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NLC BD VACUUM

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... aluminum at SLAC is 5.0E-11 Torr liter s-1 cm-2. This value ... a thermal outgassing rate of E-13 Torr l s-1 cm-2 on an, 'EX' ... E-10 Torr l s-1 cm-2 ... – PowerPoint PPT presentation

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Title: NLC BD VACUUM


1
NLC BD VACUUM
  • Thermal Outgassing Rate for
  • Various Beam Line Materials

2
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

3
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.

4
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

5
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.

6
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

7
Thermal outgassing rates for SLC ARCs
8
Effects of thermal outgassing rate - 1
9
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

10
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.
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