Bellows design for the PEP-II High Energy Ring arc chambers Page: 3 of 4
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the tip temperature could reach 91* based on 0.5 W/cm2 of
heat flux. This is far below the stress-relaxation
temperature of AL-I5 GlidCop. which is approximately
300*C [3]. GlidCop was chosen for its high thermal
conductivity in comparison to other types of strengthened
copper. GlidCop's thermal conductivity is a factor of two
higher than BeCu which is commonly used for Shield
Fingers. Therefore, the local temperature at the tip will be
higher in the BeCu. Also, BeCu over-ages and loses
strength at 250 C for high strength BeCu and 455 C for
high conductivity BeCu.
The effects of the high tip temperature are further
minimized by the independent stainless steel Spring
Fingers. These isolate the high-temperature region at the
ends of the Shield Fingers from the high-stress area at the
root of the Spring Fingers. Thus. if the Shield Fingers get
hotter than expected. they are less likely to soften and fall
away from the contact joint.
STRUCTURAL LOADING
Despite the high tip temperature. stresses in the Shield
and Spring Fingers are produced primarily by the contact
force at the sliding joint. and by the offset across the
Module due to alignment and fabrication tolerances.
The 0.65 mm thick Spring Fingers provide contact
force. so they see 205 MPa bending at their base. This is
not affected by offsets across the Bellows Module because
they are mounted solely to the stub.
However. the thin Shield Fingers are stressed only by
offsets across the Module. The chamber Flex Support
system allows up to 2 mm lateral offset, which produces a
90 Mpa stress at the root of the Shield Fingers. This
bending stress does not significantly affect the contact force
because the Spring Fingers are 15 times stiffer.
MANUFACTURING ISSUES
Two materials manufacturing issues have been
significant factors in the design of the Bellows Module.
Sliding Joint Tribology
First. the tribology of the sliding joint in vacuo is a
concern for three reasons: 1) overheating or galling at the
contact joint could cold-weld a finger to the stub. This
would destroy the finger. 2) Insufficient or excessive
lubricity from silver-plating could produce silver dust by
fretting at the sliding joint. This dust could enter the beam
passage and possibly affect the beam lifetime and stability.
3) Plated surfaces could behave below expectations during
operation. when high temperatures and high shear stresses
could cause it to flake off.
Research and testing at SLAC have shown that a
combination of 0.4-0.5 mils silver plating on the Shield
Fingers. and 0.2-0.3 mils rhodium plating on the stub
produce a good sliding joint. With the 170 gram forceexpected at the contact joint, tests have shown that the
silver plating is thick enough to endure over 200,000 cycles
at 200 *C. Thinner plating resulted in complete erosion the
plating.
The rhodium plating on the stub is likewise an optimal
thickness. Shear stresses in thicker plating reduce quality
and adhesion, while thinner plating will not contain the
image current traveling along the chambers.
Shield Finger Brazing
The second manufacturing issue is brazing. Our initial
design, and that of most other bellows modules, used BeCu
fingers. However, to attain the highest possible yield
strength, these must be precipitation-hardened after being
brazed to their retaining plate. Without this, the fingers
cannot tolerate even moderate stresses without yielding.
To avoid this failure mode. GlidCop AL-IS was
chosen as an alternative. This is a dispersion-strengthened
copper, which does not require heat-treating, and does not
overage. At room temperature, its yield strength is 380
Mpa, with 16% elongation. Experiments show only a 25%
decrease in yield strength at the brazing temperature [3].
One of GlidCop's drawbacks is its lack of ductility.
This makes it harder to form, and susceptible to fracture if
strained plastically. However, manufacturing tests show
that these problems can be avoided by smooth forming dies
and large bending radii.
FUTURE WORK
Design and production efforts are focused in two
directions. First, confirmation testing of the final sliding
joint configuration is pending.
Second, a full prototype of the entire Bellows Module
is now being prepared. This will prove out the complex
fabrication and assembly techniques, and show areas where
cost savings can be recognized.REFERENCES
[1] M. Zisman, ed., "PEP-II: An Asymmetric B Factory:
Conceptual Design Report", SLAC Report 418, 1993.
[2] C. Perkins, et al, "Vacuum System Design for the PEP-
II B Factory High Energy Ring". EPAC94 Conference
Proceedings, London, World Scientific.
[3] GlidCop is a dispersion-strengthened copper alloy
made by SCM Metal Products, Inc., Research Triangle
Park. North Carolina, USA
[4] S. Heifets. et al, "Impedance Study for PEP-II B-
Factory", PEP-II AP Note No 99.
[5] ibid.
[6] S. Heifets, op cit.3
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Nordby, M. E.; Kurita, N. & Ng, C. K. Bellows design for the PEP-II High Energy Ring arc chambers, article, August 1, 1995; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc668241/m1/3/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.