NE PCal on-site check to investigate the 0.8% mis-calibration of NE PCal power

This morning, from 9h to 9h45 UTC, we have worked on the NE PCal to investigate a possible explanation for the 0.8% power mis-calibration of the NE PCal (which I recall was renormalized on the NE NCal before the start of O4b to fix this issue).

Suzanne went at NE and had a look at the beam alignment, first on the injection Tx bench (from around 9h10), and then on the reflection Rx bench (around 9h20 UTC).

The tested hypothesis for the 0.8% mis-calibration was optical losses between the NE mirror and the NE Rx sphere, possibly due to beam misalignement on the Rx bench. On the bench, there are three optical elements: diaphragm just after the viewport, a steering 1 inch mirror and the integrating sphere with 1 inch aperture.

After visual inspection, the alignment looks fine on all optics.

In order to test if the diaphgram aperture was too small, Suzanne has increased it by a large amount but it had no impact on the power reaching the Rx sphere. The diaphragm aperture was put back to a similar value as before.

Suzanne did not notice any big dust on the viewport, but this is difficult to inspect with the surrounding elements, and with no green light.

Benches closed and power loop closed again around 9h35 UTC.

Around 9h40-45, we tried to reconnect the Hameg28 power supply for slow monitoring, but without success.

--> the NE PCal 0.8% mis-calibration is not coming from bad beam alignment on the Rx bench nor from too small diaphragm aperture.

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Measurements to study sensing noise of photodiodes and spheres

For each time, the duration is more than 60 s, more around 120 s. Times given in UTC.

• 10h05 : NE laser off (pump not disabled)
• 10h08m30s : NE and WE laser at 0.3 W, w/o loop
• 10h11m15s : NE and WE laser at 0.3 W, w/ loop
• 10h13m45s : NE and WE laser at 0.6 W, w/o loop
• 10h15m15s : NE and WE laser at 0.6 W, w/ loop
• 10h18m40s : NE and WE laser at 1.0 W, w/o loop
• 10h23m25s : NE and WE laser at 1.0 W, w/ loop
• 10h26m00s : NE and WE laser at 1.3 W, w/o loop
• 10h29m20s : NE and WE laser at 1.3 W, w/ loop
• 10h36m20s : NE and WE laser at 2.0 W, w/o loop
• 10h38m30s : NE and WE laser at 2.0 W, w/ loop
• 10h42m45s : NE and WE laser off (pump diode not off),
• 10h45mxxs : requested 2.6 W, maximum reached ~2.2 W
• 10h46m20s : NE and WE laser at 2.2 W, w/o loop
• 10h47m30s : NE and WE laser at 2.2 W, w/ loop

Back in nominal conditions (1.3 W, w/ loop, and permanent lines on) at 10h53m UTC.

The plots below show the noise spectrum of the sensors during these different measurements, first w/o loop and increasing laser power, the w/ loop and increasing laser power. The last plot show the noise spectrum with laser off.