BTeV Mechanical Design
Pixel Detector Vacuum System
& Beam Pipe Mechanical Design
Page maintained by Mayling Wong Mlwong@Fnal.gov (Engineer)
This
page contains current information regarding the design of the vacuum system for
the pixel detector and the beam pipe.
Links to web pages and
other topics are also shown.
Vacuum system for the Pixel
Detector
- Analysis of pixel vacuum
system
- Material outgassing – 5%
model
- Material outgassing –
components
- Simulated pressure
distribution in vacuum system
- End window for vacuum
vessel
- Degassing
techniques
- Flex cable
test
- Design
requirements
- Forward beam pipe
configuration
- Sandwich
composite
- Straightness of 0.008-inch wall
aluminum tube
- RICH beam pipe
configuration
- CDF Run I beryllium beam
pipe
- Electrical transition from RF
shield to beam pipe
Pixel Detector Vacuum
System
Analysis of Pixel Vacuum
System
The
original design of the pixel detector consisted of the pixel substrates being
moved in the horizontal position.
The design included thin membrane, also used as a RF-shield, that divided
the vacuum vessel into two vacuum regions.
The purpose of the membrane was to isolate vacuum pressure of the beam
line from the vacuum pressure of the pixel detector. The membrane did have a small gap so as
to allow a restrictive flow between the two chambers. An analysis of the conductance between
the beam line (“clean”) region and the pixel (“dirty”) region was
completed. The analysis showed
that, in actuality, the gap in the membrane was so large that the pressures in
the two chambers were equal. The
required pressure in the beam line vacuum chamber is 1e-7 torr. With the expected gas load from the
pixel detector to be on the order of 0.01 torr-L/sec, the beam vacuum will not
be low enough. A detailed report is
posted on the BTeV Document
Database as Btev-doc-318-v1. (password required) (21 November 2001)
The
design of the pixel detector has been modified to completely isolate the beam
line vacuum from the pixel vacuum.
To move the pixel detector with a leak-tight RF shield, a bellows is
added to the design. To accommodate
the space taken up by the bellows, the pixel substrate would be moved in the
vertical position. To achieve a
beam line vacuum pressure of 1e-7 torr in the new design, it is proposed that
the pumping speed be increased by coating the side walls of the vacuum vessel
with a NEG material, like Ti-Zr-V.
The required outgassing rate of the surfaces in the beam line volume is
on the order of 1e-10 torr-L/sec-cm^2.
(28 January 2002) – pdf
file
A
5% model of the pixel detector was built and its gas load measured. The results are explained
elsewhere on this web page. One
lesson that can be applied to the design of the vacuum system is the use of a
cryopump. When the heat sink/cable
support is cooled with liquid nitrogen, the heat sink acts as a cryopump that
pumps water at a rate of 10,000’s L/sec.
The pressure is reduced to less than 1e-7 torr. As a result, the need for a vacuum-tight
RF shield and the big bellows is gone and the pixel vacuum vessel can be one
vacuum region. This opens up the
option of having the pixel detector move either horizontally or vertically. Most importantly, the cryopump
temperature does not affect the temperature of the substrates. Before another design modification takes
place, research into other effects of the presence of the cryopump is in
progress, such as testing the effects of cold temperature on the performance of
the cable and the alignment of the carbon shell. This has been presented to the
collaboration and is posted on the BTeV Document
Database as BTeV-doc-813 (password required). (2 July
2002)
The
heat sink (cryopanel), acting as a cryopump, is likely to have some ice build-up
during the operation of the detector.
Over a year’s time, the cryopanel will have a conservatively estimated
frost build-up of 0.22 mm thick.
The frost at this thickness should not affect the performance of the
cryopanel. An analysis is
provided. (27 September 2002)
Measured
gas load from 5% model of pixel detector
An
assembly simulating 5% of the pixel detector, including 6 pixel stations, its
carbon support, and aluminum heat sink/cable support, is built and its
outgassing rate measured. The gas
load of the 5% model is 5e-4 torr at room temperature. When the heat sink is cooled to –160 deg
C and the substrates are not cooled, the pressure in the vacuum chamber was as
low as 9e-9 torr. The substrate
temperature was 20 deg C. The
results have been presented and are found in the BTeV Document
Database (password required):
·
Talk
on the results – Document 767 (17 June 2002)
·
Detailed
report, including calibration, analysis, and results – Document 812 (21 June
2002)
The
gas load was measured using the variable conductance method (15 January 2002)
(pdf file).
