KEKB INJECTOR LINAC STATUS AND FUTURE UPGRADE
T. Kamitani∗ , for the KEK electron/positron Injector Linac and the Linac Commissioning Group,KEK, Tsukuba, Japan
Abstract
This paper reports on recent status of the KEK B-factory
Injector Linac. The double-bunch injection of positrons has
been used for three weeks KEKB operation in May 2002.
The injection rate has almost doubled compared with theprevious single-bunch injection mode. Careful tuning toequalize the beam characteristics of the two bunches has
been done for a sufficient beam transmission efficiencyof both of them. Though the double-bunch injection ispromising, there is still a problem of lower peak luminosity
in this mode and the single-bunch injection is used for ordinary operation. Preliminary studies of continuous beam
injection while keeping the detectors of the collision experiment turned on, were successful. They showed that thebeam background of the injected beam was acceptable forthe detectors of the experiment.
This paper also describes the design study of the injector
upgrade for a future project of the ten-times higher luminosity machine, SuperKEKB. In this upgrade, the positroninjection energy will be raised from 3.5 GeV to 8.0 GeV
and the beam intensities of the electrons and the positrons
will be increased five times and twice, respectively.

1

Every 80 to 90 minutes, the beams are refilled. Refill injection typically takes 12 to 13 minutes in the positron singlebunch injection mode. The details are 1 minute for the detector turning off and changing the tunes for the injection,
1.5 minutes for electron injection (710 → 940 mA), 0.5
minute for the linac mode change, 8.5 minutes for positron
injection (800 → 1460 mA) and 1 minute for the detector
turning on and changing the tunes for the collision. Mostof the time during injection is spent for the positrons. In
addition to these periodic injections, the beams are injected
after sudden aborts which occur a few times a day. Afterthe aborts, injection takes a longer times to fill the maximum currents from zero.

LINAC STATUS

The KEK electron/positron linac has been operating for
injection to the KEKB collider rings [1] (8.0-GeV e- for theHigh Energy Ring (HER) and 3.5-GeV e+ for the Low Energy Ring (LER), to the Photon Factory (PF) storage ring
(2.5-GeV e-) and to the AR SOR ring (2.5-GeV e-). Details of the linac layout and its components are described
elsewhere [2]. Recently, the KEKB has achieved a peak luminosity of 7.2 × 1033 /cm2 /sec and a daily integrated luminosity of 387 /pb/day. Both of them improved the world
records. Stable and efficient beam injection from the linac
greatly contributes to minimize the dead time and to improve the time-integrated luminosity. In the following sections, the routine operation status of KEKB injection andthe two beam studies to improve injection performance aredescribed. The one of the studies is double-bunch injection
to increase positron beam intensity and the other is continuous injection without interrupting the collision experiment.

Figure 1: KEKB Daily Performance
The linac beam specifications and the performance arelisted in Tab.1. The values referred to in the parentheses
for the positron charges and its injection rates are those forthe double-bunch injection described in the next section.
Table 1: Linac Performance
items
KEKB eKEKB e+
Beam Energy
8.0 GeV
3.5 GeV
Beam Intensity1.0 nC
0.6 (1.2) nC
Emittance (normalized)
0.8 µm
2.5 µmEnergy spread
0.5 %
0.5 %
Injection rate
3.0 mA/s 1.5 (3.0) mA/s

1.2
1.1

Linac PerformanceFig. 1 shows the typical daily performance of KEKB.
The red lines show the beam currents in HER (upper) andin LER (lower) and the yellow line shows the luminosity.
The maximum currents are 940 mA (e-) and 1460 mA (e+).
∗ takuya.kamitani@kek.jp

Double-bunch Injection

Since positron injection takes about six-times longer
than electrons, the poor positron intensity is a bottleneck
for the integrated luminosity. It is difficult to increase thebunch charge of the primary electron (10 nC), because itis the maximum as for a tolerance to the wake-field effect. To increase the positron collection efficiency, large
upgrade of the positron focusing system will be necessary.

