DEVELOPMENT OF L-BAND POSITRON CAPTURE ACCELERATORSYSTEM IN KEKB INJECTOR LINAC
Shuji Matsumoto#, Mitsuo Akemoto, Mitsuo Ikeda, Toshiyasu Higo, Hiroyuki Honma, Kazuhisa
Kakihara, Takuya Kamitani, Hiromitsu Nakajima, Katsumi Nakao, Yujiro Ogawa, Satoshi Ohsawa,
Yoshiharu Yano, Kazue Yokoyama and Mitsuhiro Yoshida,
Accelerator Laboratory, KEK, Oho1-1, Tsukuba, Ibaraki, 305-0801 Japan
Abstract
In order to improve the positron beam intensity neededfor SuperKEKB project, it was decided to replace thepresent S-band structures in the positron capture section
by a new L-band (1298 MHz) accelerator system. A 2mlong TW structure of 12 MV/m gradient is now underdesign process while a 40 MW klystron will be deliveredin summer. After the klystron testing, a single L-band
accelerator unit will be constructed for the structure study.
The study is scheduled in next spring to operate thestructure under solenoid magnetic focusing field.

NEW POSITRON SYSTEM
A new upgrade phase of KEK positron/electron Linac
will start soon in order to fit our next project,SuperKEKB, in which we reconfigure the present KEKB
accelerator complex into the ultimate luminosity frontiermachine of the high energy electron (e-) and positron (e+)
collider for B meson physics [1]. The life times of thestored beams in the collider rings are to be very short(typically several hundreds sec) since the beams are
extremely squeezed at the interaction point. Thus KEK
e+/e- Linac, the injector for SuperKEKB, should provide
intense e- and e+ beams to compensate the rapidly
exhausting beams in the rings. The Linac beams shouldhave small emittances enough to be injected efficiently
into the rings. To fulfill these requirements, we will
employ a new RF Gun system for the e- beam while forthe e+ beam we will prepare a new capture section toenhance its intensity and a damping ring to improve its
emittance [2]. In the present paper the new capture
section based on L-band RF is described.
Table 1: Positron Beam Specifications

PROPOSED L-BAND SYSTEM

PresentSuperKEKB

Charge/bunch at the target (nC)14Bunch / RF pulse

2-1

Emittannce normalized (m )2.1x102-3

10x10-6

Rep rate (pps)5050Energy(GeV)

3.5

4

In the present injector Linac, the positrons are generated
at the tungsten target located at sector 2 of Linac (with____________________________________________#shuji.matsumoto@kek.jp

4GeV primary e- beam of 10nC per bunch), collected andaccelerated in the capture cavities downstream of thetarget to form an e+ bunch of 1nC. As shown in Table 1,
we need to intensify the e+ beam by factor four. We
expect the introduction of a stronger focusing magnet atthe target can give us factor two and another factor two
can be earned by enlarging the acceptance for positrons.
A natural solution to do this is use of lower frequency
capture cavities which have larger aperture and longer
wavelength instead of the present S-band cavities. To bespecific, we will use L-band cavities, whose frequency is1298MHz.
The reason of choosing this particular frequency is alittle intricated. Note that Linac uses three frequencies:
114MHz for SHB1, 571MHz for SHB2 and 2856MHz forthe main linac respectively. These frequencies are from asingle frequency f0=10.385MHz (The harmonic numbers
are 11, 55=11×5 and 275=11×5×5 respectively.) The
frequency of accelerating cavities in the collider rings is508.9MHz (49th harmonic of f0). In principle, the multibunch acceleration and injection are possible if thesebunches are separated by 96.3ns= 1/f0, and actually this istrue for the positron beam in present Linac, where thee+beam consists of two bunches separated by the timing
above.
These basic relations will be kept in SuperKEKB
accelerator system also. In order to fit the capture cavities
in this framework, one can choose the frequency of1298MHz, 125th (=5×5×5) harmonic of f0. Thanks tothese rational relations, the 2-bunch operation is stillvalid. Note that this frequency is very close to 1300MHz,
which is the most popular frequency in L-band. We may
share the established technologies or even purchasevarious devices which are commercially available.

In Fig. 1, the whole layout of the capture section at thecurrent design stage is shown, in which at least two Sband units will be replaced into L-band. The first unitconsists of two 2m long L-band accelerating structures
followed by another L-band unit which contains fourstructures. The final configuration of the positron capture
section number will be determined based on the detailstudy of the beam dynamics downstream of the target
which is currently ongoing.
Between the target and the first strucuture, there is astrong magnet to focus the positrons generated at thetarget. The structures of first two units are the travellingwave type immersed in the soelnoidal focussing magnetic
field (~ 4 kG).

The target position is moved upstream from the present
position in order to have a sufficient energy margin for e+
beam to 1.1GeV, which is the injection energy of thedamping ring which will be located in the middle of Linac.

Table 2: Accelerating Structure Design Parameters
Freqeuency

1298 MHz

Input power

15 MWGradient

12 MV/m

2a39.4 ~ 35.0 mm

t10 mmvg/c

Figure 1: Layout of new positron capture section.