Photo
and drawing gallery (2 July 2002)
·
5%
model – jpg
file
·
Model
being placed inside vacuum chamber
– pdf
file
·
Vacuum
chamber (photos and PID) – pdf
file
Estimated & measured gas load from
individual material outgassing of the pixel detector
The
total gas load of following materials was measured using the rate-of-rise method
over 150-250 hours (29 July 2002):
·
Fuzzy
carbon, Pocofoam, carbon-carbon, and Pyrolytic Graphite Sheet (PGS): candidates for the pixel substrates (pdf
file)
·
Bump-bonded
chips, a manifold of glassy carbon fiber cooling tubes, kapton HDI, G-10 circuit
boards, and carbon fiber panels (pdf
file)
·
An
assembly of a 5-chip module, a kapton tail, and a glass slide, held together
with 3M 9882 tape and Emerson & Cuming Stycast 2850FT/Catalyst 24LV epoxy
(pdf file)
·
The
empty stainless steel vacuum chamber after the material was removed from the
chamber (pdf file). (13 March 2002)
RGA
readings of the chip module/kapton/glass slide assembly were taken at different
times of the test (pdf
file). (28 February
2002)
There
are a few conclusions that can be drawn from the gas load
measurements:
·
Pocofoam
shows the lowest gas load due to outgassing when compared to the other substrate
candidates.
·
The
gas load of the bump-bonded chips is not significantly higher than the gas load
in the empty vacuum chamber.
·
The
highest gas load comes from the G-10 circuit boards, which include components
that were soldered on to the board.
·
The
assembled module/kapton tail/glass slide starts off with a high gas load but is
reduced to one of the lowest gas load.
Note that air bubbles exists in the epoxy, as seen through the glass
slide.
The
total gas load was calculated using the outgassing rate of each material in the
pixel detector that is found in literature. The total gas load is estimated at
1.36e-2 torr-L/sec inside the pixel membrane. (27 February 2002) – pdf
file
Outgassing
rate and gas load test
setup and procedure
Outgassing rates
references are shown. The
widely varied measurements are indicative of the gas load’s dependence on
material, surface treatment, temperature, and pump time (16 January
2002).
Simulated
Pressure Distribution in Vacuum System
A
model simulates the pressure distribution in the BTeV vacuum system. Included in the model are the 1.92 inch
diameter RICH beam pipes, the 1.00 inch diameter forward beam pipe, the vacuum
chamber containing the pixel detector, 80 L/sec ion pumps at the ends of the
RICH beam pipes, a 50 L/sec pump at the vacuum chamber (“clean side”), an 800
L/sec pump in the pixel detector volume (“dirty side”). A thin aluminum RF shield separates the
volumes into the clean and dirty sides.
An aperture of 0.5 cm diameter through the shield is also modeled. An estimated gas load in the dirty side
of 1.36e-2 torr-L/sec was used in the model. The outgassing rate of the beam pipes
and the clean side is 1e-10 torr-L/cm^2/sec.
The
current outgassing rate measurements indicate that the actual gas load inside
the pixel membrane can be as low as 1.50e-3 torr-L/sec. A model of the vacuum system with the
lower gas load was simulated. The
resulting pressure distribution is shown (7 Nov 2001) –pdf
file
For comparison, the pressure distribution is shown for when the beam pipe outgassing rate is 1e-14 torr-L/cm^2/sec (9 November 2001) –pdf file
End Window for Vacuum Vessel – ps file, jpg file (13 July
2001)
Formed
head: An aluminum formed head has been
designed following the guidelines in the ASME Boiler and Pressure Vessel
Code. The head thickness of 0.023
inch (0.58 mm) is the required thickness according to the Code for a head
diameter of 20 inches (508 mm). The
head profile is elliptical with about a 2:1 ratio for the diameters. It will be determined how to best
fabricate a uniformly thin-walled aluminum head with such large diameter. An analysis was performed with the
structure under an internal pressure of 14.7 psi. The safety factor for the design is
three times the yields stress of aluminum (ps file, jpg file). The maximum deflection is 0.024 inch
(0.61 mm) (ps file, jpg file). The transition to the beam pipe has a
radius of 0.1 inch. When the front
of the head sits at z=65 cm from C0, the largest thickness through which a
particle travels is 0.036 inch (0.91 mm) (ps file, jpg
file)
Flange: The current flange design is for a metal
wire seal. Research and analysis
must take place to understand the best available option to seal the window to
the vacuum vessel and how to fabricate the custom-made
flange.
