Cryostat intervention (June 2-4, 2025)
According to the decisions taken during the April 11th meeting (see the minutes here), the next intervention aims to replace the array 2/box C LNA. It is provided to do that on June 3rd.
Participants
Alessandro, Martino, Dave
Overall schedule
* End of May: NIKA2 warm-up
OK, done.
* June 2nd: Alessandro, Martino & Dave arrive to the telescope.
OK, done.
* June 3rd: Start working
Opened the cryostat from the back without moving it. A bit tight but feasible. Replaced the amplifier YL022 (probably faulty) with the official spare YL026 that was already at the telescope. After checking the connector we conclude it is pretty robust, well done and pity to cut the wire so we decide to bet on the fact that the problem was INSIDE the amplifier. Also our theory is based on the behaviour of the YL022 in past cooldowns. Not just OPEN/CLOSE, also doing strange things (high current) at intermediate temperature. In any case we have replaced, then verified that it was working (using Dave's portable VNA). We then close the cryostat but have a leak on the OR, open/close again and the leak is gone after tightening the screws in the back. The cooldown started at 18h. Goal of this cooldown is to check if the problem was really in the amplifier of the C box.
Left: 4K BEFORE the change (YL022 ampli). Right: 4K AFTER the change. PLEASE NOTE THAT THE SECOND PICTURE WAS TAKEN BEFORE CONNECTING THE IN/OUT. THIS IS THE REASON OF THE HANGING CABLES. THEY HAVE THEN BE CONNECTED BEFORE CLOSING DOWN BUT WE DID NOT TAKE THE PICTURE.
VNA Scan with -40dB attenuation on input after replacement.
* June 4th (daytime): Reserved as a backup. Martino goes TWICE to Pico Veleta in one afternoon.
* Monday 9th (daytime): Cryostat COLD, first tests.
The amplifier seems to be working but not well with the parameters suggested by Yebes. I play a bit with the parameters to find a reasonable configuration. The gain is in any case lower than all the others. For example compared to box B the ADC level, for the same parameters, is 45% versus 67% of the box B. It is no more than 3dB difference but still it's the lowest. There was a reason why I had chosen these four amplifiers, probably they were the best.
A reasonable set of parameters for the amplifier of box C (not necessarily the best, enough to have a noise comparable to other lines).
Comparison of powers on ADC for the same excitation parameters. The difference can be nulled by changing by 3dB the attenuation on output.