The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Development of A Bellows Assembly with RF-Shield for KEKB II :Abrasion and Pumping down Tests
Yusuke SUETSUGU, Ken-ichi KANAZAW A, Masaharu KAWAHARA*, Yosuke HARADA* and Motosada KANEKO*
High Energy Accelerator Research Organization (KEK), Tsukuba, lbaraki, 305 Japan*VALQUA SEIKI Ltd., Hamamatsu, Shizuoka, 431-21 Japan

Abstract
A bellows assembly with RF-spield has been designed anddeveloped for the KEK B-factory (KEKB). The RF-shieldis
a usual finger-type but has special spring-fingers to press
contact-fingers (shield-fingers) surely onto inner tube (beamtube). In a chain of resign studies an abrasion test of thecontact-fingers was performed in vacuum. A quaritity ofgenerated metal particles was estimated and expected to havelittle harm on the beam lifetime if the inner tube is coated with
The gas resorption rate and the residual gas
silver.
components of the bellows assembly were also measured as a
final bench test. The gas desorption rate of 1 - 1.5x 1o- 10 Pa ·l/s/cm2 was obtained after a bake at 150 oc for 24 hours.

estimated. The effect of silver coated spring-fingersandinnertube on the quantity of particles was also investigated. As a
final bench test, a pumping down test of the bellows assembly
was carried out to estimate the gas resorption rate and to see
The results of these
the residual gas components.·
experiments are presented here.

2 Structure

Parallel to overall resign of vacuum system, a vacuum
bellows assembly with RF-shield has been revel oped for theKEK B-factory (KEKB) 1-5 • The bellows assembly absorbs
thermal expansion and contraction of the beam tube during
beam operation and baking. The RF-shield insire it has tokeep a good electrical contact to flow smoothly the wall
current induCed by the beam while preserving smoothmechanical moving with high reliability. We reveloped abellows assembly with RF-shield of a usual finger-type but
having special spring-fingers to press securely contact-fingers(shield-fmgers) on to inner tube (beam tube). In the R&D sofar, necessary contact force, optimum finger length, HOM(Higher Order Mode) power leaked into inside of bellows havebeen investigated experimentally or by calculation3-5 •In a chain of the resign studies, an abrasion test of thecontact fingers was performed in vacuum in relation to the dust
In thetrapping problem in electron storage rings 6•7•experiment the quantity of generated metal particles wasmeasured and the influence of it on the beam lifetime was

We briefly reviews the structure of the bellows assembly at
first3-5 • For the KEKB, the bellows assembly should absorb
a maximum axial stroke of 20 mm and an offset of I mm_ The
design lifetime is I 05 times' expansion/contraction with 1 mmstroke. A typical bellows assembly designed for the KEKB is
schematically drawn in Fig.1 and the cut-model is presented inThe main components of the RF-shield are theFig_2_contact-finger, the spring-finger, and the inner tube. The
contact-fingers are pressed on to the inner tube from outside by
the spring-fingers. The electrical contact is kept at the edge of
inner tube_ The contact force is set to be larger than I 00
g/finger at the nominal position_ Even for the case of 1 mm
offset, the contact force is kept larger than 70 g/finger_The contact-finger is 0.2 mm thick beryllium-copper(Be-Cu) alloy (C1720) and has thewidthof5.5 mm. The gapof slit between adjacent fingers is 0.5 mm_ The slit length is20 mm at the nominal position. The number of contactfingers is 52 for the model in Fig.l. The spring-finger is OAmm thicklnconel-625 andis coatedwith silver(IO J..Lm). The
tip of the spring-finger has a curvature of 5 mm. The innertube is 0_8 mm thick stainless steel (the edge is 1.0 mm thick)coated with silver (10 J..Lm) to improve electrical contact.
Other parts of the bellows assembly are mare of stainless
The every component of bellows assembly wassteel.
To
regreased and then cleaned by acid before assembly_
theofassemblinghandling,
wring
prevent contamination
bellows was done in a clean room of a class less than 10000.

Fig. 1 Typical structure of bellows assembly for KEKB_

Fig_ 2 Cut-model of bellows assembly drawn in Fig.1.