As a cost-effective way to increase the positron intensity,
we have studied the possibility to increase the number ofbunches in one beam shot [3]. Due to the RF frequenciesof the linac (2856.000 MHz) and the KEKB rings (508.887MHz), the bunches have to go in 96.289-nsec intervals. For
efficient beam transport of the two bunches, careful tuning
is required. Especially, the energies of the two bunches
should be equal to fit for the limited energy acceptance.
This is mainly done by appropriate tuning of the SLED RF
pulse timing, as described in the ref [3]. It can deal withtwo bunches, but not for three bunches or more. Fig. 2
shows the orbits and the charges of the first (blue) and second (green) bunches along the linac until the end of thebeam-transfer line. The intensity of the second bunch isslightly less, but the total intensity is almost doubled andthe injection rate is increased from 1.5 → 3.0 mA/sec.

be fixed by small modifications of the data-taking components.

Figure 3: Continuous Injection Study

2 FUTURE UPGRADE
As one of the possible future projects of KEK, a feasibility study to upgrade of the present KEKB to a 1035 luminosity machine, SuperKEKB, has been started. The beamspecifications of the SuperKEKB are listed in Tab. 2.

Figure 2: Double-bunch Injection
Although the positron injection rate is almost doubled,
the peak luminosity is 10% less in this mode of operation
due to the irregular bunch spacing in the storage rings. Further investigation is necessary to improve the peak luminosity.

1.3

Continuous Injection Study

Even if the injection time is very short, the luminosity
degradation during the collision experiment is a large inefficiency factor for the time-integrated luminosity. It is inevitable because of a gradual beam loss by the beam-beamand the beam-gas collisions. One of the ideas for improving this situation is to inject the beam while keeping thedetectors of the experiment turned on. If it is achieved,
the time-integrated luminosity is improved by 20 to 30%.
However, there is a large possibility of detector trip-off by
the beam background during injection. We studied the endurance of the detectors during beam injection. The result
was fine, and no detectors were tripped. Fig. 3 shows therun-status in this study. It is clear that the LER positron
current is almost constant due to continuous injection, andthe luminosity degradation is less. There is still some problems concerning the data quality of the detectors, also thecontinuous injection mode is premature to use for routine
operation. However, these the problems are supposed toTable 2: SuperKEKB parameters
items
KEKB
SuperKEKB
Beam (HER)
8.0 GeV e8.0 GeV e+
Beam (LER)
3.5 GeV e+
3.5 GeV eStored current (HER)
1.1 A e4.1 A e+
Stored current (LER)
2.6 A e+
9.4 A eAs can be seen in the Tab. 2, the beam charge will beswitched at SuperKEKB in order to avoid the electroncloud effect in the positron storage ring, which will be crucial at low energy. Hence, although the linac will haveto inject 8.0-GeV positrons, the maximum energy gain by
the accelerator units after the positron production target is
not sufficient (4.8 GeV). We are considering two possible
schemes to increase the positron injection energy. The first
scheme is to double the acceleration field by using the Cband accelerating structures. The other scheme is to accelerate the positrons twice by beam re-circulation. Not only
the beam energy, but also the beam intensities should begreatly increased. Details of these issues are described inthe following sections.

2.1

Energy Upgrade (High gradient scheme)

In this scheme, the acceleration field gradient is increased (21 → 42 MV/m) by replacing the present S-band(2856 MHz) accelerating structures by the C-band (5712MHz) counterpart. Consequently, the RF sources also must
be replaced. A schematic view is shown in Fig. 4.
Three fourths of the positron-accelerating units are replaced by the C-band components. A positron damping
ring is added to reduce the emittance and to avoid beam loss

