Introduction -1. The main parameters in designing a vacuum system are:Conductance12.3 (D3/L)[l/s]OutgassingQTOT=(D ? Q)/L[Torr l-1 cm-2]Pumping speedS=Q/P[l s-1]Desired pressure[Torr]. Other decision criteria for choosing an UV or UHV material for particle accelerators:Electrical co
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NLC BD VACUUM
Thermal Outgassing Rate for
Various Beam Line Materials
The main parameters in designing a vacuum system are:
12.3 (D3/L) [l/s]
QTOT=(D Q)/L[Torr l-1 cm-2]
S=Q/P [l s-1]
Desired pressure [Torr]
Other decision criteria for choosing an UV or UHV material for particle accelerators:
Ease of machining
The physics driven parameters are:
Conductance, (beam line I.D)
Engineering driven parameters are:
Ease of machining
The driving engineering parameter for vacuum design is the outgassing rate. It will determine choice of material.
Through trial and error a handful of vacuum materials have evolved as being suitable for UV and UHV applications.
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
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:
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)
Comparable to stainless steel as far as the outgassing rate goes.
Hydrogen outgassing rate is not an issue.
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 hr’s, 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]
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?
Coating of inner vacuum chamber surface
Cooling the vacuum chamber
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.