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Proceedings of IPAC’10, Kyoto, JapanTHE
E LINACC UPGRAADE PLAN FOR SUPERKE
EKB
T. Sugimura, M. Akemmoto, D. Arrakawa, M. Ikeda, A. Enomoto,
E
S. Ohsawa, Y
Y. Ogawa, K.
K Kakihara,
H. Katagiri, E. Kadokkura, T. Kam
mitani, M. Kurasina,
K
M.
M Satoh, T. Shidara,S
A. Shirakawa,, T. Suwadaa,
T. Takenakka, K. Nakaao, H. Nakaajima, T. Higgo, S. Fuku
uda, K. Furuukawa, H. HHonma, H. Matsushita,
M
S. Matsumoto, T. Matsumoto,M
K. Mikawaa, T. Miura, S. Michizoono, Y. Yanoo, K. Yokoy
yama andM. Yoshida, Highh Energy Acccelerator ResearchROrgganization ((KEK),
1-1 Oho Tsukubba Ibaraki, 305-0801 Japan
Abstract
The next genneration B-facctory “SuperKKEKB” projecct [1],whose targett luminosity iss 8×1035 cm-2s-1, has just staarted.
A “nano-beaam scheme” iss introduced too the SuperKE
EKB.
In the scheeme, requireed beam eneergy, charge andnormalized emittanceeat thhe end of the injector linac are 7GeV, 5 nC/buunch and 20 x 10-6 m for electron and 4 GeV,
4 nC/bunch and 10 x 100-6 m for posiitron, respectively.
They are quiite challengingg targets for thhe present linaac. Inorder to meeet the requireements, we will
w introducee newcomponents to the linac. They are a photo-cathodde RF
gun for an ellectron beam, a positron cappture section using
L-band caviities, a newlly designed positron-matcching
device and a damping riing for a positron beam. This
paper shows a strategy of ouro injector uppgrade.

PRRESENT IN
NJECTOR LINAC
The present injector linaac provides fourfbeams too fourrings. They area KEKB Hiigh Energy Riing (HER), KEKB
KLow Energyy Ring (LER), Photon Facttory ring (PF) andPhoton Factoory Advancedd Ring (PF-AR). Table 1 showss
the beam paarameters for four rings. All
A rings exceppt for
the PF-AR normallynoperrate in “Top-uup” mode, so linac
injects three kinds of beam
m to them simuultaneously [22].
The linac is in the shappe of ‘J’ and an electron gun
g islocated at thhe end of the shorter side ofo ‘J’ (see Figg. 1).
This electronn gun has a thhermionic cathhode grid asseembly(Y796, EIM
MAC divisionn, CPI) and an electron beam
extracted by pulsed electrric potential of 200 kV fromm thecathode-grid assembly is bunched andd accelerated by a
sub harmonnic buncher 1 (SHB1, 114 MHz), a sub
harmonic buuncher 2 (SHHB2, 571 MHHz), a pre bunncher(2856 MHz)) and a bunchher (2856 MHHz). This eleectron
gun is calledd “A1-gun”. The A1- guun has four pulsedpcharge settinngs and cann change thee settings att themaximum innjection repetiition of 50 Hzz. As for the HER
beam, two electron bunchhes, each of whichw
has a chhargeof 1 nC, are accelerated ini the same RFR pulse and attainaenergy of 8 GeV at the ennd of the linaac. As for the LER
beam, two bunches
bof 100 nC beam arre acceleratedd to 4
GeV and irrradiate a tunngsten (W) taarget. An eleectron
beam generaates an electroomagnetic shhower in the target
tand emergedd positrons arre captured byb a quarter-w
wavetransformer type of matcching sectionn with an airr-core
pulse coil whhose magneticc field is 2 T and an RF fieeld ofan S-band acceleratingassection.
Connversion efficiency
from electroons to positroons is approxximately 10 %.
% Inother words, an electron beam of 10 nC is converted intoFigure 1: A plane view of the injectorr linac
ositron beam
m of 1 nC. Converted positronsparee
a poaccelerated to energy of 3.5 G
GeV and injeected into thee
LER
R. Normalizedd beam emittannces at the ennd of the linacc
are 3.0
3 x 10-4 m for the HER
R and 2.1 x 100-3 m for theeLERR. As for the PF-AR,P
the bbeam generateed at the “3T-Gun”” in the middle of the linacc is accelerateed in the linacc
and injected to the PF-AR ringg every 12 houurs. It takes afew minutes andd during thee injection, simultaneouss
injecction to other rings
ris suspennded.