1 Introduction-362-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 199.7

as3 Abrasion test
It is well known that small particles (dust) insire a beamtube have a great concern with the dust trapping problem in
The metal particles due to theelectron storage rings 6•7•
abrasion at the contact point of the RF -shield could be apossible particle source and should be controlled carefully.
The experimental setup for the abrasion test is shown inFig.3. A test piece of the RF-shield is installed in a vacuum
chamber which is evacuated down to less than 1X I o- 4 Pa by aturbo molecular pump. The test piece has five fingers and isThe
fixed to vertical position as shown in the figure.
materials of the contact-finger, the spring-finger and the plate(i.e., inner tube) is the same as the real ones except for silver
coating. The contact force is set to about 100 g/finger. The
contact-finger performs reciprocating motion with a stroke of
20 mm. The time for one cycle is about 7 seconds. The timeis on the same order with that necessary for molecules to cover
one layer on a clean metal surface (about 3 seconds). The
condition is similar to the real ring where the average pressure
will be in the orrer of 10-7 Pa and one stroke during beam
operation will take more than several thousands seconds.
The particlecounterusedis M-25s (Ushio Co. Ltd., Japan)that can count particles by a laser scattering method in vacuum(6.8 atm- lxl0- 6 Pa). The wave length of the semi-conductor
laser is 780 nm. The minimum detectable particle size is 0.27
The size of particle can be classified into 5 ranges
J..LID.
between 0.27 and 2.0 J..LID. The particles dropped from the test
piece are gathered by a cone and pass the detector of the particle

counter drawn in Fig.3.
Fig.4 is a typical result of the time variation of the total
number of measured particles (~ 0.27 J..Lm). The line (a) and(b) areforthe case with and without silver coating on the plate,respectively. The spring-fingers were coated with silver. For
both cases the quantity of particles increased rapidly at the firststage (~ 3000 sec.). That is because the metal surface isinitially covered by an oxire layer formed in air and thescratched metal particles detach easily from the surface during
mechanical motion. The quantity of particles for the case (a)was, however, much less than that of the case (b). The reduceof particles should be due to the silver coating on the plate. It
was interesting that the silver coating on the spring finger,
contrary to the plate, seemed to have little effect on thequantity of particles since the results for the case without
silver coating on both the spring finger and the plate werealmost same as (b) in Fig.4. That may be because thespring-finger touches the contact-fingerin a point, or a pair ofInconel and Be-Cu alloy is intrinsically a good combination
on reducing metal particles generated due to abrasion. Further
investigations are now undergoing on this point.
Here we estimate the influence of the particles on beam lifetime. At first we assume that 2 particles drop during one
stroke of 20 mm considering the case of (a) in Fig.4. In thereal ring, the stroke during one operation cycle is about 1 mm
and the number of fingers just above the beam orbit is one atmost. Since about 300 bellows assemblies in one ring will
expand and contract during an operation cycle, the possible
number of particles (NP) that may be trapped for one operationis11N P "" 2 X - X - X 300 = 6.
20 5

The material of the particle is assumed to be silver (the densityis 10.5 g/cm 2). The mass number (Aatom) is 108 and atomic
number (Zawm) is 47. For the particle with a diameter of0.5
J..LID, which was the most dominant size, the mass number (A P)
of the particle is 4.1xl0 11 • If the beam lifetime is determinedby the Bremsstrahlung we to the nuclei of the particle, thelifetime, -r [sec], can be calculated by 7)2)(183
1 { 16re 2 l E 1~ = 3 X 137 n /j£ n (zatom)l/3 Zatom X

Fig. 3 Experimental setup for abrasion test.

APNP _ _c__
':

:::

:

______ (b)~_it)l_()t1tJ\g_~()at!J1g_()_!l·_pl_a_te,\!
,,,

3

3.5

_______________ __________ ________ _

0.5

1.5

2

2.5

4

Time [X10 4 sec]

Fig. 4 Total number of generated particles P-: 0.27 J..LID)
for (a) with and (b) without silver coating on plate.

where re = 3xl0- 15 m is the classical electron radius, AE!E isthe RF bucket height "" 0.01, c = 3xl08 m/s is the speed oflight, C = 3000 m is the circumference of the ring, ax = 1 mm
and aY = 50 J..Lm are the width and the height of the beam,respectively. Now if all particles are completely trapped in
the beam at the same time (NP = 6) for the worst case, -r isabout 10000 seconds. The lifetime is almost the same order
as designed one determined by the pressure with beams 1•2 • On
the other hand, the experiment at the TRISTAN AR indicatedthat the metal particles would melt and evaporate immediately
in the beam and should not be trapped6 • Then if the particles
effect on the beam only wring crossing the beam (aboutlx 10-4 sec), which may be the most optimistic case, the beamloss is negligible. In summary, if the plate (inner tube) is-363-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

