NLC BD VACUUM
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NLC BD VACUUM







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NLC BD VACUUM. Thermal Outgassing Rate for Various Beam Line Materials. The main parameters in designing a vacuum system are: Conductance 12.3 (D 3 /L) [l/s] Outgassing Q TOT =(D  Q)/L[Torr l -1 cm -2 ] Pumping speed S=Q/P [l s -1 ] Desired pressure [Torr].
NLC BD VACUUM

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Slide 1

NLC BD VACUUM

Thermal Outgassing Rate for

Various Beam Line Materials

Slide 2

The main parameters in designing a vacuum system are:

Conductance

12.3 (D3/L) [l/s]

Outgassing

QTOT=(D  Q)/L[Torr l-1 cm-2]

Pumping speed

S=Q/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

Introduction -1

Slide 3

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.

Introduction - 2

Slide 4

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

Thermal outgassing rate - 1

Slide 5

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.

Thermal outgassing rate - 2

Slide 6

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 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]

Thermal outgassing rate - 3

Slide 7

Thermal outgassing rates for SLC ARC’s

Slide 8

Effects of thermal outgassing rate - 1

Slide 9

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

Effects of thermal outgassing rate - 2

Slide 10

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.

If we were to use Aluminum...


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