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NLC - The Next Linear Collider Project
NLC BD VACUUM
Thermal Outgassing Rate for
Various Beam Line Materials
Leif Eriksson
10/11/99
NLC - The Next Linear Collider Project
Introduction -1
• The main parameters in
designing a vacuum system are:
– Conductance
– 12.3 (D3/L)
• Other decision criteria for
choosing an UV or UHV
material for particle
accelerators:
[l/s]
– Electrical conductivity
– Outgassing
– QTOT=(D  Q)/L[Torr l-1
cm-2]
– Cost
– Pumping speed
– S=Q/P
– Desired pressure
– MPS considerations
[l s-1]
– Ease of machining
[Torr]
Leif Eriksson
10/11/1999
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NLC - The Next Linear Collider Project
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.
Leif Eriksson
10/11/1999
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NLC - The Next Linear Collider Project
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
Leif Eriksson
10/11/1999
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NLC - The Next Linear Collider Project
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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NLC - The Next Linear Collider Project
Thermal outgassing rate - 3
• Non-metallic materials
– Hydrogen outgassing rate is
not an issue.
• Aluminum:
– The accepted outgassing rate
for aluminum at SLAC is 5.0E11 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]
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10/11/1999
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NLC - The Next Linear Collider Project
Thermal outgassing rates for SLC ARC’s
Calculate thermal outgassing rate for SLC ARC's
Distance between pumps:
Beam line I.D:
Pumping speed:
Average pressure:
Formula
Temperature:
I.D of
350 feet = 10668 cm
12 mm = 1.2 cm
11 l s -1
2.75E-5 Torr
Q =[Px/(L/P + X/C- x^2/(2xCL))]0.75
20 degrees C
1/2L between Thermal outgassing Conductance Pumping
3
beam line
[cm]
pumps
[cm]
rate
[Torr liter s -1 cm-2]
C = 12.1(ID /L)
[liter s-1]
1.2
5334
4.04E-11
3.92E-03
Distance
Speed from pump
[liter s-1]
[cm]
11
5334
Pressure at P x
[Torr]
2.75E-05
2L
L
X
Inner Diameter
SP
Px
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NLC - The Next Linear Collider Project
Effects of thermal outgassing rate - 1
Pressure Profile Calculation for NLC Beam Delivery
Parameters:
I.D of
1/2L between
beam line
pumps
Outgassing
Conductance Pumping Distance
Pressure at P x
C = 12.1(ID3/L) Speed from pump Px=[(L/P + X/C- x 2/2xCL)Q]0.75
[Torr liter s -1 cm-2]
[liter s-1]
[liter s-1]
[cm]
[Torr]
[cm]
[cm]
I.D
L
Q
C
Sp
X
Px
1.10
1.10
1.10
1.10
1.10
1.10
1.10
1.10
3.00
3.00
3.00
3.00
3.00
3.00
3.00
3.00
650
650
650
650
90
90
90
90
1750
1750
1750
1750
240
240
240
240
1.0E-12
1.0E-12
1.0E-12
1.0E-12
5.0E-11
5.0E-11
5.0E-11
5.0E-11
1.0E-12
1.0E-12
1.0E-12
1.0E-12
5.0E-11
5.0E-11
5.0E-11
5.0E-11
2.48E-02
2.48E-02
2.48E-02
2.48E-02
1.79E-01
1.79E-01
1.79E-01
1.79E-01
1.87E-01
1.87E-01
1.87E-01
1.87E-01
1.36E+00
1.36E+00
1.36E+00
1.36E+00
30
25
20
11
30
25
20
11
30
25
20
11
30
25
20
11
650
650
650
650
90
90
90
90
1750
1750
1750
1750
240
240
240
240
9.85E-09
9.86E-09
9.86E-09
9.88E-09
9.54E-09
9.57E-09
9.60E-09
9.74E-09
3.56E-09
3.57E-09
3.58E-09
3.63E-09
3.61E-09
3.67E-09
3.76E-09
4.12E-09
Comments
Copper/Stainless Steel, 30 l/s
Copper/Stainless Steel, 25 l/s
Copper/Stainless Steel, 20 l/s
Copper/Stainless Steel, 11 l/s
Aluminum, 30 l/s ion pump
Aluminum, 25 l/s ion pump
Aluminum, 20 l/s ion pump
Aluminum, 11 l/s ion pump
Copper/Stainless Steel, 30 l/s
Copper/Stainless Steel, 25 l/s
Copper/Stainless Steel, 20 l/s
Copper/Stainless Steel, 11 l/s
Aluminum, 30 l/s ion pump
Aluminum, 25 l/s ion pump
Aluminum, 20 l/s ion pump
Aluminum, 11 l/s ion pump
ion pump
ion pump
ion pump
ion pump
ion pump
ion pump
ion pump
ion pump
2L
L
X
Inner Diameter
SP
Px
Leif Eriksson
10/11/1999
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NLC - The Next Linear Collider Project
Effects of thermal outgassing rate - 2
• Two identical vacuum systems,
one of stainless steel, one of
aluminum, will not achieve the
same base pressure.
• What are the main ways to
lower the thermal outgassing
rate?
– Cleaning
• 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.
– Bake out
– Coating of inner vacuum
chamber surface
– Cooling the vacuum chamber
Leif Eriksson
10/11/1999
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NLC - The Next Linear Collider Project
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.
Leif Eriksson
10/11/1999
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