KAGRA Logbook

MIF (ITF Control)

masayuki.nakano - 22:39 Thursday 10 October 2019 (10958)

ALS DARM loop investigation

[Ushiba, Takano, Nakano]

We continued to check waht's happening in the ALS DARM.

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Experimental condition:

Lock the ALS DARM and CARM by using the guardian.
The laser power is ~350 cnts at PDA1_DC on the bright fringe.
Hold the Xarm on IR resonance, and the Yarm on IR off-resonance.

Check procedure:

Blocked the beam to PDA1 with a beam damp.
-> Oscillation went away
Blocked the beam to PDA2 with a beam damp.
-> Nothing changed
Blocked the beams to PDA1 and PDA2 with a beam damp.
-> Same with situation 1.
Removed a beam damps
-> Oscillation came back
Disconnected all of the RF cables connected between the RF PDs (PDA1 and PDA2) and the demod boards.
-> Nothing changed
Turned RFQPDs power off.
-> Nothing changed
Changed the switch on the RF PD interface to the test mode.
-> Nothing changed
Turned the RFPD interface power off
-> Oscillation went away
Turned the RFPD interface power on
-> Oscillation came back
Disconnected the Dsub cable between the interface and the PDA1.
-> Oscillation went away
Changed the interface channel for the PDA1 from Ch.1 to Ch.2
-> Oscillation came back
Spark!! (Ushiba-san will report)

Result:

Our hypothesis is that the ground of the optical table and the k1als0 rack are connected via the RFPD and this makes an unexpected loop. (detail will be described in Ushiba-san's report.

We hope the disconnection of the RFPD ground from the optical table solves this problem and the PRFPMI can be locked.

Comments to this report:

shinji.miyoki - 8:09 Friday 11 October 2019 (10960)

In CLIO, all PDs, whose housings are metal and connected with the ground of connected BNC cables, are isolated with the optical tables with ceramic spacers.

Also RF LP, HP, notch filters to pass modulation/demodulation signals and to reject higher harmonics, and isolation transfers were inserted.
However, too much such filters did not always bring good results, sometime the sensitivity for higher frequency range got worse.

In addition to this, one point ground was realized because single end cabling was used, not differential transfer.
The one grounding point was maybe the innolight laser controller ground.

Actually, some time we felt electrical shock when we touch the optical tables and vacuum tubes.

According to my poor memory, a teflon sheet roll was in CLIO per-arm optical table for rejecting grounding. I don't remember that I brought it to KAGRA before or not.

In KAGRA, isolation washers and plastic spacers for screw bolts and black coated spacers are used to reject contacting with the optical tables.
I am a little bit anxious about the isolation effect of black coated plates because the black coating is too thin and has unexpected capacitance btw the optical tables.
Another anxious point is that the screw bolts might scratches the side of the black plates and electrical connection might be there.

keiko.kokeyama - 9:07 Friday 11 October 2019 (10963)

I have bought teflon washers for this purpose. I think they are now on the REFL table.

Black coatings are too thin and most of the PDs are actually connected to the optical tables.