Acceptable LHC pilot bunch intensity

1
Acceptable LHC pilot bunch intensity

• The issue is discussed using some plots of the
  attached presentation at the Pixel 2005 workshop
  (Bonn, Sept 2005).
• Pixel detectors, being quite radiation hard, are
  expected to survive some level of beam accidents
• Summary of the results
• Calculations (M.Shupe, D. Bocian) show that for
  pilot beam intensities of 5 109 the dose
  deposited in one pixel module in the worst
  scenario considered (beam scraping the TAS) is 5
  10-3Gy in few nsec.
• Experiment shows that ATLAS pixel modules survive
  to 200 catastrophic events, each of these
  corresponding to 3 Gy over 42 ns
• i.e. instantaneous dose higher x30 than pilot
  bunch grazing the TAS
• and/or total dose x 600 than
• A pilot bunch of 5 109 should be safe for pixel

EB106 P.Grafstrom L.Rossi
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Effect of accidental beam losses on the ATLAS
Pixel detector Pixel2005, Bonn, September 6th
2005 C. Gemme and L. Rossi, INFN, Genova K.
Einsweiler, LBNL A. Andreazza, INFN and
Dipartimento di Fisica, Milano P. Sicho,
Institute of Physics (FZU), Praha
3
Outline
Beam losses are known to be dangerous for vertex
detectors. The ATLAS pixel detector has been
designed to sustain large integral doses. It is
therefore expected to be robust against
accidents, but is anyhow better to check.

• Beam scenario losses in the LHC IR1
• The experiment done at CERN PS setup and results
• Comparison between the experiment and the beam
  losses in LHC

4
Beam losses scenarios in the LHC IR1

• Given the very high energy stored in each
  circulating beam of LHC, several systems will be
  implemented to guarantee safe operation of the
  accelerator and the detectors beam monitors,
  beam bumps, collimators and beam absorbers.
• A study (D.Bocian, LHC-note 335 and references
  therein) has been performed to understand the
  consequences of beam losses in the vicinity of
  the ATLAS experiment.
• Multi-turn losses
• quenches of superconducting magnets or failures
  in power systems gt
• beam dump system is activated and the beam is
  extracted in 1 ms. Additional inductance added
  to the supplies of the magnets should slow down
  the rate of the beam orbit change to give the
  beam dump system time to act and prevent any
  damage to accelerators and detectors.
• Single-turn losses
• at injection from SPS at 450 GeV

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Beam losses at injection pilot beam

• Losses due to wrong settings at the injection
  (due to incorrect equipment settings or equipment
  malfunctioning) cannot be detected early enough
  and then taken care of by the LHC interlock
  system.
• The consequences can be limited by the adoption
  of a pilot beam of intensity lower than nominal
  not to damage LHC equipment or experiment
  detector but still high enough to be detectable
  by the LHC beam instrumentation for tuning
  purposes. The pilot beam has the same
  characteristics of the nominal injected beam
  apart the intensity.

• PILOT beam
• Bunch 1
• Protons/bunch 5x109
• Proton Energy 450 GeV
• Stored energy 360 J
• RMS bunch length 11.2 cm
• RMS beam size 375 um

• LHC
• Bunches 2808
• Protons/bunch 1.15x1011
• Proton Energy 7000 GeV
• Stored energy 360MJ
• RMS bunch length 7.6 cm
• RMS beam size 17 um

• Once the machine is tuned with the pilot beam,
  injection of standard intensity bunches can begin.

6
LHC optics at IR1
MCBX, MCBY, MCBC - crossing angle/parallel
separation scheme, D1 - separation dipole
magnet, D2 - recombination dipole magnet, Q1, Q2,
Q3 - low-? triplet.
In order to travel correctly through the first
turn, the parameter of all magnets must be set to
the correct values. Assuming that only one magnet
at a time is set to the wrong value, three
magnets have been identified in the IR1 region to
be potentially dangerous for ATLAS MCBX, D1 and
D2.
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Pilot beam on the detector

• Bocians study has considered different
  combinations of wrong settings of the MCBX, D1
  and D2 but due to the presence of an absorber
  (TAS) at each end-cap of the ATLAS experiment,
  the pilot beam can never hit directly the ID.

• Thus the most dangerous case is when the wrong
  magnets setting is such that the beam scrapes the
  incoming TAS and then hits the outgoing TAS.
• Scraping 3m long TAS, only an indirect flow of
  particles hits the ID in few ns.

8
Pilot beam on the detector

• If such a pilot beam loss happens, the estimated
  radiation dose delivered to the b-layer is 5x10-3
  Gy, i.e. 10-2 J energy deposition (M. Shupe
  calculations presented by Polesello at EB98).
• Considering a specific heat capacity of 300J/(kg
  K) the temperature increase will be negligible (
  2x10-5 degrees).
• The radiation dose is also negligible as the
  detector is designed to survive to a radiation
  dose of 5x105 Gy, that corresponds to 108 pilot
  beam losses.
• The radiation dose and the temperature increase
  are not of concern but the energy is deposited in
  a very short time and corresponds for few ns to
  O(MW), a value so high that needs to have an
  experimental proof that the detector can really
  survive to these conditions.
• Thus, a module was exposed to a beam of intensity
  comparable to the one expected in case of a pilot
  beam accident. This was possible using the CERN
  PS fast extracted beam of 24 GeV protons.

