JAERI-Conf 95-021

PA24
Development of a bellows assembly with an RF-shield for the KEKB
K.Ohshima, Y.Suetsugu* and K.Kanazawa*
Irie-Koken Co., Ltd.,740-5 Aza-Ohnohara, Shimoakasaka, Kawagoe 350-11, Japan*National Laboratory for High Energy Physics,
1-1 Oho, Tsukuba, lbaraki 305, Japan

2. Structure

Abstract
A bellows assembly with an RF-shield has been
The RF-shielddesigned for the KEK B-factory (KEKB).
suppresses the excitation of the higher order mode (HOM) atOur newthe bellows gap and reduces the impedance.
springextra
hasbuttypefinger
usualadesigned RF shield is
betweencontact
electricalsufficient
fingers to ensure theThe wearing test is
contact fingers and beam duct.
performed using a experimental model over 5000 expansioncontraction cycles with a 20 mm stroke and any mechanicalproblem is not found except for the dust production due to theabrasion. Since the abrasion depends on the contact force,the heating of the model is measured transmitting 80 kW of508 MHz microwave in atmosphere for several contact forces
and the necessary contact force, 50 g/finger, is obtained
experimentally.

1. Introduction
The collider of the KEK B-factory (KEKB)
consists of two rings, High Fnergy Ring (HER, for 8.0 GeVelectrons) and Low Energy Ring (LER, for 3.5 Ge V positrons).
The average beam currents for the HER and the LER are I. I A
and 2.6 A, respectively. The designed bunch length (Oz) is
4 mm for both rings, which gives the strict limitation on theFor easy
impedance of vacuum components (!].
installation and alignment, about 750 bellows assemblies areto be installed in each ring and, therefore, are Iikely to be oneof large impedance sources in the ring. The RF-shield has arole to flow the wall current as smoothly as possible andreduce the impedance [2-4].
The main criteria in designing the RF-shield is to keep
electrical contact while absorbing the expansion andgoodaFailure to meet
contraction during the beam operations.
this criteria will result in electrical discharge and possible
melting on the contact point [5]. Large enough contact
force without excess wearing is required, but there has beenno practical data for the contact force so far.
A bellows assembly with an RF-shield for theKEKB collider has been specifically designed to meet theserequirements. The RF-shield is a usual finger type but has a
special spring fingers to press surely the contact fingers on
to the beam tube. The wearing test was performed using aexperimental model. To find the necessary contact force,
furthermore, the heating of the RF-shield was checked
transmitting 508 MHz microwave through the model. We
report here the structure of the bellows assembly and theseexperimental results.

A schematic drawing of the bellows assembly forthe LER is shown in Fig. I. The RF-shield is composed ofthree parts, a inner tube, a contact finger and a spring finger
The contact finger is positioned between(slide finger).
Since the contact finger
the spring finger and inner tube.
is outside of the inner tube, the impedance of the bellows
For I mm thick inner tube,
assembly is kept very small.
for example, the calculated broad band impedance (Z/n) andthe loss factor (k) of a bellows assembly are 4.23x I o-6 Q and2.5xlo-3 V!pC, respectively for the LER. The electricalcontact between the contact finger and the inner tube is
Each spring finger is inensured by the spring finger.
contact with one contact finger with an appropriate force.
The st~ture can be applied for not only the circular beam
racetrack ortube but the non ·circular one, such asrectanglular shape. Cooling water channels in the drawing
should be attached to absorb the heat flux due to the reflectedsynchrotron radiation power, the joule loss and the higher
order mode (HOM) loss [3].

Fig.!. A schematic drawing of the bellows assembly
Figure 2 shows the outlook of an experimental model
The model isofthe bellows assembly (without bellows).
The innersection.
for the LER and has a circular crossspringThemm.
160
isdiameter is 94 mm and overall lengthandmm
0.4ofthickness
finger and the contact finger haverespectively.
mm,5.5and0.2 mm, and widths of 4.6 mm
In total
The gap between each contact finger is 0.5 mm.
50 contact fingers surround the inner tube. RF-shield has amaximum expansion stroke of20 mm and transverse offset of-91-

JAERI-Conf 95-021
GoodI mm.
The inner tube has I mm thickness.
elasticity is required for the spring finger to maintainsufficient contact force.
Furthermore, high thermal
conductivity is necessary for the thin fingers to avoid excess
heating. Both the spring finger and contact finger areThe contact
therefore made of Berylliwn-Copper (CI720).
point of spring finger is coated with about I J.IIllTIN to reducewearing. Both the contact finger and spring finger wereheat treated at 315 °C.
All other pans of the bellows
assembly including the inner tube are made of Stainless Steel(SS304).

Fig.2. The outlook of an experimental model

3. Experiments
3.1 Mechanical test

undergoing for several promising materials such as inconel;titanium or hastelloy. The second effective solution is tochoose a sufficient but not excess contact force. We have
tried to find the suitable contact force experimentally as
described in the following section.