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Proceedings of IPAC’10, Kyoto, Japan

Table 1: Present Beam Specifications
Particle

Energy(GeV)

Charge(nC)

ee+ee-

8.0
3.5
2.7
3.2

1.0
1.0
0.1
0.1HERLERPF
PF-AR

Bunch(bunch/RF-pulse)2
2
11Top-up
Yes
YesYesNo

BEAM REQUIREMENT FOR
SUPERKEKBThe KEKB team has been proposing an upgrade plan
multiplying the luminosity by factor of about 40 for
several years [1]; however, a base design of theSuperKEKB has changed radically in 2009 [3, 4]. Since
the luminosity is proportional to the product of beam
currents and inversely proportional to the beam size, there
are cursory two choices to upgrade luminosity, i.e. tomultiply the beam current or to reduce the beamemittance. Contrastively the previous luminosity upgrade
plan depended mainly upon the beam current upgrade; a
new upgrade plan will have been achieved by the beam
size reduction with an assist of beam current increase of
merely about factor of two. Table 2 summarizes required
injection beam parameters at present. Compared to thepresent linac parameters, the bunch charge increases by afactor of 4 to 5, while the beam emittance reduced by one
order of magnitude.
We are also considering a simultaneous injection to all
four rings in order to maintain the currents of theSuperKEKB rings. Without this simultaneous injection
scheme, the SuperKEKB rings will lose their beam
currents rapidly during the PF-AR injection due to theirshort life time; roughly about 10 minutes. In this scheme,
we are proposing to change the beam particle of the PFAR from electron to positron.
Table 2: Required Beam Parameters

Energy (GeV)
Charge (nC/bunch)
Bunch/pulse-6

Normalized Emittance (x10 m)

HER

LER

7.0 (- 8.0)

4.0

542

2

20

10

Repetition (Hz)Energy Spread δE/E (%)

500.080.07

LINAC UPGRADE PLAN
Electron BeamAn upgrade plan for the electron beam has a very simple
strategy. We will install an additional electron gun next toTHPD007

the present “A1-gun”. The new electron gun is a laser
driven photo cathode RF gun. An RF gun cavity and a
1m-long accelerator section will be installed in an
accelerator room and a laser system will be installed in
the A1-gun room. Both of the accelerator room and theA1-gun room locate on the underground level. A present
pulsed power supply for the A1-gun will be moved to an
extended ground floor and an RF source for the RF gunwill be installed also on the same floor. The A1-gun andthe power supply will be connected with a high-voltage
coaxial cable.
As for the cathode material, we are considering to takeadvantage of Cu with a longer operating life. Assuredly it
is often said that quantum efficiency of Cu is about twoorders of magnitude less than that of Cs2Te. It is notedthat the quantum efficiency of Cu for the laser wavelengthof 193 nm is about two orders of magnitude lager thanthat for 266 nm [5]. A laser wavelength near 200 nm
will bring Cu favorable quantum efficiency comparable toCs2Te. A rough estimate, using the quantum efficiency
near 200nm measured at CERN, gives necessary laser
pulse energy of 0.1 mJ/pulse for 5 nC electrons at the Cu
cathode.
As for the cavity design, we use Kim’s method [6, 7]
assuming practical parameters. Kim’s equations show that
accelerating field strength in a cavity has a strong
influence on total beam emittance, so that we will use asingle cell cavity with shorter filling time with an
expectation of higher accelerating field strength. In orderto reduce production costs, we will utilize the present RF
sources, thus, the RF gun cavity will be operated at
frequency of 2856 MHz.

Positron Beam
In order to increase positron-beam charge, we will
improve capture efficiency four times [8, 9] byintroducing a new adiabatic matching device and new Lband accelerator structures [10] in the capture section
locating at the downstream of a positron generation target.
We have two candidates for the matching device. One isa flux concentrator and the other is a superconducting
solenoid. Generally, a flux concentrator consists of twocoils. A pulse current in the primary coil induces eddy
current in the secondary coil and eddy current gives off astrongly concentrated magnetic field near the centre of thecoil just after the positron generation target. The magnetic
field will be 3-5 times stronger than that of the present
matching solenoid. A prototype flux concentrator, which
can achieve a magnetic field of 10 T, has been developedin collaboration with BINP, Russia. The primary and thesecondly coils of the prototype has combined into a singleconductor, which properly acts as the primary and thesecondly coils by carrying the primary and secondary
currents in different paths in the same conductor. A
preliminary operation test at BINP was successfully
finished with results of no mechanical failure. The
prototype flux concentrator will be shipped from BINP
and will be installed in a test stand at KEK in June, 2010.
An operation test will be started soon.

