JAERI-Conf 95-021

2a3-1

VACUUM DESIGN FOR KEKB
Ken-ichi KANAZAWA, Yusuke Suetsugu, Hiromi HISAMATSU, Masayuki
SHIMAMOTO, Shigeki KATO, Nobuhiro TERUNUMA, Mitsuru
NAKAGAWA, Masayuki SATO, and Hajime ISHIMARU.
KEK, National laboratory for High Energy Physics.
1-1 Oho, Tsukuba, Ibaraki, 305 JAPAN.
Abstract
In designing a vacuum system of a new
asymmetri cal collider for KEKB, the vacuum
ducts deal with intense heat from synchrotro nradiation because of high stored current. Dueto the short bunch length, the requiremen t forthe smoothnes s of the inner surface is tight.
OFC is adopted for vacuum chambers. Acidetch or chemical polishing is applied to clean
extruded surface. Using NEG strips as the mainpump, pumping speed is designed as 100 1 s-1
m -1 . When the photo-deso rption coefficient is10-6 a pressure of 10-9 Torr will be realized.
All chambers are baked before installation .
By adopting "dry-hood" technique, in situ bakeout will be omitted. Pumping slots are backed
up by grid to prevent the penetration of beam
induced field which causes heat up of pumpelements. A gap between flanges are filledusing Helicoflex as a vacuum seal. Contact
force· of an RF finger in a bellows is assured bya spring finger.
1. Introductio n
KEKB is a project to study B meson
physics as a second stage of TRISTAN. The
accelerato r consists of two rings with one
intersectio n where electrons and positrons
collide with different energies[!) . The vacuum
system deal with high beam current to obtainhigh luminosity . Described in the following
are general aspects applied for the most part ofrings which includes the regular arc, thewiggler straight and the RF cavity sections.
The design outline of KEKB vacuum system aresummarize d in Table 1.

Table 1

Beam energy
Beam current
Circumfere nce
Bunch length
Bending radius
Total Power of SR
Critical Energy of SR
Total photon flux of SR
Local maximum of the linear
power density

LER
3.5
2.630164
16.31
2117 (arc)
5.84
7.35xlo21
14.8 (bend)

HER
8.0
1.1
f-f--

104.463817
10.9
7.llxlo21
5.8 (bend)

GeVAmmmmkW
keV
photons s-1
kwm-1-13 (wiggler front)-10 (wiggler)(mm)
OFC (6)f-Duct material (thickness)
year- 1
7rad<10<105
surfaceductondoseRadiation·c
not estimated120 (bend)
Maximum temperatu re%
not estimated-0.15 (bend)
Maximum strain
duct
beamofwall
f-Surface exposed to SR
photons s-1 m-1
3.2xlo1B
3.3xlo18
Average linear photon density
Torr-10-9
f-beamwith
Average pressureTorr-10-10
f-Average base pressure
10-6
molec. photon-1
f-Photo desorption coefficient
1 s-1 cm-2
Torr-10-12
f-Static outgassing rate1m- 1
s1
1OO(target)
f-Linear pumping speed
racetrackcircle
Cross section of duct
1 s- 1
52.3
102
Conductan ce ofbeam duct (lm)integrated
array of port
Arrange of pump
NEG strip
NEG cartridge
Main pumpotron Radiation.
SR=SynchrRing,Energy
HER=H1gh
LER=Low Energy Ring,- 197-

JAERI-Conf 95-021

2. Effect of Synchrotron radiation
In designing pumping system, we
assume the desorption coefficient reaches 1o-6
after about a1000 Ah operation for the LER. In
recent high current rings this is an attainablevalue [2]. It is considered important to set a
static base pressure to be one order lower thanthe operating pressure.
Because of the high local linear power
density, OFC is adopted as a material for
vacuum chambers of the LER. It is known
copper works as a good self-shield against yray[3]. So it is used also for the HER
In the LER, rather large strain of-0.15% is induced locally by synchrotron
radiation from a dipole magnet. Applying a0.15% strain over 108 cycles does not cause
cracks on a half or quarter tempered OFC[4].
However, localized heating will lead toannealing of the material which can reduce
the local mechanical strength[5]. Annealed
OFC can still withstand 104 cycles of 0.5%
deformation at 150'C[6]. But it is considered
preferable to keep the local temperature below
140'C at KEKB.
2. Fabrication of copper chamber
The grade of OFC is ASM C 10100(oxygen-free electronic copper) for vacuum
surface, and C10200 (oxygen-free copper)
elsewhere. An oxide layer on the inner surfaceof an extruded pipe contains a large amount ofcarbon compositions, which are released as
CO and C02 in photo-desorption . To avoid
frequent conditioning of NEG duringcommissioning, it is necessary to remove thisfirst oxide layer and to produce a new oxide
layer with much less carbon. This treatment
will be done by using a commerciallyavailable chemical polisher which contains
H2~ and H2S04[?]or by applying standard
acid etch with H2S04, HN03, HCl, and water.
Both treatment can reduce the total amount of
the desorbed gas to a one order value compared
to an untreated case.
Joining together Cu pieces is possible
by TIG welding, EBW and Brazing. TIG welding
in uncontrolled atmosphere will give adamage on vacuum surface. The use of TIG
welding must be limited. The joint between Cu
and stainless steel which is necessary to use
stainless steel flanges is possible by various
ways, direct welding along a lip structure[8],welding using third material, explosive
bonding, HIP, and brazing.
3. Pumping elements

