[Koji / Kiwamu]
We tried finding a possible clipping in the vertex part.
We couldn't find an obvious location of a clipping but found that the recycling gain depended on the horizontal translation of the input beam.
We need more quantitative examination and should be able to find a sweet spot, where the recycling gain is maximized.
(what we did)
+ locked the carrier-resonant PRMI.
+ with IR viewers we looked at the inside of ITMX, ITMY and BS chambers to find an obvious clipping.
=> found two suspicious bright places and both were in the ITMY chamber.
(1) POY pick off mirror : looked like a small portion of a beam was horizontally clipped by the mirror mount but not 100% sure whether if it is the main beam or a stray beam.
(2) The top of an OSEM cable connectors tower : although this is in the way of the SRC path and nothing to do with PRC.
+ Made a hypothesis that the POY mirror is clipping the main beam.
+ To reject/prove the hypothesis we shifted the translation of the incident beam horizontally such that more beam hits on the suspicious mirror
+ Realigned and relocked PRMI.
=> Indeed the recycling gain went down from 6 to 0.8 or so. This number roughly corresponds to a loss of about 50%.
However the MICH fringe still showed a very nice contrast (i.e. the dark fringe was still very dark).
Therefore our conclusion is that the POY mirror is most likely innocent.