1 / 55

Anomalous HV Currents in the D0 Central Calorimeter

Anomalous HV Currents in the D0 Central Calorimeter. a.k.a. Malter Currents. Outline. What information do we have Lots: too much show if all What effect does it have on data Systematic change in the detector response Can we explain what we see?

caine
Download Presentation

Anomalous HV Currents in the D0 Central Calorimeter

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Anomalous HV Currents in the D0 Central Calorimeter a.k.a. Malter Currents Dean Schamberger

  2. Outline • What information do we have • Lots: too much show if all • What effect does it have on data • Systematic change in the detector response • Can we explain what we see? • With some hand waving arguments agreement with many of the observed effects can be explained Dean Schamberger

  3. Malter Process • ions accumulate on oxide layer • field extracts electrons from base metal increasing current • oxide eventually breaks down and discharges surface Dean Schamberger 3

  4. D0 readout Cell Dean Schamberger

  5. Is U Oxide like Al Oxide? • Resistivity of Al Oxide is 1 e14 ohm cm • Resistivity of U Oxide at 83 K is 1.4 e16 ohm cm • From D0 note 1154 by M. Lynn Stevenson (1991) • Similar enough to cause Malter Currents Dean Schamberger

  6. Excerpt from D0note780, RLM Nov,1988 • Current then about 25 times higher than Uranium decay rate would predict. Dean Schamberger

  7. HV Current Monitoring • Both Test Beam and Run I HV current monitoring data is no longer available • Starting in about 2002 “5 minute” HV currents sampling on the 32 CC HV supplies is available • Occasionally in 2010-2012 high sampling rate data (> 1Hz) was taken for more detailed studies • In the fall of 2008 one HV supply was split into the 16 individual wires to “fix” the Purple haze noise. This allows use to study the current drawn from both ends of the same HV gang Dean Schamberger

  8. CC Currents – no beam Dean Schamberger

  9. ECN currents – no beam Dean Schamberger

  10. ECS Currents – no beam Dean Schamberger

  11. CC current – with beam Dean Schamberger

  12. EC currents – with beam Dean Schamberger

  13. CC HV “phi symmetry” Dean Schamberger

  14. Malter Breakdown Dean Schamberger

  15. End of store Current drop Dean Schamberger

  16. Resistance Measurements Measurements (plot) are for ten surfaces in parallel Table lists the single surface Resistance Expected ~0.4 from the Nim Article description Now understand we should have expected ~1.5 +/- 25% Resistance essentially unchanged from when we built the detector Dean Schamberger

  17. Current from the 2 ends Dean Schamberger

  18. Changes for the better? Dean Schamberger

  19. HV sag at twice the “typical” current draw at 3x1032 • Profile assumes uniform current per unit area • Max voltage change is ~400 Volts • Converting from voltage drop to signal decrease has never been measured Dean Schamberger

  20. Long Term Behavior • The upper right plot is the current for one channel over a 10 year period. • The current increases by ~4 • The maximum increase is ~7 for channel 0 • The lower right plot is one year of running for an EC channel • The jumps in the blue trace are thought to be resistive shorts which come and go • There is no long term trend Dean Schamberger

  21. CC HV Current continues rising • Since 2002 the “no beam” current in the CC has risen by about a factor of 4 % raise in current Dean Schamberger

  22. HV Currents (cont) • In the 7 months following the last long machine downtime the current rose an additional 40%, with typical currents 175 Amp Dean Schamberger

  23. HV Currents (cont) • In the next 6 months only rose an additional 10%, with typical currents 200 Amp • This is about a factor of three less than the previous 6 months Percentage Dean Schamberger

  24. Current vs Luminosity • Current increase is proportional to delivered luminosity • All channels have a similar behavior • Two points at L~7 and 12 are 2 measurements during the the same shutdown • indicates the size of the error bars Dean Schamberger

  25. Turn on Current • The current draw in the CC is very different from the EC • The CC (upper plot) takes ~2 days to reach equilibrium current while the EC is less than 5 minutes • Note the very good exponential fit to the CC data CC current draw. Red curve is fit EC current. The plot covers 2 days and the current goes to full scale in one 5 minute sample period Dean Schamberger

  26. EC-CC difference • Most likely difference between the EC and CC is in the Uranium plates • The EC had the UO2 removed with a high pressure water jet before assembly while nothing was done to the CC plates • The readout plates were processed in the same manner for both detectors • Shape is different (long-thin in CC, more square in EC) • The UO2 coating can explain the unusual CC behavior Dean Schamberger

  27. Other Features • After about 1 day some channels break into stable oscillations • The oscillations persist in a store • period decreases • Sharp drop at 11.3 hours is end of store • 2 hours between store with 1 oscillation • Vertical bar is losses from scraping • Then current climbs with new store Dean Schamberger

  28. Other Channels • Other channels have a more complex pattern • Plot on right shows 2 frequencies • By using a numerical derivative technique one can do a frequency analysis on these channels Dean Schamberger

