[Dan, Yuzurihara, Tanaka]
## Abstract
We tried and succeeded in the MICH lock with both the 3f and 1f signals without the change of ITMY alignment. The open loop gain of 3f MICH seems reasonable because the contrast is 10% lower than the previous one. However, the open loop gain of 1f MICH is 8 dB lower than the previous one. We're not sure the reason why for now (ITMY alignment?, clipping on POP table?). So we need more investigation.
## What we did
- First, we aligned BS with the current ITMY alignment to hit the beam on REFL PD. (Note that ITMY alignment may not be best because GRY flash was not confirmed for some reason, but not so bad because GR beam reached 3 km with current ITMY alignment.) We aligned ITMY and BS, misaligned ITMX with each guardian, then we roughly oscillated BS IM stage in both P and Y degrees of freedom with IM OPTICALIGN (+/- 1000 cnts) and found the flash on REFL PD at some alignment. We adjusted BS to that alignment and tuned the BS alignment to maximize the power on the REFL PD manually.
- After that, we aligned ITMX with the guardian, then we tweaked the BS alignment to increase the fringe amplitude. Fig. 1 is the fitting result of free-swinging. The current MICH contrast is 82%. We stopped here today.
- We requested the VERTEX guardian to go to "MICH_3F_LOCKED" in that state. However, the VERTEX guardian stopped at "LOCK_PREP" because the RMS of BS PIT motion (0.025) was slightly larger than the threshold (0.02). Ushiba-san said that the turbo-pomp is working now and it may shake BS at high frequency. So we decided to increase the threshold value to 0.05 for the MICH trial. After that, the VERTEX guardian locked MICH with a 3f signal.
- We measured the open loop TF of 3f MICH. Fig. 2 shows the result. The gain of the TF seems to be 1 dB lower than the previous one. This is because the current contrast (~80%) is 10% lower than the previous one (~90%). The phase differs from the previous one because the current roll-off frequency increased from 100 Hz to 300 Hz. So 3f MICH seems to be reasonable.
- Second, we requested the VERTEX guardian to go to "MICH_LOCKED" and the VERTEX guardian succeeded in transitting and locking MICH with a 1f signal. We measured the open loop TF of 1f MICH. Fig. 3 shows the result. The phase is different from the previous one for the same reason as the 3f MICH case. On the other hand, the gain of the TF seems to be 8 dB lower than the previous one. For now, we are not sure of the reason. One possibility is that ITMY is not the best alignment so the beam was clipped somewhere up to POP RFPD. Anyway, tomorrow, ITMY alignment will be performed with GRY flash. After that, we will try the MICH lock again.
