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Report from COT Aging Committee

Report from COT Aging Committee. Bob Kephart March 4, 2004 CDF Executive Board Meeting. Committee Membership & Charge. Internal members Bob Kephart (Chair) Bob Wagner Aseet Mukherjee Morris Binkley David Ambrose Kevin Burkett Chris Hays Rob Roser Del Allspach Kevin Lannon

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Report from COT Aging Committee

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  1. Report from COT Aging Committee Bob Kephart March 4, 2004 CDF Executive Board Meeting

  2. Committee Membership & Charge Internal members Bob Kephart (Chair) Bob Wagner Aseet Mukherjee Morris Binkley David Ambrose Kevin Burkett Chris Hays Rob Roser Del Allspach Kevin Lannon Muzaffer Atac External members Fabio Sauli Jerry Va'vra Carsten Niebuhr Marcus Hohlmann John Kadyk CDF Spokespersons Nigel Lockyer Luciano Ristori • The Committee has met twice: • Feb 20, 2003 • March 3, 2003 (yesterday) • We are very much in “input mode” as we try to digest the known facts • Presentations, meeting minutes, and the charge to the committee can be found via the following web link: http://www-cdf.fnal.gov/upgrades/cot/aging_committee.html RD Kephart CDF Exec. Board

  3. What do we know ? • The BAD NEWS: • The COT tracking chamber has exhibited a reduction in gain in the inner super-layers that is faster than that which we expected due to wire aging processes in Ar-Ethane-Isopropanol. • The pattern of aging in the chamber is asymmetric in phi and z and exhibits non-linear time and luminosity dependence. These patterns are unexplained at this time • So far the COT group has been unable to identify cause of unexpected high rate of aging or to significantly reduce the aging rate observed in the chamber RD Kephart CDF Exec. Board

  4. Reminder of basic design and geometry • 1/6 Section of COT End Plate (Super Layer / Super Cell Design) Incorrect shaper positions RD Kephart CDF Exec. Board

  5. Width versus cell () (Axial Layers) • Run 149663 (very early run) • Run 168820 (just before Fall ’03 shutdown) Width is proportional to pulse height, Largest loss in gain is in inner SL at the bottom RD Kephart CDF Exec. Board

  6. Width versus z (Axial Layers) • Run 149663 (very early run) • Run 168820 (just before Fall ’03 shutdown) Loss in gain is larger at positive Z than negative Z RD Kephart CDF Exec. Board

  7. 3-D Aging pattern in SL2 RD Kephart CDF Exec. Board

  8. Time DependenceWidth .vs. Run Number • Rate at which gain is lost is not constant in time • Some regions in time show very little change • Some evidence for gain recovery after a shutdown • Attempts to correlate the knees in the plots with “events” related to COT, gas system, beam, silicon system, etc. so far have not been fruitful • Does it depend on integrated or instantaneous Luminosity ? RD Kephart CDF Exec. Board

  9. Width .vs. Integrated Luminosity (before fall shutdown) Ave. Loss in gain does not appear to be simply proportional to integrated luminosity RD Kephart CDF Exec. Board

  10. Time Dependence SL2 Width .vs. Region in chamber RD Kephart CDF Exec. Board

  11. Time Dependence SL2 Width .vs. Region in chamber Loss in gain in SL2 at bottom and Z>0 appears to be continuous, Normalize to green RD Kephart CDF Exec. Board

  12. Time Dependence SL2 Width .vs. Region in chamber Same plot, normalized to green= region of SL2 that shows little or no damage RD Kephart CDF Exec. Board

  13. Other Facts and observations: • z dependence: • Gas input at z = -150 cm and exits at positive Z = 150. • Is the flow rate too small? “Contaminants” flowing toward exhaust ? • Magnetic field points in + Z direction, protons go that way also… •  dependence: • Chamber currents do not indicate that there is more radiation on the bottom. • Are there gas flow variation with ? We don’t know. • We observe temperature variations in the chamber (bottom of COT inner tube is colder) Maybe this drives convection currents dragging contaminants downwards ? Plan = study this possibility. • Are there places cold enough to condense alcohol ? (6 F dew point) • What about the Gas Monitor Chambers? • These chambers are located such that they sample the inlet and exhaust gas • No significant aging observed in these chambers. Why ? 79 77 68 72 RD Kephart CDF Exec. Board

