain.
We locked the PRMI on carrier. The carrier recyclying gain was measured to be 35 at highest. However, since the alignment was not perfect, it probably would go up. To be continued.
After playing with the gain settings, we eventually became able to lock the PRMI on carrier. However the alignment was not stable to keep it locked with high build-up. I think this needs more study to understand what is going on. Anyway, so far, the highest buidl-up in POPAIR_A_LF we had tonight was about 3.5x104 uW. When the simple MICH without power-recycling was locked, POPAIR_A_LF was about 30 uW. Assuming that there is no mode-mismatch and Tp=0.03, we get a recycling gain of 3.5x104 / 30 * Tp = 35.
LSC settings:
Attached are the OMC scan results for a PRMI sideband lock, compared to a scan from a single-bounce beam. The first plot shows the results of three single-bounce scans and three PRMi scans (100 second ramps of PZT2); the second plot has averaged the traces. The PRMI data appears to be shifted upwards compared to the single-bounce data, by about 1V in the PZT2 output. We expect some drift and hysteresis in the PZT, but the single-bounce data was taken immediately after the PRMI lock, and a shift of this size is...surprising.
Using Kiwamu's expression from alog:14532, I calculate the PRC gain of the 45MHz sideband to be about 11.8 or 14, depending on which sideband peak you use. I think this is lower than we expect. The PRMI sideband lock was quite wobbly with a lot of angular motion, we might get a more robust measurement by locking the OMC to a particular mode and maximizing the transmission.
Here is a table of peak heights:
With a Schnupp asymmetry of 9.5cm the gain, for example for the upper 45MHz sideband, is (4.7/0.31) * (0.03*0.5*0.5) * (1/sin(2*pi*0.095*5*9100230/c)**2) = 13.9.
Just for a book-keeping purpose:
Two weeks later from this entry, we have measured the recycling of the carrier with the ASC loops fully engaged. We measured it to be 45 (see alog 15793).