Research has shown that
the outgassing rate of material varies with cleaning techniques. A proven method to degas a vacuum
chamber is to bake it at temperatures greater than 150 degrees Celsius. However, the silicon in the BTeV Vertex
Detector cannot withstand temperatures greater than 80 degrees Celsius. Thus, methods are being investigated to
degas the beamline vacuum without elevated temperatures.
A
test is being run to understand how well low-temperature bake-outs can degass a
vacuum chamber. A chamber that is 6
liters in volume is heated at various temperatures between 50 and 150 degrees
Celsius. Its outgassing rate
is measured using the rate-of-rise method after a 24 hour bake-out and cool
down. RGA readings are also
recorded of the cleaned chamber.
The current test results are shown (pdf file). (13 March 2002)
The
effects of cold temperature on the performance of a prototype power flex cable
and signal flex cable are being tested.
Photos of the test setup are shown (pdf file). The flex cables are immersed in liquid
nitrogen and flexed repeatedly. 10
mA runs through the signal cable, and 1 A runs through the power cable. The current and voltage are
recorded. The cables are positioned
so that there is a 2.5-cm bend radius when the supports are 2-cm apart. The supports slide apart a maximum
distance of 4-cm. (23 August
2002)
Beam Pipe Vacuum
System
Design
requirements (updated 17 September 2002) –pdf file
The
beam pipe system extends from the end of the pixel detector to the low beta
quads. There are several components
making up the system. This table
(ps file, jpg file) lists the layout
of the components in terms of the location from C0. (15 October 2001)
An
assembly drawing of the beam pipe assembly from the end of the pixel detector to
the ion pump (z=8m) is posted on the BTeV Document
Database as Btev-doc-318-v1 (password required) (4 April
2002).
The
transition of the forward beam pipe to the RICH beam pipe is a low-mass
flange. The analysis of the design
is posted (pdf
file) (20 May 2002).
Comparison
of forward beam pipe configurations (table
updated 14 September 2001) – ps file, jpg
file
In
order to meet the design requirement of a minimum clear line-of-sight, the
forward beam pipe must be supported at the location of z=125 inches (317 cm)
from C0. The calculations for this
conclusion is shown (pdf file). (18
September 2002)
Sandwich
composite panel – 3-point bending test (5 April 2002) –pdf file
One
of the candidates for the material of the forward beam pipe is a sandwich
composite made of aluminum foil and Rohacell (a polymethacrylimide) foam
core. Another option is a sandwich
composite made of aluminum foil and a fuzzy carbon core. A test was run to measure the mechanical
properties of both types of composite.
The results show that the composites are 44-60% efficient as
expected. However, both composites
are more than adequately strong if either are made into a tube that contained an
internal vacuum.
Straightness of 0.008-inch wall, 1.0-inch
inner diameter aluminum tube
6061-T6
aluminum tubes were manufactured so that the wall thickness was nominally
0.008-inch and the inner diameter nominally 1.0-inch. The drawn tube lengths varied from 31 to
120 inches. As a potential BTeV
forward beam pipe, the straightness of the longest tube is adequate for a clear
line-of-sight for the beam to pass through if the tube is supported along its
length. A detailed report documents
the measurements (3 April 2002) - pdf file
Comparison of RICH beam pipe configurations (table updated 31
August 2001) – ps file,
jpg
file
The
CDF Run I beam pipe can be modified so that it can be used as the beam pipe
through the RICH detector. The
two-inch diameter beryllium part of the pipe is about 244 inches long. The wall thickness is 0.020”. Drawings of the CDF pipe and the BTeV
RICH beam pipe are posted on BTeV Document
Database as Btev-doc-323-v1 (password required). (4 April 2002)
Transition
from Pixel Detector’s RF Shield to Beam Pipe
For
electrical continuity, an aluminum transition must be in place connecting the RF
shield to the beam pipe. One
concept is proposed here. The
transition is made of two halves to accommodate movement of the RF shield. For each half, one end is circular,
matching the beam pipe, and the other end is L-shaped, matching the RF shield
(pdf file). Slots are cut into the pieces in order
to increase the flexibility during movement of the RF shield and during
installation of the beam pipe (pdf file). (A line sketch is also shown: pdf file) As this is a concept, much more work is
needed to optimize the design by minimizing the amount of material, increasing
strength, determining the method of electrical and mechanical connection, and
finding an appropriate manufacturing process (1 November
2001).
Links
Joe Howell's B-Tev Mechanical Design Web
Page
Fermilab - Particle
Physics Division – Mechanical Support Department
Pressure Accumulation inside
CDF-CLC photomultiplier tubes due to Helium Permeation (2 April 2002) – pdf
file