Revision as of 18:56, 26 July 2012

July 26, 2012 FDC meeting

Agenda

1. Cathode corrosion
   • Status [1], [2], [3], [4], [5], [6], [7], [8], [9], [10],[11], [12]
   • Microscope results
   • Testing chambers
2. Production (Dave)
   • Extra cathode production
   • Second package refurbishment
   • Grounding scheme (Vlad)
3. Engineering (Bill)
4. Electronics (Chris, Nick)
5. Gas system (Beni)
6. Test set-up in 126 (Beni)
7. Other

Minutes

Participants: Fernando, Bill, Eugene, Dave, Nick, Chris, Simon, Beni, Vlad, and Lubomir

Cathode corrosion

- 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 by installing the same cathode on the top and this cathode never showed problems, 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.