SLAC-PUB-3354
June 1984
(1)
A SANDWICH STRUCTURE BEAM PIPE
FOR STORAGE RINGS*
GORDON
B.
BOWDEN,
H.
DESTAEBLER
CHARLES
T.
HOARD,ALVIN
E.
JOHNSTON
Stanford Linear Accelerator Center
Stanford University, Stanford, California 9&?05
ABSTIXACT
A beam pipe fabricated from aluminum foil and plastic honeycomb has been
used in the DELCO detector on the PEP storage ring at SLAC for one year. The
pipe has a radiation thickness of 5.8
X
lO-3Xu, a failure pressure of 3.5 atm and
was baked for high vacuum service.
Submitted to Nuclear Instruments and Methods
* Work supported by the Department of Energy, contract DEAC0376SF00515
Detector performance at storage rings is often limited by multiple scattering
of particle trajectories as they pass out through the wall of the vacuum pipe or
by photon conversion into e+e- pairs in the pipe wall. To minimize this the pipe
should have the smallest radiation thickness possible without collapsing. On the
other hand the vacuum pipe wall should not be so transparent to synchrotron
and x-ray radiation that the detector is flooded with low energy background
radiation characteristic of storage ring operation.
MECHANICAL DESIGN
The failure mode for pressurized pipes depends on whether the pressure is
external or internal. An internally pressurized pipe is elastically stable and fails
when the circumferential wall stress reaches the yield strength of the wall mate-
rial. For long thin-walled pipes of radius R and wall thickness t at differential
pressure p the stress is
R
a=p t
0
(1)
Externally pressurized thin-walled pipes such as an evacuated beam pipe often
become unstable and collapse at pressures far below the limit set by the yield
strength of the wall material. Failure is then caused by elastic instability. For
long thin-walled tubes without end reinforcement, the critical collapse pressure(l]
1 E
t3
PC
=-.- -
0
4
l-u2 R
(2)
Here E is the material elastic modulus and v is its Poisson’s ratio. Yield strength
of the wall material does not appear in this equation. In many cases the tube
becomes unstable and collapse begins before the wall stresses reach the yield point
of the material. Collapse pressure is often limited by the tube’s elastic modulus
E and its resistance to deformation rather than its ultimate yield strength. As
an example a long thin-walled high strength aluminum tube of
50
mm radius and
2 mm wall collapses from external pressure at
l/8
the pressure it could withstand
internally.
To resist unstable collapse under vacuum load and to minimize the pipe’s
radiation thickness, a high elastic modulus, low Z material such as beryllium is
.
2
often used
[2].
A tube fabricated from beryllium is both exceptionally stiff against
colla.pse and transparent to radiation.
The product of elastic modulus times
radiation length for beryllium is
17
times that of aluminum. Another method of
stiffening the pipe against collapse is to note the (t/R)3 dependence in eq.
(2)
and increase the bending stiffness of the wall by a change in construction. Beam
pipes for the ISR at CERN have been made of stainless steel tube with bellows
convolutions to stiffen against collapse (31. The DELCO design reported here
employs sandwich construction to increase the wall stiffness. External pressure is
supported by a tube wall made of two thin metal skins laminated to a core of low
density honeycomb [fig. I]. The bending stiffness of such a sandwich structure
increases as the second power of the core thickness [4,5]. The stabilizing effect of
the low density core can raise the elastic collapse pressure to the point where the
tube’s failure pressure is the same for external pressure as for internal pressure
and is limited only by the yield strength of the load bearing skins. Instability is
no longer a problem.
MATERIALS
For the skins of the sandwich, aluminum foil was chosen over beryllium be-
cause it is easily welded and bonded and is less expensive. Moreover it is less
transparent to x-ray radiation. Calculations of the penetration of scattered syn-
chrotron radiation through various materials showed that it was not necessary
to add a higher Z liner to the DELCO pipe as it was in the case of the beryllium
beam pipe for the MkII detector [2].
The function of the core material in a sandwich structure is to stabilize the
metal skins so they can bear compressive loads without wrinkling or bending.