coated with silver as our resign the generated particles at the -of the upstream sire (Ch-I) and oownstream sire (Ch-2) arecontact-finger will have little hann on the beam lifetime.
about 3100 cm 2 and 3400 cm 2, respectively, without theWithout the silver coating, however, the short lifetime less bellows assembly. Two BA gauges (BA-1 and BA-2) are set
than one hour may be observed for the worst case (NP "' 20).
to each chamber. The bellows assembly and a quadrupole
mass analyz:er (QMA, surface area"' 900 cm 2) are attached toCh-I. The gas resorption rate of Ch-I (q1) was obtained by
4 Pumping down test
the through-put method, i.e., q1 can be calculated by
As seen in Fig. I, the bellows assembly has a very
complicated structure and has a large surface area For thetypical one with the length of 160 mm and the diameter of94
mm, the inner surface area is about 2500 cm2 which is about
5 times larger than the normal beam tube with the samelength.
The gas resorption Tate, therefore, should besufficiently lower than that necessary for the usual beam tube.
The nominal residual pressure without beams should be lessthan 5x I o-s Pa for the ring 1.2. That requires the gas desorption
rate of less than about Ixl0-9 Pa·l/s/cm2 , where an average
pumping speed of 80 //s/m was assumed The goal of gas
desorption rate of the bellows assembly, therefore, was set to
2x10· 10 Pa·l/s /cm2 in N2 equivalent.
The experimental setup for the pumping oown test ispresented in Fig.5. The test chamber is evacuated by two
turbo-molecular pumps in series. The chamber is separated totwo chambers (Ch-I and Ch-2) by an orifice with aconductance ofCori =4.9l!s in N2 equivalent. The surface areaq, =cori- - ( ~ -P2 )s,where S 1 [cm2] is the surface area ofCh-1, P 1 [Pa] and P2 [Pa]are the pressures of Ch-I and Ch-2, respectively. The gas
desorption rate ofCh-1 without the bellows assembly (q0 ) was
about 6.6x 10· 11 Pa ·l/s/cm2 after a bake at 200 °C.
Fig.6 is the pumping oown curves for Ch-I (P 1) and Ch2 (P2) with the bellows assembly, The bake was at 150 oc for
24 hours. The P 1 and P 2 at 100 hours after the bake were
1.6xl0-7 and 3.0xl0-8 Pa, respectively.
Then the gas
resorption rate of Ch-I including the bellows assembly was
about 9.8xl0- 11 Pa ·l/s/cm 2 (S 1 =6000 cm 2) from the equation
above. For reference, considering the % obtained previously
for Ch-I without the bellows assembly, the gas resorption
rate of only the bellows assembly was estimated as about
I.5xl0- 10 Pa · l/s/cm 2•
In any case the measured gas
desorption rate was found to be less than the aimed one.
The residual gas components were also measured after thebake and the most oominant gas component was H2• Othermain gases were H20, CO and C0 2 and any significant
contamination was not detected.

5 Conclusions

Fig. 5 Experimental setup for pumping down test.

The abrasion test of the contact-fingers indicated thequantity of metal particles generated during mechanicalmotion would have little hann on the beam lifetime if theinner plate was coated with silver. Silver coating was foundto be effective on not only keeping good electrical contact but
also rewcing particles due to abrasion in vacuum. The gas
desorption rate of the bellows assembly was 1 - 1.5x I o- 10 Pa ·l/s/cm 2 after a bake at 150 °C for 24 hours and the bellowsassembly showed a good vacuum performance.

References
10-3

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102

103

Time [hours][1] K.Kanazawa et al., "Vacuum Design for KEKB", Proc.
lOth Sympo. Ace. Sci. & Technol., Hitachi-naka,Japan(1995) 197.
[2] H.Hisamatsu et al., Vacuum, 4 7 (1996) 601.
[3] K.Ohshima et al., ''Development of a bellows assemblywith an RF-shield for the KEKB", Proc. lOth Sympo.
Ace. Sci. & Techno!., Hitachi-naka, Japan (1995) 91.
[4] Y.Suetsugu et al., Vacuum, 4 7 (1996) 629.
[5] Y.Suetsugu et al., Rev. Sci. Instrum., 6 7 (1996) 2796.
[6] S.Kato et al., ''Dust TrappingExperiments at TRISTAN
AR", Proc. lOth Sympo. Ace. Sci. & Technol.,Hitachi-naka, Japan (1995) 200.
[7] F.Zimmermann, ''Trapped Dust in HERA and Prospects
for PEP II", PEP-II AP Note 8-94 (1994).

Fig. 6 Pumping oown curves for Ch-I (P 1) andCh-2 (P 2 ).

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