Crystal collimation for LHC

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Crystal collimation for LHC

• Valery Biryukov
• IHEP Protvino
• Vincenzo Guidi
• Ferrara University and INFN
• Walter Scandale
• CERN
• CERN, Geneva, 24 April 2003

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Borrowed from Ray Flillers talk at Paris EPAC
2002
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Crystal Channeling
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Beam line (70 m long) made of 3 crystals, IHEP
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• Beam focusing by crystal

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Crystal design as used at IHEP Protvino and RHIC
Crystal is 3 to 5 mm along the beam
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Crystal
SQUEEZE
Beam Direction
5mm
Crystal Courtesy of IHEP, Protvino
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New crystal design (strip) gave 85 efficiency
at IHEP
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Typical beam phase space at crystal location, IHEP
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1- circulating beam, 2- extracted beam, IHEP
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Crystal extraction efficiency as measured since
Dec 1997.85 is measured even when all stored
beam is dumped onto crystal
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Deflected (left) and incident (right) beams as
seen downstream of the crystal

• Prior to the test, the crystal was exposed in the
  ring to 50-ms pulses of very intense beam (about
  1014 proton hits per pulse).
• No damage of crystal was seen in the test, after
  this extreme exposure.

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Beam profile at collimator face with NO crystal,
70 GeV
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Misaligned x
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Crystal collimation
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Effy vs Energy
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45 GeV
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12 GeV
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Crystal lifetime is order of 51020 proton/cm2
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RHIC Crystal Collimator Setup
8 Upstream PIN diodes
4 Downstream PIN diodes
Data fill focus on upstream PIN diodes
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Layout of RHIC experiment on crystal collimation
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RHIC measurements, EPAC 2002
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Simulations of LHC crystal collimation
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Simulations with smaller bending, 0.1 mrad
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Two bending options compared 0.2 and 0.1 mrad
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Efficiency vs bending angle
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Background suppression factor vs crystal bending
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FNAL simulations for Tevatron crystal scraping,
PAC 1999
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Conclusion

• Simulations and experiments promise
• 10-fold improvement in backgrounds
• at TeV accelerators if bent crystal is used as
  primary scraper.
• No problems with high intensity or lifetime.

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Extraction parameters

• Protons
• Energy at 1.3-70 GeV
• Intensity 1012 protons in spills of 2 s duration
• Efficiency greater than 85
• Equivalent to 1000 T dipole magnetic field

Extraction efficiency vs. crystal length at 70
GeV
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Structure of the bending crystal

• Dimensions 0.5?2?50mm3
• 1/R? is the curvature experienced by channelled
  protons

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Bending device

• Bending exploits anticlastic effects due to
  anysotropy of crystalline Si
• For the (111) direction the sample takes the
  shape of a saddle

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Preparation of the Si samples I

• Starting material is prime-grade, (111) oriented
  525-?m-thick silicon wafer
• In previous runs there came out that a surface
  layer as thick as 30 ?m was rich in scratches,
  dislocations, line defects and anomalies that
  would reduce channelling efficiency
• Such a layer originated in the mechanical cutting
  for manufacturing the samples
• Thus we attempted removal of the layer

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Preparation of the Si samples II

• Preliminary cleaning to organic and metallic
  impurities from the surface of the wafers by
  H2O2, NH4OH, HF, HCl,...
• Coverage of the largest surfaces by Apiezon wax
• Cutting of the samples by a diamond-blade saw
  avoiding alignment with major crystalline axes.
• Planar etching (HF, HNO3 and CH3COOH, 2155)
  with a timing set for 30 ?m thinning.

More info in Rev. Sci. Instrum. 73 (2002)
3170-3173
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Images of the beam deflected through
mechanically treated (left) and chemically
polished crystals (right)