[Yamamoto, Ushiba]

Abstract:
We locked MICH with 3F signal at REFL (REFL 51 Q).
UGF of the loop is about 22 Hz.
Also, we prepared MICH_3F_LOCKED state in VERTEX guardian, so we can lock MICH with 3F signals from the guardian now.

Detail:
Since ALS_DARM was locked (klog21659) and BS motion was drastically reduced (klog21662), we moved to the lock trial of central part: MICH.
This time, we only use TM stage, so design of the filter is very simple: zero at 1 Hzand pole at 100 Hz with 2-order eliptic low-pass filter at 100 Hz.
To obtain large DC gain, we also implemented DC boost filter from 10 Hz.

Figure 1 shows the time series data of error and feedback signals of MICH.
As you can see, MICH is locked.

After that, open loop transfer function of MICH loop was measured.
Figure 2 shows the result.
UGF is about 22 Hz and phase margin is about 40 degrees.

Then, we engaged it to the vertex guardian, so we can lock the MICH by requesting MICH_3F_LOCKED state to the guardian now.