1 Stony Brook Update More on Voltage Asymmetry T.K. Hemmick

2 Reasoning behind Voltage Asymmetry It is well known that even when individual elements in a multi-stage gain system hold a particular Voltage/Gain that the system as a whol is not guaranteed to operate at the same point. The concept behind Voltage Asymmetry is that if one can isolate the weak point, one can relieve the gain burden on that point and take it up elsewhere. COMPASS investigated this.

3 From COMPASS NIM A, 479 (2002) What’s OK for a single GEM is not necessarily OK for the triple or double stack.

4 NIM A, 479 (2002) The COMPASS Solution: ∆V = V Bottom – V Top At fixed gain, one can scan asymmetry to find the most table point. 2 Orders of magnitude improvement.

5 Measuring discharge rate Take a voltage signal off bottom of resistor chain AC couple to see fast discharges Feed into standard NIM electronics and count for set time period But these signals look familiar…

6 Measuring discharge rate II Spectrum could give info on which GEM shorted in a trip, based on pulse height Pulse heights dependent on magnitude of discharge

7 GEM Substitute Dangling wire can be shorted to simulate single strip discharge

8 Adjustable Asymmetry Parameter Potentiometers allow adjustment of top and bottom GEM dV Up to 32% asymmetry Sacrifice accuracy of current measurements due to low resistance of pots Can measure asymmetry in one direction only (positive asymmetry), but that is best for hadron blindness

9 Saturday Triple GEM stack, all new production 3 GEMs: –Csi Au…Hold 500V well, 520 V some discharges, 540 V…does not hold. –Cu GEM…stable at 540 V. –Cu GEM…stable at 540 V Gain of 2200 at dV = 500V. Stack operates relatively stably at 500 V, will not go higher… Question: –Where is the weak point…CsI or bottom GEM?

10 Next Step New stack formed: –Bare Au GEM –Bare Cu GEM Regimen: –Test all bare stack and determine whether top GEM remains as the weakest link. –Investigate Asymmetry on stability with Matt’s adjustable chain. –Coat with CsI and repeat. Determines: –Whether CsI is a source of Voltage instability. –Whether CsI still affects gain on new production GEMs –Where the weak link in the chain is.

11 Possible Outcomes Identify that the weakest link is bottom GEM and use graded triple stack for stability. Identify CsI coating as source of instability (perhaps simply from higher gain?) and use graded triple stack. Identify equal weakness in whole stack and use 4-GEM solution (only one known to work at this time).