KAGRA Logbook

Yokozawa, Tanaka

## Abstract

We tried to lock OMC in air and OMC can be only locked in very short time (~1 min) due to the DCPD saturation by ~850 Hz oscillation. So we need to turn off the instruments around OMC like klog25990 if you want to enhance the lock duration.

## What we did

This morining, we aligned OMMT1 with OMMT2 TRANS QPD1 and aligned OMMT2 and OSTM with OMC in-vac QPDs. In this time, we flipped each sign of each alignment control with their QPDs because each actuator sign was flipped accoriding to klog30052(OMMT1), klog30056(OMMT2), and klog30057(OSTM).
Then, we started the OMC lock trial but we could not see the any flash even though we tweaked the beam position on in-vac QPDs. Then, we realized that the cable between DCPD and the OMC rack was disconnected.
This noon, Yokozawa-san connected the cable and we could see the flash on the PD. However, we couldnot see the flash with the camera because the transmitted beam position in the OMC TRANS path on AS table was shifted the +Y direction from the each center of the optics in the path (upper (Fig. 1) and lower mirror of a periscope, a lens (Fig. 2), a camera (Fig. 3)). We need the camera image if we lock OMC with the OMC_LSC guardian. So we tweaked the OMC length with the PZT HV1 manually to keep flashing the transmitted light, and then, we moved the camera to hit the beam on the center of the camera using the flash (Fig. 4).
- In this time, we did not touch other optics because we didn't have enough time for this work and the duration of flashing was too short to take their alignment. Therefore, we just moved the camera so that we could at least lock OMC with guardian. We have to align the OMC TRANS path after we can lock OMC for much longer time.
After that, we could lock OMC by using the guardian. However, the lock duration is very short, at most ~1 min. We found that two DCPDs' input was almost saturated due to the 850 Hz oscillation (the timeseries and the spectra of two DCPDs are in Fig.5 and Fig. 6). This is the known issue as reported by klog22900. If we want to enhance the lock duration we need to turn off the instruments like klog25990.