  29. Frequency Analysis • Least Squares fit a parabola to 100 consecutive data points • Analytic derivative at center of fitted region • Rolling fit over entire data set • Averages out small data fluctuations Derivative for HV Chan 0 Time in minutes Dean Schamberger 29

  30. Channel 25 • Pattern suggests 2 different frequencies • Can extend this method to more complex distributions Derivative for HV Chan 25 Time in minutes Dean Schamberger

  31. Channel 0 Data No store Store 7261 Dean Schamberger

  32. Oscillation Cycle Times • Cycle times are an hour or more • Cycle times in a store increase by ~50% over the store • No store cycle times are longer and constant Dean Schamberger

  33. Change with Time • Six channels had oscillations in 2002 • 22 had oscillations in 2011 • Channels in 2002 had one or two frequencies • Almost all channels in 2011 have multiple frequencies Dean Schamberger

  34. Beam On/Beam Off Slope • Slope (increase in current/day) is much larger during shutdowns than during beam. • Plots on right are the slope between two shutdowns and the average of the two slopes in the shutdowns Blue: No beam slope in µA/Day Pink: Average slope during stores in µA/Day Dean Schamberger

  35. Change in Current with HV off • Took 24 days of data with HV on at end of Tev. • Then turned the first 16 channels off for 60 days and then back on for 18 days • Following plot deletes the 60 day off period and plots the data as if the HV was always on • a few day overlap is put in to help guide the eye. • If the current continued to increase, we would expect the second part of the plot to be higher than the first Dean Schamberger

  36. The red curve is current after the HV was turned back on • The green curve is the approximate average increase in current experienced by the 16 channels that were left on • If current were increasing during the HV off period, we would expect the turn on to exceed the green line for some of the channels • All pre turn off data is at or above the turn on data • Conclude that there is no increase in current with HV off Dean Schamberger

  37. Malter Breakdown • Lower fig. shows breakdown detail • FWHM of peak is ~200 s • Downward slope time is ~70 sec • RC time constant of HV supply is ~10 s • Not a supply affect (see below) • Neutralizing surface charge takes longer than bringing the charge through the oxide • Little E field in transverse direction so charge movement is very slow Dean Schamberger

  38. Start Of Store Scraping structure supports RC of ~10 s (power supply resolution) Dean Schamberger

  39. Exponential Turn on Curve • DC resistivity of UO2 can also explain this • Model the U LAr system as a capacitor with the UO2 as the dielectric. • Model as a circuit with a capacitor in parallel with a resistor • Current increases as the capacitor charges up Dean Schamberger

  40. Analysis k = current from Ar ions V/R = ohmic leakage current through UO2 Equation is the same as R in parallel with C Solution with V=0 at t=0 Dean Schamberger

  41. let I = total current through oxide and f= fraction that reduces voltage on the oxide layer • Fraction of current into the Argon is then • The argon current is what we measure Dean Schamberger

  42. Current Increase • Linear current increase with no beam is just Ohm’s law • UO2 must have a decreasing resistance • Decrease must be reduced when beam is present • Most likely due to semi conductor properties of UO2 Dean Schamberger

  43. Semi Conductor Properties • 1.3 volt band gap - similar to silicon • Conduction is NOT the same as silicon • No Hall effect observed Dean Schamberger

  44. No Hall voltage means that current is not conducted by free carriers Dean Schamberger

  45. Polarons • Charge transfer is by charge carrier hopping from one donor atom to the next. • UO2 is p type so charge carrier is a hole • As electron moves through the oxide, it polarizes the other atoms in the material creating a retarding field which slows down the electron. • Combined affect is called a polaron Dean Schamberger

  46. Polaron Theory • Small polaron theory gives the conductivity as • T= temperature, E=activation energy, k=Boltzman’s constant • Plot of σT versus 1/T on Log x axis should be a straight line • P. Nagels et al,Solid State Communications 1 35-40 (1963) Dean Schamberger

  47. Conductivity VS oxygen • Obeys polaron theory for 8 orders of magnitude • Data to 90 Kelvin • Conductivity is strong function of oxygen content • Change from UO2.0000 to U2.0064 (0.3%) changes the conductivity by 5357 (Nagels’s paper) • We need only a factor of ~5 • Linear extrapolation would change the O content by ~6 parts per million Dean Schamberger

  48. O2 in LAr • Measured concentration at end of Tev is 131 parts/billion • Current increase is a function of L and not time so O is likely made by the beam • Candidate is the resistive coat which is epoxy based • Basic epoxy bond is C O Dean Schamberger

  49. Model • Argon ions build up on the surface of the UO2 from beam or from U238 decay products • Voltage increases with time which increases the current flow with time • Some regions with oxide layers that are thin or sufficiently conductive build up enough voltage to cause Malter breakdown Dean Schamberger

  50. Model cont • O is released into the LAr by the beam • O is ionized by radiation and drifts to the UO2 surface and is absorbed decreasing the resistivity • HV off stops this process and this is confirmed by the data • Uranium decays keeps the diffusion process going with no beam so this explains the continued increase in current • This must stop as the O is removed from the LAr but we did not wait long enough to observe this Dean Schamberger

More Related