RF Distribution System
We adopted WR650 waveguide for the high powertransmission (a=165mm, b= 82.6mm). Note that thetypical peak power to be handled in the L-band system isa few tens MW with a few micro seconds pulse width.
The whole waveguides to transmit the high power is kept
in vacuum.
One concern is that the weights of the waveguidesystem components: one needs to use waveguides strong
enough to sustain ambient atomospheric pressure.
Suppose we use copper waveguides with 10mm thickness,
their unit weight is about 50 kg/m. Cares should be paidin this respect since the weight of those components givesome impact on their support system as well as the actual
work for construction. We may use Aluminium alloy forsome of the waveguides once we convince ourselves thatthey work fine under vacuum condition. We will do thegas emission test for a sample Al alloy waveguide.
There are some vacuum flanges commercially available
for WR650. Among these, we will adopt MO Flange [3],MOF-650. The MO flanges are unisex. We haveconfirmed MOF-650 made from SS to be usable in oursystem.
Most of the RF components necessary for our projectsuch as directional couplers, vacuum ports, dummy loads,
or 3dB hybrids, are commercially available. We haveordered those components.

Accelerating Structure
The electric design work of the structure has been done[4]. The design parameters are listed in Table 2. It is adisk loaded 2m-long constant gradient travelling wave
structure of 28 cells operated with 2π/3 mode. The
coupler dimensions were determined by Kyhl method.
The whole structure will be made from OFC class1. To
allow the insertion of solenoid magnet after thecompletion of the structure, the maximum transverse
dimensions of the structure are specified to be within
350mm in diameter around the beam axis.

0.61-0.39 %Filling time

1.32 μs

Attenuation

0.261

Q~20000

Length2m

Klystron
There is no commercial klystron to meet perfectly with
our required specifications. We ordered to constract a newL-band klystron based on a 40 MW S-band klystron(2856 MHz) in such a way that we share its gun design
while the RF circuits are newly designed. This brings us
advantages not only to save the design effort of theklystron but we need not to modify the modulator which
supplies the pulsed power to the klystron. The whole
length of the L-band klystron will be slightly increased tofit the lower operating frequency. One needs to optimize
the solenoidal focussing field and a new magnet will beprepared. The klystron was delivered in the end of August2010 (See Table 3 for Klystron design parameter).
Table 3: KlystronFreqeuency

1298 MHz

Output power

40 MW

Pulse width

4 μs

Rep rate

50 pps

Gain>50 dB

Efficiency>40 %Cathode Voltage

350 kV

Perveance

2 μP

LLRF
It is simply supposed that LLRF for our L-band systemshould be as stable as S-band system and it can be. Thereis no further requirement on LLRF in our system.
However, in order to handle the L-band RF signals in amore sophisticated way, we have a plan to introduce acompact LLRF unit based on IQ modulator/demodulator

with an FPGA controller instead of our traditionalIφΑ module. See Fig.2. This new unit handles the RF
signal for a driver amplifier (600 W solid-state amplifier)as well as the monitor signals from the klystron output for
the interlock (VSWR) and feedback.

surface area per unit length of L-band components isabout twice of that of S-band, it is preferable to lower thegas emission from the surface. The rate from Aluminumalloy surface will be available soon and we can actually
start to design the detail configuration of the pumps tomeet the vacuum requirements.

TEST PLAN

Figure 2: Diagram of a proposed compact LLRF unit for
KEKB Injector L-band RF.

VacuumDesign work on the vacuum system is ongoing. The
volume, the sureface area and the vacuum conductanceare the primary parameters for the design. Those numbersof WR650 waveguide and the accelerator structure areshown in Table 4. From these numbers, we can sketch theL-band vacuum system. For example, the coupler of theaccelerating structure is known as the most vulnerable toRF breakdowns therefore it is critical for stable operations
to lower the pressure around the coupler as much as
possible. If we assume the gas emission rate from theaccelerator structure or the waveguides is lower to besome 2 × 10-8 Pa m3/s/m2, we need more than 50 l/s
evacuation at the coupler area in order to maintain thepressure below 2×10-6 Pa.
Table 4: Area, Volume and Conductance
WR650 Waveguide(unit length )Accelerator Structure(2m long)

The whole devices and components will be tested
before they are assembled to be the L-band capture
section of Linac. The test of the klystron will be started
soon. We will check its performance and if it is sucessful,
we are going to do the high power testing of RFcomponents such as a dry RF load using the power
provided by this klystron.
The accelerating structure will be delivered in January
next year 2011. One accelrator unit will be constructedwith this structure. The L-band klystron will be moved tothe sector 1-4 connected to the modulator and a newtransmission line will be constructed to the acclerator
structure which will be set in KEKB Linac tunnel. Its highpower test will be done in two stages: in the first stage,
the test will be done for the structure alone and in thesecond it is with the focussing magnetic field (thestructure will be immersed in the focussing field) to check
whether the application of the field induces additional
breakdown events in the structure.

CONCLUSION
Most of the specifications of the e+ beam in Linac are
considered to be already fixed and work for a new L-band
accelerator system for the positron capture section in
Linac has started. The klystron power test will be started
in this autumn. We expect the key devices such as
klystron and accelerating structure will be ready soon. We
also expect the RF components needed to form a powertransmission line to be ready. Those devises andcomponents will be assembled for the first half RF unit of
the accelerator within this JFY2010 and high power
testing of the accelerating structure will be done in nextyear 2011.

Surface Area

0.50m2

Volume0.014m3

REFERENCES

Conductance270 L/s

Surface Area

1.9m2

Volume0.05 m3

Conductance

3 L/s[1] M. Masuzawa, “Next Generation B-factories”,
IPAC10, Kyoto, Japan, May23-28, 2010.
[2] T. Kamitani, “Injector Linac Upgrade forSuperKEKB”, MOP011, these proceedings.
[3] http://mo-ohtsuka.co.jp/moflange/index.html[4] K.Saito, et. al., “Design of L-band positron capture
accelerating structure” in Proc. of the Particle
Accelerator Society of Japan, Himeji, Japan, Aug.
2010 (in Japanese).

The gas emission rates from the surface are thefundamental data for the vacuum design. Since the