  14. Other Facts and observations: • Evidence that gain of inner superlayers goes up during a store. • SL2/SL8 width and currents increase during a store • SL8 gain is stable  SL2 goes up • Does it charge up with ions ? • Fix SL1 to have same current per unit length as SL8… observe that SL1 change in gain tracks gain change of SL2  temperature effect ??? Model seems to agree • If yes  3 degree C temperature rise in the gas near inner SL when beam is present,(dG/G=6% x dT/T where T = abs temperature ) • Corresponding temperature rise on inner bore tube is < 1 degree. • Does this matter ? Convection ? SL2/SL1 SL2/SL8 Aseet’s fit using input power to SL2, volume, and specific heat RD Kephart CDF Exec. Board

  15. The Good News • The outermost super-layers exhibit very little aging. • There are portions of the innermost super layers that exhibit aging that is small or non-existent even though those portions of the chamber operate in the same gas and with particle fluxes and gas gains that are identical to portions where significant aging is observed • The aging has not been uniform in time. There are times when aging in some parts of the chamber seems to stop. The observed aging is not simply proportional to time or integrated luminosity.  there is a good chance that if we can understand these phenomenon it will lead to a fix. RD Kephart CDF Exec. Board

  16. Other Tests & Results • Is it a contaminate of the gas or alcohol? • Can’t rule this out, but no large aging effects observed in gas monitor chambers located on the inlet and exhaust lines • Analysis of inlet gas samples and alcohol in bubbler show nothing unusual • Setting up a pump system to take samples of exhaust gas to send away for analysis • Will try to condense alcohol from exhaust gas and analyze that in an attempt to concentrate possible aging sources • Observe Gain Recovery after shutdowns. Why ? • We don’t know: Feb 17 COT guys introduced N2 into the chamber, and CDF warmed the silicon detector for ~ 4 stores. At most COT aging was reduced by x 2 during this time. RD Kephart CDF Exec. Board

  17. Aging “Budget” • How much aging do we observe vs what we expect? • Drop in SL2 corresponds to ~ 500%/coul/cm • Gas monitor chambers age at < 2%/coul/cm • How much aging can we allow ? • Not a lot more in SL2, in some places near the bottom width=pulse height is ~ half of that when chamber was new • Limited ability to reduce thresholds (noise) • Limited ability to raise HV, other cells in SL2 limit how much it can be increased… could split top and bottom on different HV supplies… but it’s a lot of work and would only be a temporary fix if aging continues RD Kephart CDF Exec. Board

  18. Current Status & Plans • Damage Control: • Friday February 13th: Turned off the HV on SL1,2 and reduced the HV (gain) on SL3,4,5. XFT changed to mask axial SL2 on and to allow 3 misses instead of 1 miss in SL4. • Goal = limit further damage to inner superlayers until the aging problem can be understood • March 3: Direction of gas flow to chamber was reversed • SL1,2,3, 6,7,8 back at nominal voltages • Goal is to determine if aging pattern in Z follows direction of the gas flow in the chamber.. If yes then this mode also buys us more time to diagnose the problem • March 15: Ten day shutdown is planned to warm up A4 house… • COT will use this time to do more tests (next slide) RD Kephart CDF Exec. Board

  19. March 15 Shutdown Plans • Gas Flow rate: • Flow rate to the chamber was increase ( Jan 12) from 20 to 30 SCFH  one volume change per 15 hrs • Other experiments have observed aging due to insufficient flow rates in chambers • Plasma Chemist consultant (Dr. Yasuda) confirms this and says build up of heavy hydrocarbons downstream in gas flow pattern could result in deposits of heavy cross-linked materials on wire • Plan is to add a recirculation pump to “stir” the chamber at about x5-10 times the present flow rate • Operations Dept. will try to get this installed during the shutdown (three PPD engineers currently working on it) RD Kephart CDF Exec. Board

  20. March 15 Shutdown Plans • Extract a wire plane from SL2: • Plan is to pull one or more wire planes and to examine surface of field sheets visually to check for damage • Extracted wire samples will be examined visually with optical microscope and sent off to be viewed with an Scanning Electron Microscope (SEM) • SEM provides information on the nature of deposits on wire • Fourier Transform Infrared Spectroscopy is planned for analysis of the chemical composition of deposits (molecules) • Extracted wires will be installed in a test chamber to study means for reversing the observed aging (eg adding CF4, O2, CO2, H2O, etc) RD Kephart CDF Exec. Board

  21. Summary • COT Aging Committee has started its work: • Input mode… Have no firm recommendations • Have made a few suggestions for various tests. The COT group is doing a good job at chasing down leads • Data from the gas reversal and wire extraction should be very useful in understanding what is going on • The problem is clearly serious, but the data indicate that there are reasons to be optimistic that a solution can be found… however…. The clock is ticking…. • COT group is small… a few dedicated people added might make a big difference in how fast this problem is resolved RD Kephart CDF Exec. Board

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