[Takahashi, Tanaka, Ushiba, Komori]

Abstract:

We changed the IY (IX) height by approximately –3.68 mm (+0.42 mm), but unfortunately, the DARM sensitivity worsened with more centering on IX.

Detail:

Continuing from the work reported in the klog:33941, we attempted to improve sensitivity by adjusting the ITM heights.
The basic strategy was to lower the IY height, causing the BPC for IY to follow and thereby centering the beam spot on IX.

The procedure involved repeating the following two steps:

• We adjusted the setpoints (calibrated in micrometers) of the IY GAS control filters in the negative direction until the corresponding COILOUT reached approximately 30,000 counts, which is close to saturation. Note that the signs of the coil output differ among the GAS filters.

• While maintaining these setpoints, Takahashi-san moved the corresponding fishing rods until the COILOUT again reached approximately 30,000 counts, but with the opposite sign.

Using this method, we successfully lowered the IY height to –3.68 mm
 However, when attempting to lower it further, the IY suspension began to make contact with the earthquake stop beside the recoil mass, so this value is near the practical limit.

At the final stage, we slightly raised the IX height to further change the relative beam axis alignment without the offload.
We measured the DARM sensitivity in four different configurations, as shown in the attached figure.
The setpoint values for each configuration are listed in the following table.

In contrast to the expectation, the sensitivity got worse with lowering the IY height, in other words, centering the beam spot on IX more.
In each phase we tweaked the P2L drive align value and ASC offsets, but we did not observe significant difference.
It seems that the jitter couping got larger, and more important factor might be the beam spot on IX, rather than on IY.

The next step is raising IX more, and if we cannot find good height, we will go to the opposite drection.