The core material should be of low density and needs little strength or stiffness
in the plane of the sandwich. Only transverse compressive strength and high
shear modulus are important for supporting the load bearing skins. HRH
10
0X-3/16-1.8
honeycombl’l was used for the sandwich core. This material is a
[‘IHexcel Corp., Dublin, CA 94566
3
I
flexible honeycomb structure made from 50 pm thick Nomex12j , a nylon fiber
base paperlike material. The material has 5 mm hexagonal cells and an average
density of 0.029. Its transverse compressive strength is 9 kg/cm2 and it retains
60%
of this up to
2OO'C.
CONSTRUCTION
The beam pipe has dimensions inner dia.
143
mm, outer dia.
163
mm, length
560
mm, (fig.
1).
The inner wall of the pipe must have good electrical conduc-
tivity and be free of sudden changes in diameter in order to minimize heating
from beam induced image currents. High vacuum requires an all metal inner
wall without seams or adhesive contamination. Unalloyed-hardened-aluminum
foil 0.25 mm thick was used for the inner skin of the pipe. The foil was rolled
onto a collapsible aluminum form and fused into a cylindrical tube by electron
beam welding the longitudinal butt joint. After welding, the tube wall thick-
ness was reduced to 0.15 mm by chemical etching. Weld zones are differentially
attacked by acid and must be masked off during this process. Aluminum-to-
stainless-steel transition rings were then electron beam welded to each end. An
alternate method for fabricating the inner wall is to machine it from thick-walled
extruded tubing while supported in suitable fixtures.
The sandwich core of 9.5 mm thick honeycomb was bonded to the outside of
the etched aluminum tube with a layer of MA4518 film adhesiveI This material
is a 0.3 mm thick film of precatalyzed high-temperature epoxy which must be
stored under refrigeration and protected by plastic foil until use. Bonding was
done in a hot air oven at 175OC. Before heating, the longitudinal honeycomb
seam and gaps between the honeycomb and end transition rings were filled with
Hysol Thermofoam 3050 core splice adhesive foam.[41 Compression of the sand-
wich onto the form during bonding was provided by wrapping the structure with
2 layers of heat shrinking plastic film. Compression was uniformly distributed by
12jDupont Corp, Wilmington, DE
19898
i31McCann Manufacturing, Oneco, CT 06373
141Hysol Div., The Dexter Corp., Olean, NY
14760
.
4
a
0.4
mm thick protective aluminum caul sheet wrapped around the assembly be-
fore heat shrinking film was applied. To avoid wrinkling of the outer skin of the
sandwich tube wall as the honeycomb core slides into melting adhesive, bonding
was done in
2
steps. The honeycomb core was first partially cured to the inner
beam welded foil. The sandwich was then completed by bonding a 0.15 mm foil
of high strength aluminum foil to the outside using a simple overlapping seam.
Before final welding of vacuum flanges and expansion bellows to the pipe ends the
inner supporting form was removed and the inner wall of the pipe was cleaned
for vacuum service by painting on a gelled form of chromic acidi and rinsing
with distilled water. The finished pipe was evacuated and baked at 80°C for 3
days after which its base pressure at 20°C was
1 X lo-'
Torr.
PERFORMANCE
Since pipes were relatively inexpensive to fabricate once tooling and pro-
cedures had been developed, three prototype pipes were tested to destruction.
Loaded axially as a column one pipe failed at a load of 24500 N. Under exter-
nal pressurization a crease or wrinkle developed in the outer aluminum skin at
3.5 atm differential pressure. At this point any further increase in pressure re-
sulted in deformation of the wall but the pipe did not collapse catastrophically.
Calculated stress levels in the outer aluminum skin of the sandwich were 6.9
MPa at failure. Small dents in the pipe wall did not reduce its strength. In
general sandwich construction appears less sensitive to this type of damage than
a thin solid wall of beryllium because of the much greater geometrical thickness.
One pipe was subjected to lo6 rads of radiation from a 3 MeV electron beam.
Another pipe was heated for
1
hour to 175OC while under vacuum load. Neither
of these tests caused any reduction in proof pressure.
The finished pipe contains 0.097 gm/cm2 of plastic core and adhesive and
0.082
gm/cm2 of aluminum in the skins. The total radiation thickness of 0.58%
X, is a significant reduction compared to 2.22% X, of the 2 mm thick solid walled
aluminum pipe it replaces and is comparable to the 0.56% X9 of a 1.5 mm thick
i5jPASA JEL 105, Semco, Glendale, CA
91206
5