July 26, 2012 FDC meeting
- Cathode corrosion
- Production (Dave)
- Extra cathode production
- Second package refurbishment
- Grounding scheme (Vlad)
- Engineering (Bill)
- Electronics (Chris, Nick)
- Gas system (Beni)
- Test set-up in 126 (Beni)
Participants: Fernando, Bill, Eugene, Dave, Nick, Chris, Simon, Beni, Vlad, and Lubomir
- After opening the first package for refurbishment last Friday we found imprints on the copper (many pictures linked above) corresponding to the two sides of the O-ring (EPDM). We tested the resistance between the strips and the output connectors and it turned out that all of the cathodes had about 50% of the channels with a resistance higher than 100 Ohms. Bill took samples from one cathode, both copper and O-rings. Pictures with optical microscope showed that some kind of corrosion has started and at some places along these traces the copper was removed completely. Also some flakes (maybe copper) are visible on the O-ring. See pictures at: /u/group/halld_engineering/FDC/cathode\ corrosion (Linux) or M:\halld_engineering\FDC\cathode corrosion (Windows), and in the subdirectories there. First package was tested from January to June in 126 EEL. Right before moving it to Blue Crab, Vlad tested all the cathodes with a 55Fe source and a scope (except for the bottom cell, not accessible with the source) and all the channel were working fine showing similar signals. Fernando explained that if the resistance is increased the signals would be longer (same total charge) and Vlad would have seen it.
- We immediately opened the second package: only the top cathode was severely damaged while the rest were either without bad channels, or only with a few. After completely testing the second package at the beginning of the year, it was put aside with closed gas connections (no gas flushing) and was always at Blue Crab. Some traces corresponding to the O-ring are visible also on the two type-3 cathodes of the first production cell which we tested at 126 in the summer of 2011 for about 3-4 weeks. No traces are visible on the cathodes of the full-scale prototype. It's different from the production packages that it didn't have deadened area; therefore the deadening is the main suspect so far. Another observation is some discoloration of the copper on those cathodes that are facing the top of the wire frames, the pattern follows exactly the shape of the epoxy (Epon) that holds the wires. Most of us think it's not related to the corrosion problem, we used exactly the same epoxy in the full-scale prototype, except there was longer time between building the wire frames and installing them in the package. For the first production cell tests the wire frame was built with Epolite.
- Olga looked at the samples with electron microscope and found substantial amount of sulfur! She made measurements at 9 different places (look at the "SEM scan of foil 7-25-2012" subdirectory) starting from outside the gas volume (1) then at the outer trace (2-6), in between the traces (7), at the inner trace (8) and inside the gas volume (9). One sees high amounts of sulfur inside the gas volume and at the traces, but almost nothing in between the traces or outside of the gas volume. Sulfur may come from the electroplating procedure which uses copper sulfate water solution, or possibly from the EPDM materials, there's a paper reporting damages on the copper from the EPDM.
- Long discussions and many hypotheses explaining different features of the above observations. Fernando: sulfur may react with a gas coming from the EPDM (explains the place of the corrosion). Bill: EPDM leaks, creating flow of some gas containing sulfur that is deposited there. Lubomir: all the packages were tested cell by cell but installing the same cathode on the top. For the second package this cathode never showed problems during the tests, but was severely damaged when we opened it several months later. Therefore it's possible the corrosion started developing later when the packages were not connected to the gas, say due to the moisture in combination with the sulfur deposited somehow inside the chamber.
- We already started a test with the testing chamber in the following configuration: Lexan sheet, spacer ring with Viton and Apiezon, new cathode type-2, wire frame #27 (7.8cm deadened area) with Viton and Apiezon, spacer ring with EPDM, cathode type-1 with EPDM (from the second package but the dark traces at the O-ring were covered we new foil at many places), and Lexan on the top. HV was applied on 07/25 and we will wait for about a month before opening it to look for corrosion effects comparing EPDM and Viton. At the same time we will start tests with an old prototype, Simon's chamber. It's small and hopefully we can get results faster. The plan is to replace the wires using our technology and deaden them, then again to mount EPDM at one side Viton O-ring at the other side, and test it under HV. Eugene proposed also to do tests with high intensity radioactive source. Eugene required to prepare a document describing the tests.
- In one hand, we have 13 cathodes with, say 30-70% of the channels having resistance higher than 100 Ohms; these are all the cathodes from the first package and the top from the second. The rest 11 cathodes of the second package: 4 are OK, 5 have one channel above 100Ohms, 2 have 4-7 bad channels. In the other hand we have 8 spare cathodes and spare foils that we can use to build 14 new cathodes, in total 22 cathodes. The plan is to use first the spare cathodes while building new cathodes at the same time. For that we need also cathode frames and daughter cards. Casey already proofed on one type-1 damaged cathode that we can remove the daughter cards and the foil from the frame. We should try the same also with type-2 cathode and then build new cathodes using the old frames and daughter cards. If this is successful, we can continue building new cathodes, if not we will have to order also new frames/daughter cards, Fernando says they are not expensive.
- We discussed briefly if we can start stacking the second package, basically we have the parts ready for three cells, siting on tables. Everybody preferred to wait for more results from the electron microscope and from the testing, but since it may take a month or two, we agreed we can stack three cell without tightening the package for now.
- Many questions remain unanswered, some of them urgent, like do we want to clean (and using what chemical?) the cathodes from the second package to prevent further corrosion. We decided to flush the third and forth package with Nitrogen, although we haven't seen problems there.
Due to the long discussion we had to stop at this point. Vlad has prepared an Excel file describing the grounding scheme which is linked for those interested.