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CF4 vs. ArCO2: A Frustrating Week

This update highlights the challenges faced with CF4 gas in operating GEM stacks, while showcasing the stability and success with ArCO2 gas. The need for potential changes in HV supply and acceptance of single bad strips on GEMs is discussed.

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CF4 vs. ArCO2: A Frustrating Week

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  1. Stony Brook Update:Another Frustrating week...However difference is isolated: CF4 T.K. Hemmick for the Tent Crew

  2. Picking up where we left off… • We had a “bad” stack that developed problems while in CF4, trying to operate as a stack (not individual GEMs). • Burned off the problem from bottom GEM and tried something new: • Ar-CO2 (Full gain) • Trip after 5 min…no damage. • Trip after 25 min…no damage. • Trip after 3 hours…no damage. • Trip after 6 hours…no damage. • Did not trip for an additional 13 hours. • CF4 (try for gain=1000) • Trip after 30 seconds…bottom GEM drawing current…burn. • Trip after 1 min…no damage. • Trip after 1 min…middle GEM draws current…burn. • Next trip = no recovery possible, restack the stack…

  3. Try the next stack (replace both Cu) • Usual procedure w/o changes. • Ar-CO2 (Full Gain) • No Sparks in 5 hours. • CF4 • Could not sustain gain of 1000 for more than a few seconds…several sparks while trying… • Spark on middle GEM makes damage…burn. • G=600 for 20 hours, calms after initial sparks. • G=1400 for one day, calms after initial sparks. • G=3200 no sparks…1/2 day. • G=4800 minutes only…sparks increase in frequency…back to G=1400 to hold stable (no longer stable at 2500). • Still in box…we’ll see…

  4. Conclusions • The real difference to last year is uniquely identified: • ArCO2 is easy, stable, and operates well. • We can build GEM stacks all day long and have great results in this gas. This is what we did last year! • CF4 is the difference that makes for nearly zero progress in several weeks…stable operation of a perfect stack at G=5000 is simply a rare event even though it does happen sometimes. • CF4, we believe, is the one reason why we were able to do production quickly last year and now we are at a snails pace. • Remember that the “stable performance” in the IR of about ½ of the stacks, was nearly always on stacks w/ at least one GEM having a dead short or w/ non-linear current draw. • Is my memory correct…1 perfect stack out of 24??? • Our present requirement of perfection of a stack under long term CF4 tests of all three GEMs in a stack at G=5000 very rarely produces a good stack. • If CF4 is forever the gas of choice: • We may need a quadruple stack. • We may need a HV supply system that CAN tolerate some current draw w/o losing gain (i.e. no R-chain, but instead direct power). • We may need to accept single bad strips on individual GEMs as an acceptable level of performance.

  5. Options for direct HV power • With direct power we would NEVER EVER have to change resistors during a run. • With direct power, we could condition individual GEMs from the counting house even while the beam is present. • With direct power we would have nominally zero current draw and could trip directly and sensitively on a spark. • A single 1469 power supply provides: • 24 outputs at 3 voltages. • Trip relay ALREADY located behind the filter cap. • If we used multi-strand HV cable we could power the whole HBD and simply note with interest current draws WITHOUT losing gain. • 1469 does not seem to have the FPGA code bug. • 1471 (present) requires 6 power supplies. • 1469 (future?) requires 7 power supplies, but limited flexibility due to “coupled” voltage settings. • PHENIX likely has plenty of 1469 power supplies.

  6. Not-so-good news • Our scroll pump used on the gain test box failed over the weekend. • We are using an ordinary oil-based pump temporarily while a rebuilt unit of better manufacture (Varian TriScroll like the one on the HV storage) is acquired by Rich. • This oil-based pump has a special (new & high tech) particle filter for particles above 6 microns in size. • The filter limits pumping speed and ultimate vacuum, but should keep the GEMs clean from the lesser pump, particularly if we avoid pumping to ultimate vacuum.

  7. Everything old is new again. • The past two stacks have been 100% GEMs from new production. • The behavior of these GEMs is DRAMATICALLY different from last year’s production: • The voltages in the NIM article are the ones required for these GEMs (close to 3800V on the chain in CF4 instead of 3500-3600V). • These GEMs do not show the charge-up saturation of the gain curve, they look more like they drop over time! • Reminder: • Sauli criticized us last year saying that our results were somehow related to the absence of his personal supervision of the GEM manufacture process. • Was he more involved now? The GEMs are dramatically different.

  8. We may have water trouble soon. • The glovebox has started a slow rise in water level indicative of the filter filling up. • The glovebox CANNOT be regenerated with the HBD open as the water content on the GEMs would rise. • We are soon near the point where we will have to take emergency action: • Stow “open” GEMs into the vacuum storage. • Close up the HBD and put on independent flow. • Regen the glovebox filters. • Open back the HBD. • This would cost us most of one more week.

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