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ATLAS Pixel Modules

• The ATLAS pixel detector is a mosaic of 1744
  identical modules.
• A module consists of a Si sensor tile of 16.4 mm
  x 60.8 mm active area connected via high-density
  bump bonding to 16 FE IC. Each FE read-outs 2880
  channels organized in a matrix of 18x160
  channels. Each pixel has a 400mm x 50mm area.
• The 16 FE chips are controlled by a Module
  Controller Chip (MCC). It decodes data/cmd
  signals, generate control signal for 16 FEs,
  collects data from FEs and accumulate in FIFOs,
  checks event consistency, builds module event and
  sends to DAQ, handles errors.
• A Flex-Hybrid circuit glued on the sensor
  backside provides the signal/power routing
  between the 16 FE chips and the MCC.

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The test module

• A pixel module was randomly chosen to the
  standard production line after having passed the
  QA tests.
• The module has separate digital (DVDD2.0V) and
  analogue voltages (AVDD 1.6V) with a total power
  of 3.5W. The sensor is biased at 150V with a
  leakage current in the mA range at room
  temperature.
• The module was connected to a test board equipped
  with coaxial cables to remotely monitor the
  electronics low voltages (DVDD and AVDD) and the
  sensor high voltage (Vb).
• It was glued to a C-C cooling support to keep the
  module temperature to 30 0C during operation
  (higher than in ATLAS standard conditions).

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Module in the beam

• The module was placed parallel to the beam
  direction. Beam particles crossed the full active
  length of the sensor (6.04 cm) in order to
  maximize the energy deposition and to mimic the
  barrel geometry that is the most unfavorable one
  in case of beam loss accident with beam scraping
  the TAS.

• Using a low mass cable the module was connected
  to a prototype of the final PP0. The electronics
  was powered by a prototype of the voltage regular
  system to be used in ATLAS.
• Lengths of cables were as the final ones so that
  the powering system provided an accurate model of
  the electrical environment seen by a module in
  ATLAS.
• The coaxial cables were brought to the control
  room to be monitored during the beam extraction.

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The beam

• The module was exposed to the CERN PS fast
  extracted beam of 24 GeV protons
• Shots delivered at T7 area user selected shots
  of n-bunches separated by 256ns (n ranging from 1
  to 8)
• Bunch width 42ns
• Number of protons in each bunch 1011
• Beam spot profile was measured by an optical
  stimulated luminescence (OSL) film plus Al foil
  activation in the vicinity of the module.

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Beam profile

• Beam profile of 1011 protons/bunch the beam
  maximum intensity was 2.85x1010 p/cm2/bunch in a
  central area of 5x3 mm2 (pink region).
• The average flux over the module was 1.5x1010
  p/cm2.

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Fast Extraction tests

• Characterization before/after
• Standard tests were performed on the module after
  installation and after the extraction test,
  including measurement of noise, threshold and
  leakage current pixel per pixel.
• During Fast extraction tests (90 min)
• Low and high voltages were monitored with an
  oscilloscope using 10MOhm probes. In between the
  shots the status of the module was verified
  measuring pixel threshold and noise.
• A signal provided by a 1 cm2 diamond detector
  placed in front of our test set-up was used to
  synchronize measurements with the beam.
• A total of 213 bunches organized in single shots
  or in shot trains (10 shots of 8 bunches each at
  a shot period of 19.2 s) crossed the pixel
  module.
• Important comment
• 3x1015 carriers are generated by each bunch in 42
  ns 10 orders of magnitude higher than in normal
  operation high luminosity. This charge generates
  a peak current of O(200mA) on each pixel. The
  power supply system can not provide the requested
  currents and therefore keep the set voltages.

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Test during the extraction low voltages

• Low voltages suffer a sudden drop and then come
  back to the set value with time constant that
  depend on the characteristic of the power
  supplies and on the electrical parameters of the
  module (50ms for DVDD and AVDD). The digital
  drop DVDD causes the loss of configuration data.
  The module was then reconfigured between the
  shots.

16
Test during the extraction high voltage

• The sensor remains undepleted for a relatively
  long time (O(ms)) as the rise is driven by the
  large (O(nF)) decoupling capacitance placed on
  the flex to dump the noise contribution due to
  fluctuations in the power supply voltage.

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Test after the extraction

• Total dose can be computed
• 1.5x1010 p/(cm2 bunch), sensor cross section
  4x10-2 cm2 gt 3x1015 carriers
• 3x1015 carriers gt 3 Gy/bunch, 200 bunches gt 600
  Gy.
• Module mean values of threshold, noise are not
  changed after irradiation. The mean pixel leakage
  current has increased of 1.5nA/pixel that is
  compatible with the accumulated dose.
• The increase is not uniform on the module area
  integrating the contributions over the beam
  direction (i.e. along the columns), we get a row
  distribution that indicates that the increase in
  current is higher were the dose was higher.

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Conclusions

• The worst beam loss scenario for the pixel
  detector is represented by the pilot beam
  scraping the TAS during injection.
• A fast extraction test was performed at CERN PS
  to study the response of an ATLAS pixel module
  to high instantaneous dose.
• Each bunch in the PS experiment delivers to the
  ATLAS module a dose of 3 Gy in 42 ns, that is
  higher than the dose that the LHC pilot beam
  could deliver to a b-layer module (5x10-3 Gy in
  few ns). The ratio between the integral
  (instantaneous) doses in the two cases is 600
  (30).
• The results obtained at the PS fast extraction
  test therefore indicates that the loss of a LHC
  pilot beam of 5x109 protons should not make any
  permanent damage to the performances of the ATLAS
  pixel modules.