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THPD007

Proceedings of IPAC’10, Kyoto, JapanThe superconducting coil is another idea to apply astrong magnetic field as the matching device. It is a main
issue to understand whether the superconductive coil canmaintain superconductivity or not in the electromagnetic
shower near the target. Quench limit measurement underenvironment near electromagnetic shower has just started
using the present electron beam at the end of the linac.
Preliminary result shows that quenching is not sensitive to
peak heat density by pulsed irradiation. After moredetailed irradiation studies with a test solenoid, we will
start designing a prototype solenoid taking into accountthe irradiated average heat density and field strength in
the solenoid wire.
The final decision for the matching device will be madeafter performance studies of the flux concentrator and thesuperconducting solenoid.
The S-band capture sections will be replaced by L-band
capture sections, whose first disk’s aperture diameter ismore than 30 mm. Fabrication of the first prototype Lband section has just started at Hitachi, Ltd. Table 3
shows specifications of the capture sections.
Table 3: Capture Section Specification
PresentNew

Accelerating field(MV/m)

1410

aperture diameter ofstructure (mm)2030

RF frequency
(MHz)

2856

1298

Structuresconfiguration

1mx2+2 m x22mx6

As for an RF source of L-band, a new 40 MW class
klystron will be developed. The present modulator power
supply for S-band klystron will be utilized for the L-band
klystron.
As for an emittance reduction of positron beam, we will
introduce a damping ring in the middle of the linac, wherepositron beam with energy of 1 GeV is available. Thebeam emittance of 2 x 10-6 m at the injection point will bereduced to 17 x 10-9 m in horizontal and 5 x 10-9 m invertical at exit of the damping ring. For the details of thedamping ring, please refer to the reference [11, 12].
The positron generation target will be moved to theposition where the primary electron beam energy
becomes 3.5 GeV, while the present target locates at theposition of 4.0 GeV. This relocation is required in orderto have sufficient energy margin of the positron beam for
1 GeV injection to the damping ring. Though, theconversion efficiency from electron beam to positron
beam will be reduced by lowering the primary electron
beam energy, a new matching device and new capture
sections will eventually increase positron yield.

SUMMARY
In order to achieve the luminosity of 8 ×1035 cm-2s-1 in
the SuperKEKB, the injector linac needs to provide
higher charge and lower emittance beams for theSuperKEKB rings. As for the electron beam upgrade, wewill introduce a photo-cathode RF gun with a Cu cathodeilluminated by laser with wavelength of near 200 nm.
With respect to the positron beam upgrade, we will
prepare a new matching device with much higher
magnetic field and new capture sections with much larger
aperture. There are two candidates for the matching
device, one is a flux concentrator and the other is asuperconducting solenoid. Both of them will provide
several times stronger magnetic field than that of thepresent matching device. The development of these newdevices has been just started or will be started soon. It
may take a certain amount of time to complete the studies.

REFERENCES[1] K. Abe, et al. “Letter of Intent for KEK Super BFactory”, KEK Report 2004-4.
[2] M. Satoh, et al., “Present Status of the KEK Injector
Upgrade for the Fast Beam-Mode Switch”, inProceedings of LINAC2008, Victoria, BC, Canada,
2008, p.416[3] H. Koiso, “Design of SuperKEKB based on theNano-beam Scheme”, in this conference[4] Y. Ohnishi, et al., “Optics Design in Nano-BeamScheme for KEKB Upgrade”, in this conference[5] E. Chevallay, et al. “Photocathodes tested in the dc
gun of the CERN photoemission laboratory”, Nucl.
Instr. and Meth. in Phys. Res. A 340 (1994) 146-156[6] Kwang-Je KIM, “RF AND SPACE-CHARGE
EFFECTS IN LASER-DRIVEN RF ELECTRON
GUNS”, Nucl. Instr. and Meth. in Phys. Res. A275
(1989) 201-218[7] Christian Travier, “An introduction to photo-injectordesign”, Nucl. Instr. and Meth. in Phys. Res. A 340(1994) 26-39[8] N. Iida, et al., “Design of the Positron Transport
System for SuperKEKB”, in this conference[9] N. Iida, “Beam transport lines for SuperKEKB”,
http://www-kekb.kek.jp/MAC/2010/Report/Iida.pdf[10] S. Matsumoto, et al., “L-band Accelerator System in
KEKB Injector Linac”, in this conference[11] M. Kikuchi, et al., “Design of Positron Damping
Ring for Super-KEKB”, in this conference[12] M. Kikuchi, “Design of Damping Ring”,
http://wwwkekb.kek.jp/MAC/2010/Report/Kikuchi.pdf

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