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
Cold block assembly

Beam pipe

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
CDF Run IIb 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 was 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. 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 effects of cold temperature on the carbon shell has been documented and shown to not affect the alignment of the pixels. Thus, the LN2-cooled cryopanel has been incorporated into the design of the detector in order to pump water vapor. Two cryopanels, one on the top surface of the vessel, one on the bottom, will be used. In addition, liquid nitrogen will be used as part of the temperature control system for the pixels. A secondary benefit of the temperature control system is that the surfaces that are cooled by liquid nitrogen act as cryopumps. Also, the strain relief structure for the cables, essentially a set of aluminum ribs, are thermally connected to the liquid nitrogen cooled surfaces. Through thermal conductivity, the strain relief structure is an added cryopump. The total pumping speed of water vapor due to all of the liquid nitrogen cooled surfaces is 290,000 L/sec. A liquid helium cryopump system is added to the vacuum system in order to pump non-condensable gases such as nitrogen (7500 L/sec) and hydrogen (2000 L/sec). The conceptual design has been presented to the collaboration, with documentation posted on the BTeV Document Database as BTeV-doc-2005 (password required). (3 October 2003)

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)

· Outgassing rates, calculated by the surface area of the overall volume (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)

· Outgassing rates, calculated by the surface area of the overall volume (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.

Degassing Techniques

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)

Flex Cable Test

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)

Cold Block Assembly

Photo of a complete cold block assembly (jpg file). This device will be used to record thermal distribution and LN2 flow characteristics. The entire assembly is 1.3-m long. There are 30 copper blocks that are each 4cm x 4cm x 0.6cm. The copper blocks span a total distance of 1.2-m (centerline of block 1 to centerline of block 30). The blocks are brazed to a stainless steel tube 1.6-cm (0.62-inch) diameter (sectioned view). (27 January 2004)

Photos of cold block assembly – details of test setup (jpg files):

o Photo 1: One heater clamped to each tab of cold block assembly. 150W, 120V cartridge heater is used.

o Photo 2: End view of cold block assembly with heaters clamped to tabs.

o Photo 3: Top view of cold block assembly with heaters clamped to tabs.

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-322-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)

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. The report is posted on BTeV Document Database as Btev-doc-1513.

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

CDF Run I beam pipe

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)