NLC BD VACUUM

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NLC BD VACUUM

• Thermal Outgassing Rate for
• Various Beam Line Materials

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Introduction -1

• The main parameters in designing a vacuum system
  are
• Conductance
• 12.3 (D3/L) l/s
• Outgassing
• QTOT(D ? Q)/LTorr l-1 cm-2
• Pumping speed
• SQ/P l s-1
• Desired pressure Torr

• Other decision criteria for choosing an UV or UHV
  material for particle accelerators
• Electrical conductivity
• MPS considerations
• Cost
• Ease of machining

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Introduction - 2

• The physics driven parameters are
• Conductance, (beam line I.D)
• Desired pressure
• Electrical conductivity
• MPS considerations

• Engineering driven parameters are
• Cost
• Ease of machining
• Outgassing
• Pumping speed
• The driving engineering parameter for vacuum
  design is the outgassing rate. It will determine
  choice of material.

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Thermal outgassing rate - 1

• Through trial and error a handful of vacuum
  materials have evolved as being suitable for UV
  and UHV applications.
• Stainless steel
• Copper
• Aluminum
• Glass
• Ceramics
• They all have one thing in common, a low thermal
  outgassing rate.

• The origin of the thermal outgassing rate is
• Hydrogen trapped in the molecular structure of
  the material and from decomposition of water
  vapor.
• Other sources of thermal outgassing are
• Surface contamination from lack of proper
  cleaning
• Gases trapped on the material surface such as CO
  and CO2
• Virtual leaks
• Design

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Thermal outgassing rate - 2

• The other sources of outgassing for the various
  UV and UHV materials are dealt with through
• Surface contamination - Refined Cleaning Methods
• Water Vapor - Outgassing
• CO and CO2 - Glow Discharge, Electron Bombardment
  
• Virtual Leaks - Engineering
• Design - Engineering

• Typical thermal outgassing rates for various
  materials
• Stainless Steel
• For stainless steel the outgassing is small. An
  accepted value for baked and cleaned, 304 or 316
  stainless steel is 1.0E-12 torr liter s-1 cm-2.
  (Source AVS)
• Copper
• Comparable to stainless steel as far as the
  outgassing rate goes.

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Thermal outgassing rate - 3

• Non-metallic materials
• Hydrogen outgassing rate is not an issue.
• Aluminum
• The accepted outgassing rate for aluminum at
  SLAC is 5.0E-11 Torr liter s-1 cm-2. This value
  is based recent PEP-2 experiences. (Source
  Private conversations with Dan Wright and Julia
  Weinberg)
• How does the value hold up to other measured
  values of the thermal outgassing rate for
  aluminum?

• 1. Hajime Ishimaru, KEK, measured a thermal
  outgassing rate of E-13 Torr l s-1 cm-2 on an,
  EX processed, UHV cleaned and baked system.
  E-10 Torr l s-1 cm-2 before bakeout.
• EX process AL part dipped in sodium hydroxide,
  rinsed in de-mineralized water, thermally
  processed in an oxygen/argon atmosphere for 24
  hrs, the water forms an hydroxide layer which is
  transformed in to a thin oxide layer during the
  thermal treatment. Source AVS Series 8, ISBN
  0-88318-756-6 p. 139

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Thermal outgassing rates for SLC ARCs
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Effects of thermal outgassing rate - 1
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Effects of thermal outgassing rate - 2

• Two identical vacuum systems, one of stainless
  steel, one of aluminum, will not achieve the
  same base pressure.
• The aluminum system will always be approximately
  two orders of magnitude higher in pressure than a
  stainless system.
• If the thermal outgassing rate on aluminum could
  be lowered, it would be close to an ideal beam
  line material.

• What are the main ways to lower the thermal
  outgassing rate?
• Cleaning
• Bake out
• Coating of inner vacuum chamber surface
• Cooling the vacuum chamber

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If we were to use Aluminum...

• The beam line aperture must be maximized.
• Provide for in situ bake out
• Consider a design with an ante chamber to
  maximize pumping speed.
• Re-evaluate the current pressure specifications
  for Collimation and Big Bend.

• What to do next?
• Determine the exact vacuum specification for NLC
  BD.
• Discuss maximizing beam line aperture.
• Quantify MPS specification
• Quantify Electrical properties.
• Do a cost evaluation of Aluminum alternative.
• Investigate the consequences of a cooled beam
  line.