Specifications of the RF box for the
upgraded LHCb vertex detector
date: 2014-04-17
authors: MD,JK,PW,RW,TK,WH (Nikhef/VU)
Contents
1 Introduction
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2 Design
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2.1
3D model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2.2
Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
3 Choice of material
3
4 Summary of requirements
4
5 Time-line
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1
Introduction
The LHCb experiment is one of four particle physics experiments at the Large
Hadron Collider, a TeV proton-proton collider, at CERN. The heart of the detector is
a so-called ’Si-strip vertex detector’ (with the acronym ’VELO’ for ’vertex locator’),
a detector to track charged particles close the proton-proton interaction point. The
vacuum around the LHCb vertex detector is isolated from the LHC beam vacuum
by thin aluminium envelopes, called “RF boxes”. At the end of the next LHC run,
lasting from 2015-2017, the LHCb detector is ready for an upgrade and the vertex
detector and RF foil will be replaced [1].
The RF box serves three main purposes:
1. The RF box separates the VELO detector vacuum from the LHCb beam vacuum;
2. The RF box suppresses heating due to strong wake-fields, because it allows
a mirror current to travel parallel with the beam. When this mirror current
along the beam lines is interrupted, or significantly changed in phase, beam
power will be deposited at that location;
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3. The encapsulation by the conducting RF box reduces the RF Interference.
RFI from the pulsed proton beams will affect the electronics of the VELO
detector.
Particles originating from the vicinity of the proton-proton interaction point travel
through the RF box foil before they are measured in the VELO detector. The
traversing of the foil leads to multiple scattering which reduces the accuracy with
which the position of the particles at the interaction point can be determined. Even
a aluminium foil with 0.25 micron thickness affects the performance of the VELO
significantly.
The distance between the Si sensors and the foil is at some points only 800 micron.
Since the sensors are very sensitive, the foil should not touch the sensors under
any circumstances. Therefore, it will need to be built very precisely. Furthermore,
pressure differences of up to 5 mbar may occur between the inside and outside of
the foil. The foil needs to maintain its shape under these pressure differences, in
particular in the vicinity of the detectors.
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2.1
Design
3D model
As the dimensions of the upgraded VELO detector have not yet been finalised, the
final design of the RF box can not yet be made. For prototyping a first 3D model
was made in CATIAv5. The Velo detector consists of two parts, on opposite sides
of the proton beams, called the A-side and C-side respectively. The step files for the
corresponding boxes can be found at
https://www.dropbox.com/s/jrtrr2ll7zl51og/stepfiles20140405.zip
Figure 1 shows a view of the RF box model for the C-side. The corrugations in
the foil reflect the position of the Si sensors of the VELO detector: they appear at
regular intervals of 25 mm. The corrugations of the opposing A-side RF box are
shifted by 12.5 mm in the beam direction with respect to the mounting flange.
2.2
Dimensions
Some of the relevant dimensions of the RF box are:
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Figure 1: View of the prototype design for the C-side.
Minimum distance of foil to beam-axis
Minimum distance to VELO sensors
Period of corrugations in z
Depth of corrugations
Outside dimensions of the flange
Maximum distance of flange to beam
Minimum distance of flange to beam
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3.5 mm
0.8 mm
25 mm
8.0 mm
270 mm x 1175 mm
310 mm (TBC)
90 mm
Choice of material
For the choice of material the following properties are important:
1. High electrical conductivity (mirror current, RFI: Be, Al or Cu metal)
2. Long radiation length (multiple scattering: low Z material)
3. Vacuum tightness (vacuum poisoning: non-porous metal, low vapour pressure)
4. Strength (shape stability)
5. Machining (good milling properties)
6. Welding (if additional wake field connector clamps are needed)
The RF box for the existing VELO detector foil made from a 0.3 mm thick stretched
AlMg3 foil. Several successful milling tests have been performed with AlMg4.5Mn0.7
(EN AW-5083), which is therefore our preferred material.
To provide an electrical insulation between box and sensors the inside (VELO vacuum) of the box will be painted with a Torlon coating, which is hardened by heating
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to a temperature of 200° C. To improve the beam vacuum, the outside (LHC vacuum) of the RF box will be painted with a NEC coating. To activate the coating
the box will be baked out at about 200° C for about 30 minutes. For the above
treatments it will be important that no mechanical stress is present in the produced
RF box.
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Summary of requirements
The RF box needs to satisfy the following requirements:
• Assuming the box is made of AlMg4.5, the maximum thickness of the part of
the foil facing the beam is 250 micron. The thickness of the sides, front and
rear of the box can be 500 micron;
• Differences in thickness between the actual foil and the design should be less
than ±10%;
• In the absence of a differential pressure, differences between the position of
the actual foil and the design should be less than ±200 micron;
• For a 10 mbar differential pressure, differences between the position of the
actual foil and the design should be less than ±500 micron;
• The vacuum leak rate of the box may not exceed 1·10−6 mbar l/s He-equivalent;
• The box must return to its original form after a 200 degree bakeout;
• The box must withstand multiple temperature cycles between 20 degrees and
its normal operating temperature of 5 degrees. These cycles happen a few
times per year, or about 50 times for the entirely lifecycle of the box.
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Time-line
A rough time-line of the entire RF box project is:
2014:Q3–2015:Q3 proof-of-concept, prototyping
2015:Q3–2016:Q3 production of 4 RF boxes
2016:Q3–2017:Q3 transport to CERN, application of Torlon and NEC coatings
2018:Q3–2018:Q4 installation in the VELO tank
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References
[1] The LHCb Collaboration, LHCb VELO Upgrade Technical Design Report,
CERN-LHCC-2013-021. LHCB-TDR-013, Nov. 2013.
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