Figure 4: High gradient Scheme
in the C-band structures with smaller apertures. The Japan
Linear Collider C-band group has been working to develop
the C-band components, the klystron, the pulse modulator, the RF compressor, the accelerating structure and so on[4]. For example, a 50-MW class C-band klystron is already commercially available. However, their component
designs are optimized for the specification of the linear collider. Thus, we have started designing the components optimized for the SuperKEKB injector. The design of the pulse
modulator is in progress to fit for the limited space in theklystron gallery. The design of the prototype C-band accelerating structure will be based on the presently used S-band
2π/3-mode traveling-wave structure, except that the dimensions will be scaled according to the frequency. Concerning
the RF compressor, we first thought about a scaled-down
version of the present S-band SLED type TE015 cavity, but
found that the Q value is not sufficiently high at the C-band.
We are considering to adopt the TE038-mode cavity, usedfor CERN LEP injector linac (LIPS cavity) [5], which has
higher Q. Prototypes of the components will be fabricatedthis year and high-power tests of them will be performed
next spring.

2.2

Energy Upgrade (Recirculation scheme)

In this scheme, the positron beam is re-circulated to upstream through a damping ring in synchronous to the next
RF pulse to arise, as shown in Fig. 5. The beam-line layout and the beam operation are rather complicated. The
re-circulated positrons and the newly arrived electrons are
set in a certain bunch-spacing which gives the optimum acceleration phase for both of them.

the next positron beam, and the positrons bypass the target.
The positrons are combined with the newly produced lowenergy positrons after a few acceleration units. Further, after a few units, the low-energy positrons are extracted by
the fast kicker and sent into the damping ring, and the highenergy positrons are accelerated up to the end of the linac.
In this scheme, upgrades of the accelerating structures andthe RF sources are not necessary. However, the operationis complicated and careful tuning will be required for beam
separation and combination, as well as for the accelerationof two positron beams with different energies in a common
beam-transport line.

2.3

Beam-Intensity Upgrade

The second issue in the upgrade is to increase the beam
intensities. As shown in Tab. 2, the electron stored current
will be increased by 8.5 times and the positron current by
1.6 times. This unbalance comes from the charge switch
and it release the requirement for the positron intensity.
Double-bunch injection is supposed not to be compatible
with use of the damping ring. Instead, we are considering an upgrade of the positron focusing system. Provided,
the present 2.3-Tesla pulsed solenoid coil is replaced witha 6.0-Tesla flux-concentrator type of tapered solenoid, similarly to the SLC positron source, the positron yield is expected to be almost doubled. The design of the flux concentrator and the prototype fabrication will be performed
later. Concerning the large increase of the electron intensity, there is no problem to produce such a high-intensity
beam. Already, 10-nC electron beam is produced from theelectron-gun as a primary beam to produce positrons. However, the worse beam quality of the high-intensity will be a
problem for injection. The 5-nC intensity is a good compromise of the beam quality and the injection rate. An injection study of high-intensity electrons will be performed
to investigate the level of the beam background.

3REFERENCES[1] ”KEKB Accelertor Papers”, KEK Preprint 2001-157, December 2001, Chapter 1 ”Overview of the KEKB Accelerators””KEKB B-Factory Design Report”, KEK Report 95-7, August 1995
[2] ”KEKB Accelertor Papers”, KEK Preprint 2001-157, December 2001, Chapter 9 ”The KEKB Injector Linac”

Figure 5: Beam Recirculation Scheme[3] Y. Ogawa, et al. ”Two-Bunch Operation of the KEKB Linacfor Doubling the Positron Injection Rate to the KEKB Ring”,
presented at 2001 Asian Particle Accelerator Conference,
September 2001, Beijing, China[4] http://c-band.kek.jp/

The positrons and electrons are separated before the 180degree beam turn, since they go through different bendinglines because of their opposite charges. They are recombined after the beam turn and further accelerated up to thepositron production target. Just before the target, they areagain separated. The electrons hit the target to produce[5] A. Fiebig, et al. ”Design Considerations, Construction andPerformance of a SLED-type Radiofrequency Pulse Compressor using Very High Q Cylindrical Cavities”, CERN/PS
87-45 (RF), March 1987, presented at 1987 Particle Accelerator Conference, March 1987, Washington D.C.