Peter, Keita, Stefan

The ISS is still acting up, so we tried to do some diagnostics.

First we had to adjust H1:PSL-ISS_REFSIGNAL to -1.78V to avoid running out of actuation  range. (The diffracted power RMS still seems bigger than before the upgrade - we now fluctuate between ~8% and ~23%). We also had to increrase the ISS gain to 9dB.

The ISS now initially seemed stable. But we were sitting at 22W into the IMC. Of course, that rotation stage is after the IMC. But nevertheless we noticed that when we reduced the selected power an oscillation appeared in the ISS, that sometimes manages to break the loop.

The attached plot shows the two ISS PDs with 3 traces each:

- Now with an oscillation at 2.5kHz

- Now, with no oscillation

- Before the laser upgrade (and with only the first loop closed)

Additionally, the out-of-loop diode (PDA in plot), sees a significant excess in sensing noise below 100Hz. And there are new peaks at ~950Hz and ~4.2kHz.

We are not really sure what couses all this, but one suspicion is scattered light from the high power beam dumps. Excess scatter noise could explain the difference between in and out-of-loop. Also, the abount of scattered light would likely be reduced when less power is dumped. We do not yet understand the 2.5kHz oscillation, which also seems to be moving around somewhat.