Since the space of dipole magnets fills
only 5% and 31% of the standard cell in theLER and the HER, respectively, a build in
sputter ion pump is not effective as a
distributed pump. We adopt, instead, NEG [9]strip (St707) for distributed pumping. Allpumps of LER are attached to pumping ports(see next section). A distributed pumping speedof 1001 s-1 m-1 is possible by installing a 200 1
s-1 pump to a port with a 200 1 s-1 conductanceat every 1m. A special cartridge arranged withshort NEG strips is designed for the LER. At theHER, a beam duct is pumped through side slots
by long NEG strips.
In addition to NEG, 200 1 s- 1 ion pumps
are installed at every 1Om in the arc. These ion
pumps are of recently developed type which
can keep their nominal pumping speed down
to w-10rorr[l0]. Roughing pumps are attachedat every 40m. Roughing unit consists of amagnetic bearing turbo-molecular pump and ascroll pump. It is essentially oil free. The
pressure of the ring must be monitored atintervals less than 20 m to find a vacuum leak
which affect an average pressure over the ring.
Cold cathode gauges will be equipped at thesame pumping port as an ion pump.
All chambers are baked 24 hour at150'C before installation fully equipped withvacuum components to check an outgassing
rate. Mter bake out they are filled with dry N2and closed with flanges for storage. In thetunnel chambers are connected using "dryhood" technique like ALS[ll] and will not bebaked in situ.
4. Impedance-related structures
Pumping slots must be backed up by agrid so that penetration of wake fields whichcauses a heat-up of pumping elements may be
reduced. Unfortunately no reliable estimation
is available on the magnitude of the radiationthat propagates through pumping slots.
Consequently, the vacuum system design must
be prepared for an unexpectedly high power
penetration especially in the LER which has
higher current. This is done by making it
possible to add a further grid through apumping port.
The connection with standard conflat
flanges will leave a gap between the gasket andthe flange where beam induced field can betrapped. There are 1500 flange connections inthe LER arc. The contribution for broad band
impedance is not negligible. The use ofHelicoflex[12] with the same inner diameter asa beam duct can drastically reduce the loss
factor to an acceptable level.
All bellows have an RF -shield inside toconnect a duct aperture smoothly. In our- 198-

JAERI-Conf 95-021

design, contact fingers are pressed by spring
fingers to ensure a contact force between thecontact finger and an inner tube. The detail of
this bellows will reported by Ohshima[1 3].

6. Schedule
LER chambers will be fabricated from1995 to 1996. The fabrication of HER will startin 1996 and continue to 1997. Installation will
start in the beginning of 1997. During 19971998 other special chambers used in theinjection section and the interaction region
will be constructed .

0-chombe r

Acknowled gment
We would like to thank not only thecompanies noted in the text but also thefollowing companie s for their sincere
collaborat ions and valuable informatio n:
Ishikawaj ima-Harim a Heavy Industry.
Mitsubish i Heavy Industry, Mitsubish i
Electric Corporatio n, and NEOS.
We also thank KEKB stuff for their help
on designing , especially Dr. Y. H. Chin for
impedance calculation and DrY. Namito forradiation dose estimation .
Notes and References[1] The design will be published as a KEK reportin 1995.

Fig. 1 Typical vacuum chambers of the LER

5. Vacuum chambers
In the current design three types ofbeam ducts will be fabricated. The first is the"B chamber" which is used with dipole bendmagnets and supplied with pumps. In B
chamber the linear power density is highest.
The second is the "Q chamber" which isused for quadrupol e magnets. It includes abeam position monitor and a mask, but nopumping port. Q chamber is fixed to aquadruple magnet through the support of thebeam position monitor. It is separated fromother chambers with bellows of 16 em long toavoid strong force from outside. It has asymmetric al cross section which is necessary
to causes the eddy current with a good leftright symmetry during an active modulatio nof the excitation of quadrupole and sextupole
magnets for the beam-base d alignment.
The third is the straight "S chamber"which has a cooling channel, a mask andpumping element. It is used between Qchambers. Typical Q chamber and S chamberof LER are shown in Figure 1.

[2] Y. Hori, M. Kobayashi, andY. Takiyama, J.
Vac. Sci. Techno!. A 12(4) (1994) 1644
[3] R. Ballion, J. Boster, W. Giesske, H. Hartwig,
D. Jagnow, R. Kose, J. Kouptsidi s, G.
Schumann , and M~ Schwartz, Vacuum
41(1990)18 87[4] private communic ation from Hitachi
Works[5] heat cycle test was carried out by Hitachi
Zosen using laser beam.
[6] private communic ation from Hitachi
Cable.
[7] the use of chemical polisher was proposedby Toshiba.
[8] collaborat ion with Sumitomo Heavy
Industry reported in I. Taguchi, 0 Morishita,

Y. Suetsugu, and K. Kanazawa, Proc. 9th UHVTechnique s for Accelerato rs and Storage
Rings, KEK, March 3-4, 1994, KEK proceeding s94-3, p385. in Japanese.
[9] Non Evaporable Getter, product of SAESGetters.
[10] the use of the new ion pump is suggested byULVAC.
[11] CERN Courier, November 1994, p14[12] Helicoflex delta of Le Carbone-L orraine
Cefilac will be used.
[13] K. Ohshima, Y. Suetsugu, and K.
Kanazawa, this proceeding s.

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