A high voltage test for the qualification of RPC gas volumes for the ATLAS experiment P' Camarri Uni

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A high voltage test for the qualification of RPC
gas volumes for the ATLAS experimentP.
Camarri(University of Roma Tor Vergata and
INFN Roma II)on behalf of the ATLAS RPC
CollaborationLecce, Napoli, Protvino, Roma II
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Summary

• Motivations for a gas-volume acceptance test
• Description of the gas-volume qualification
  procedure
• Further steps introduced to recover (if possible)
  initially rejected gas volumes
• Comments and present status.

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Motivation

• High number of gas volumes to be produced (3320),
  with requirement of stable, long term operation

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Test location and setup

• The test is performed at General Tecnica (the
  gas-volume manufacturing firm)
• The high voltage for the gas volumes is provided
  by a Bertan (4 kV, 40 mA) and a CAEN N570 (15 kV,
  1 mA) power supplies
• The gas volumes are grouped in bunches of 23-24
  units soon after they have been closed
• The total current flowing through each gas volume
  is measured it is given by the voltage drop
  across a 100 kW resistor on the HV return line
• The measurements are performed using NI DAQ PCI
  boards and LabVIEW

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Test procedure

• A pre-conditioning of the gas volumes is
  performed before varnishing, in order to
  eliminate inner surface impurities.
• After the linseed oil varnishing, the final
  selection is performed by measuring the
  current-voltage characteristics of the gas
  volumes.

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ATLAS RPC glossary
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Step n. 1 Argon conditioning

• A total current drop is observed when keeping the
  gas volumes filled with pure Argon at 2.1-2.2 kV
  for 2-3 days. This is due to the inner surface
  conditioning during the treatment.
• The current through the gas volumes is initially
  set at 80-100 mA/m2 by changing (if necessary)
  the resistance value on the HV return lines by
  means of variable resistors
• This step is meant to optimize the inner gap
  surfaces before the oil varnishing.

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Step n. 1 Argon conditioning

• Duration 2.5 3 days
• Initial drop of current and subsequent
  stabilization

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Before and after the conditioning
Initial scan
Comparison After the treatment the discharge
regime starts at higher voltage
Final scan
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Step n. 2 HV scan with binary mixture
(TFE/iC4H1090/10)

• The gas-volume certification is based on the
  measurement of the I-V characteristics with a gas
  mixture similar to the ATLAS one, except for the
  SF6 for practical reasons
• The acceptance criterium is I lt 2 mA/m2
  _at_ (9 kV, 20 C, 1 bar), but the complete I-V
  behaviour is also important

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Step n. 2 HV scan with binary mixture
(TFE/iC4H1090/10)
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Step n. 3 re-conditioning of the suspended gas
volumes

• The gas volumes (now with oil !) are
  re-conditioned with Argon so that the current
  does not exceed 20-30 mA/m2, for 2-3 days
• After that, the I-V characteristics is measured
  again with the binary gas mixture to see if any
  of the initially suspended gaps can finally
  fulfil the acceptance criteria.

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Recovery results so far

• Extremely effective for the BML-D (126 volumes
  recovered).
• 10 BML-A recovered (starting, however, from a
  much better initial acceptance percentage with
  respect to the BML-D).
• Ineffective for the BML-E, which showed several
  additional problems (bad spacer gluing, bulk
  resistivity out of the specifications in many
  cases).
• Good result for the BMS-E (18 recovered in one
  session).

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Average gap resistivity evaluation

• It is possible to compute the average gas-volume
  plate resistivity by measuring the slope of the
  linear region (at V gt 2.2 kV) of the Argon I-V
  characteristics
• To have common reference conditions for all the
  measurements, we use the empirical scaling law in
  order to estimate the resistivity at 20 C
  

S gas-volume surface d plate thickness
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Average gap resistivity evaluation
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Resistivity measurements vs. production serial
number BML-A and BML-E
BML-A average plate resistivity within the
required range, except the units from 220 to 260
which have higher average values
BML-E average plate resistivity mostly outside
the required range, with many average values
below 1010 W cm
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Resistivity distributions BML-A and BML-E
Mean value 26.2 GW cm St. Dev. 11.7 GW cm
Mean value 15.6 GW cm St. Dev. 15.3 GW cm
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Current test integral trend (only accepted gaps
Oct 7, 2003)
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Test summary (October 7th, 2003)
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Conclusion

• A gas-volume selection technique has been
  introduced to certify the RPCs for the ATLAS Muon
  Trigger Detector
• Additional careful iterations of the test allowed
  the recovery of a big amount of gas volumes
  rejected at the end of the ordinary selection
  procedure
• So far we observed low acceptance percentage only
  with a specific gas-volume type (BML-E) this is
  probably in connection with more problems related
  to the BML-E
• The ATLAS RPC gas-volume qualification test at
  General Tecnica has reached 45 of the final goal
• Due to the present acceptance rate (about 33 gas
  volumes/week), one more year is needed to
  complete the gas-volume selection.