- Instructions for Bluejeans meeting connection
- Meeting ID: 290664653
- To join via a Web Browser, go to the page  https://bluejeans.com/290664653.
Thu 9/2/2021 2pm
- Run preparations
- FDC: FDC1 C1
The main discussion was about the problem with the first FDC chamber (FDC1 Cell1):
- In January 2020 (just when we got the first pulsed beam in the 2020 run) the pressure inside the chamber went down from 65 to 60Pa within couple of hours, while the pressure in all the other chambers slightly increased. From that time on, the pressure in that chamber remained always bellow the rest in the first package. 5Pa itself is not a big deal, but turned out to be indicative of a bigger problem.
- Since then the wire efficiency of the chamber started dropping slowly down to ~0.9 at the end of the 2020 run in September last year. The efficiency was dropping with the time even when the chamber was not in operation during the COVID downtime March-July 2020. The pseudohit efficiency went down to 0.7 (0.9*0.9*0.9) as it requires in addition hits from the two cathode planes. Alex prepared two animations from the FDC_Efficincy plugin, aboout the wire and pseudohit efficiency:
- Beni proposed the hypothesis that the gas input to that chamber got clogged and the pressure inside is coming from the output gas connection. As a result there's no flow through the chamber and it starts contaminating due to the outgassing inside the chamber.
- During the 2021 downtime we measured the oxygen contamination in the four packages 1-4 to be about 100ppm, 200ppm, 40ppm, 50ppm. Such levels would not produce the observed drop of efficiency. This also most likely excludes the other hypothesis that we have some leakage, otherwise it would result in higher oxygen levels.
- Naomi proposed to look at the charge vs drift time (or distance to the wire).
- Alex re-run (immediately after the meeting) the FDC_Efficiency plugin for four runs during 2020, where he calculated the chamber efficiencies vs distance. The results confirm that we have contamination in the first chamber that is worsening with the time:
which is not the case for the other chambers:
- Action plan
- As the working hypothesis is that the input of that chamber got clogged, we decided to reverse the gas flow in the first package, i.e. to swap the input and output tubes at the gas panel down in the Hall.
- If indeed, the input is blocked or heavily clogged, when reversing the gas flow the pressure in the chamber will start increasing and we may damage it.That's why we decided to do it first on the FDC spare package.
- Tim, Scot, Mark S., Beni, and Lubomir met in the Hall (on the next day after this meeting) to discuss the plan. The spare package will be moved top EEL126 where the tests will be done. To simulate the problem, we will install a needle valve on the input of the first chamber. Then we will start closing it till we get the pressure difference of 5Pa b/n the first and second chamber. Then we will reverse the flow and see how high the pressure in the first chamber will go. Scot will take care of all the parts needed for this test. He will also design a system of valves to swap the input and output gas lines of the first package. It will be installed as an extension to the gas panel in the Hall, from the side where the FDC gas lines are coming.
- Long term concerns
- If all the above is true, then we have a problem with the gas purity (w.r.t. particulate material). It might be the same green stuff that was found in the CDC MFC that caused the clogging in the chamber. FDC still uses the same MFCs, while the in the CDC it was replaced with a rotameter.
- One chamber (out 24) has a minor effect on the tracking efficiency (Simon), however, this may happen with any other FDC chamber at any time now if the source is the MFC. We may consider replacing the FDC MFCs with rotameters.
- Depending on the results with reversing the flow and identifying the material inside the gas, we may consider reversing the flow also for the other packages, or use any other possible way to purge the chambers.