3.2 Heating test
Excess heating at the contact points was checkedby transmitting 508 MHz microwave for several contact
forces using the experimental model. The microwave wasapplied because it can simulate more realistic wall current
excited by the bunched beam than the DC current. The CW
508 MHz microwave was supplied from a 10 MW klystronused for the RF cavity of the 1RISTAN Main Ring. Theaverage and the peak beam current of l.ffi are 2.6 A and 156
A. which are induced by 14.3 kW and 600 kW of 508 MHz
microwave power, respectively. In the experiment themicrowave power up to 80 kW was transmitted and was
sufficient to investigate the heating due to the average beam
current.
Figure 3 shows a schematic diagram of the experimental
apparatus. A 50 n coaxial transmission line was formed
using the model bellows assembly and a brass rod as an innerTheconductor. The transmission mode is TEM mode.
wall c~nt, therefore, has only axial component that is justthe same as the real wall current in the beam tube.
The
bellows model is similar to that descried in Sec.2 but has thelength of200 mm and the body of Alwninwn alloy. Spring
fingers under five different loads investigated, that is, load I(0 to 3g), load 2 (15g), load 3 (50 g), load 4 (90g) and load 5(160g). The bellows assembly is thermally isolated by acover to simplify the experimental condition.
The input
power was increased in step of 10 or 20 kW and maintainedat each power level for about 10 minutes.
Temperatures
were monitored using three thermocouples positioned at theinlet, at the middle and at the outlet of bellows assembly.

Testing was carried out using a model with theModelsame parameters described above under a pressure of i xI o-2
Bellows Assembly
Torr. The contact force per finger was set in the range from150 g to 170 g. The RF-shield was expanded and contracted
5000 times over a stroke of 20 mm, which corresponds toTrans1ormer
I 00000 times with a stroke of I mm that will be experienced
by the real bellows assembly during I 0 years beam operation.
Directional'One stroke took approximately I0 seconds.
CoolingCoupler
Insulating
We observed finally no kinking .or sticking ofWater
Covercontact fingers and found no mechanical problem. However,metal panicles due to the wearing of contact pans wereSOBMHz CW
observed.
The mean size of particles was approximately Microwave' ---7
50 J.IIll.
Such particles have been found to reduce the beam
Fig.3. A schematic diagram of experimental apparatus
lifetime or broaden the beam size when passing though ortrapped to the beam [6 ).
Since the wearing is inevitable
Figure 4 shows the increase rate of temperature ("C/sec)
for finger type RF shield, the particle generation is one of theversus
input microwave power.
The temperature is theimportant problems in the present design.
An approach tOofthree
thermocouples.
Thestraight line is theaverage
reduce the particle is to choose appropriate combination ofof
theminimum
power
loss,
that is, without
resultfor
thecasethe materials or the coatings for the contact finger, the springanycontact
resistance,
calculatedusing.thesurface
resistancefinger and the inner tube.
The wearing test is ·now
and the specific heat of the model. From this figure it can-92-JAERI-Conf 95-021
seen quite clearly that abnormal heating occurs for the load
To see more definitely the effect ofless than 50 g/finger.
contact force, the power loss per 10 f<W input power isplotted versus the average contact force in Fig.5.
Observation of the fingers after the experiments revealed
arcing spots on the contact fingers with a load less than 50g/finger.
From the results it can be concluded that the contact
force larger· than 50 g/finger is necessary to provide thesufficient electrical contact.
Considering manufacturing
and setting errors, however, the spring finger shOuld bedesigned to provide the contact force of l 00 glfinger at least.
This value is much less than that in the mechanical test (170g/finger), and the problem associated with metal particles
may be reduced Be careful, however, that the experiments
were done in atmosphere where the water vapor in the air will
Furthermore, the frequency of wall
work as a lubricant.
current in the experiments is far low compared to those in thereal beam tube. The more realistic investigation will benecessary.

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4. Summary
A bellows assembly with an RF"shield has beendeveloped for the KEKB. The RF shield is a usual finger
type but has the special spring fingers to press the contact
finger without fail.
The minimum contact force wasinvestigated experimentally by transmitting microwave up to
80 kW though the bellows assembly in atmosphere.
Acontact force above 50 g/finger was found to be necessary toConsidering manufacturing andavoid abnormal .heating.
setting errors, however, the spring finger should be designed
to provide the contact force of 100 g/finger at least. A test
using 2856 MHz microwave to check the arcing in vacuum is
now in preparation.
Further tests to find the best
combination of finger materials will be also carried out inhigh vacuum.

Acknowledgment
The authors would like to thank the members of
1RISTANVacuum Group in KEK for their useful discussions.
The authors also express their deep gratitude to H. Nakanishi
of 1RISTAN RF Group for his valuable suggestions
instruction during the RF experiments.

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Wang and A.Zolfaghari,Proc 1993 US PAC. p.3851,~ 25 ········-················t·····-·-··-·······-·····:·········c··················t·························- Washington,D,C. ( 1993) .
.....
~(5] TKoh etal.,Technology ofvacuum. p.947 (1990).
c.
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~62( II), I 99 I~

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050

100

150

200

Contact Force [g/finger]
Fig.5. The power loss per 10 kW input .power versus theaverage contact force. The dotted line is the calculated
minimum power loss.

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