2023-03-15
Start up the machine as usual.
Plan of Action
It is expected to take 4 hours to break the vacuum, fix whatever is wrong with the FAB, make the seals, turn on the first-stage of pumps then turn on the turbo pumps. It will then take about an evening for the turbo-pumps to bring the vacuum back down to usable levels. If we break the vacuum now, we will make no measurements today, but there is enough time to fix the FAB in the afternoon and still have the vacuum back by tomorrow morning.
For this reason we have decided to try and take some data with TORA in the morning. We will continue the experiment with TORA if results look OK, but if not, we will open the vacuum in the afternoon and fix the FAB.
Mid-Day Update
We can't see any peak at Tora, and because we do not have much time to do more, we change the vertical scale of the scope to 2mV/div range to try to see a signal. The noise level at Tora is much weaker compare to the FAB concerning the RF.
The 2mV/div does not allow us to see the signal.
We break the vacuum at 12:15pm.
FAB Maintenance
The connections from the feedthrough went: Feedthrough > RG58-like-cable > BNC-gender-adaptor > Short RG-58-like-cable > inner conductor connected to FAB.
Maintenance
After the vacuum was broken and the blanking flange at the FAB removed, we noticed several problems with the FAB:
The inner conductor of the BNC cable which was connected to the FAB felt loose. When the fab was eventually removed from the vacuum chamber this cable broke off entirely without any tension being directly applied.
The BNC gender-adaptor was very close to touching the FAB because it was "resting" on top. If these were touching, the FAB would have been shorted to the vacuum vessel via the outer conductor of the RG-58-like cable
The end of the FAB would have been very close to the blanking flange.
To resolve these issues we:
Connected a new wire to the FAB and verified that it was a strong connection.
The BNC-gender adaptor was covered in heat-shrink to prevent any shorting of the FAB via the outer conductor of the BNC.
Kapton tape was applied to the ends of the FAB to prevent any shorting of the FAB at the flange.
Before restarting the vacuum pumps, we used a multimeter to verify continuity from the feedthrough to the FAB and isolation between the FAB and the vacuum chamber.
Capacitance Measurements
Before the vacuum was re-established, the capacitance of the FAB was measured with the LCR meter. The meter was set to slow measurement without averaging with a 100 kHz signal.
Before the blank flange was connected, this was performed by connecting the kelvin clips to the FAB and to the vacuum chamber directly. The reference measurement (just the leads) gave 0.5 pF and the measurement with the fab gave 228.5 pF, giving a capacitance of 228 pF.
Since this measurement would not be possible after the blank flange was connected, we verified that the same result was obtained via a coaxial cable at the feedthrough. With the coaxial cable disconnected from the feedthrough the measured capacitance was 307 pF and with the coaxial cable connected to the feedthrough it was 534 pF, giving a FAB capacitance of 227 pF. With this difference, the measurements are considered consistent.
The coaxial cable measurement was repeated after the blank flange was connected. With the cable disconnected from the feedthrough, the capacitance was 300 pF and with the cable connected to the feedthrough the capacitance was 523 pF, giving a FAB capacitance of 223 pF. Adding the blank flange would be expected to increase the capacitance, so it's surprising that the measurement has reduced, however the change is still small, so could still be attributed to different positions of the kelvin clips or moving of the FAB cable whilst installing the flange.
Pumping Down
After the vacuum seals were remade, the vacuum started to pump down as expected. The turbomolecular pumps were activated and now we cannot take any more data until the vacuum is back to an operational level.
Delay 47ms above ^^^
Delay 77ms above ^^^