KEK e+/e- INJECTOR LINAC
T. Natsui†, M. Akemoto, D. Arakawa, H. Ego, Y. Enomoto, K. Furukawa, T. Higo, N. Iida,K. Kakihara, T. Kamitani, H. Katagiri, M. Kawamura, S. Matsumoto, T. Matsumoto,
H. Matsushita, T. Miura, F. Miyahara, H. Nakajima, Y. Okayasu, I. Satake, M. Satoh, Y. Seimiya,
T. Shidara, A. Shirakawa, T. Suwada, M. Tanaka, D. Wang, Y. Yano, K. Yokoyama,
M. Yoshida, T. Yoshimoto, R. Zhang, X. Zhou, KEK, Ibaraki, Japan
Abstract
The KEK injector linac feeds the beams into four rings.
It is called J-Linac. The SuperKEKB main rings are highenergy rings (HER) and low-energy rings (LER). Thelinac injects a 7.0 GeV electron beam to the HER and a
4.0 GeV positron beam to the LER. It also injects electron
beams into the two light source rings. We successfully
performed this simultaneous four-ring injection. We
achieved this complex simultaneous injection using twoelectron guns, a positron source with a flux concentrator,
and pulsed magnets. In SuperKEKB phase 3 operation,
2.0 nC electron and 3.0 nC positron beam injections wereachieved.

INTRODUCTION
The KEK electron/positron injector linac was designed
to inject different types of beams into four different rings.
This injector achieved simultaneous four-ring injection at
50 pps. SuperKEKB has two rings: the HER and LER [1].
The other rings are the light source rings of the PF andPF-AR rings. An electron beam with an energy of 7.0
GeV and a positron beam with an energy of 4.0 GeV are
required for the HER and LER, respectively. The energiesof the PF ring and PF-AR are 2.5 GeV and 6.5 GeV, respectively.
The injector linac consists of eight sectors (sector A-Cand 1-5) and a bending sector (J-arc) with a total length of
600 m. This shape resembled that of J, as shown in Fig. 1.
One sector has eight klystrons, and one klystron drives
four 2-m accelerating structures. The energy is adjusted
from pulse to pulse by switching the accelerating or
standby mode of each accelerating structure.
We used two types of electron gun: a photocathode RFgun and a thermionic cathode DC gun. An RF gun with ahigh-power laser was used to generate a low-emittanceelectron beam for the HER injection [2]. The RF gun
charge and emittance design values were 5.0 nC and 6mm-mrad, respectively. Positrons were generated byhitting a primary electron beam onto a tungsten target.
These positrons were focused with flux concentrator (FC)[3] and accelerated with large aperture S-band (LAS) [4]accelerating structures. We obtained a 4.0 nC positron
beam with a 10 nC primary electron beam. The generated
and accelerated 1.1 GeV positron beam was injected intoa damping ring to reduce the emittance. A pulse-bendmagnet merged these two electron gun lines, and thesebeams were injected into a common acceleration beamline. A thermionic gun was used as the electron source forthe light source rings.
Sector A to 2 has common optics that use DC magnets.
However, sector 3 to 5 has independent optics using pulse
magnets. These pulse quadrupole and steering magnet
systems were developed for the SuperKEKB project andcan change the optics at 50 pps.
We achieved this complex four-ring simultaneous injection using two types of electron guns: a positron sourceand pulsed magnet system [5].

LINAC BEAM STATUS FOR SUPERKEKBThe SuperKEKB phase 3 operation began in 2019. Thelinac beam quality was gradually improved. Table 1 lists
the current beam status and final goal. The energy was set
to the required value, and a sufficient energy margin wasmaintained by providing standby units. The amounts ofbunch charges in operation didn't reach the target values.
However, these were almost sufficient for 2022b. The
emittance was improved in a step-by-step manner. Simultaneous injection to four rings and damping ring wascompleted.

Figure 1: KEK injector linac.
___________________________________________† takuya.natsui@kek.jp

Table 1: Beam Status of the Final Goal and Current Performance
2022abFinal goal
Beame+
ee+
eEnergy [GeV]
7.0
4.0
7.0
4.0
Bunch charge [nC]
(1st, 2nd)
(1st, 2nd)
(1st, 2nd)
(1st, 2nd)
3.0, 2.5
2.0, 1.5
4.0, 4.0
4.0, 4.0
Normalized emittance(Hor., Ver.)(Hor., Ver.)(Hor., Ver.)(Hor., Ver.)[mm-mrad]120, 520-50, 20-50
100, 1540, 20Electron beam
Figure 2 shows the one-year history of the electron
beam charge for the HER. 2022, stable 2 nC beam generation was achieved using a laser feedback system [6]. The
amount of charge was almost sufficient in the current
situation. However, the HER sometimes requires a higher
bunch charge. We must increase this for the next
SuperKEKB operation.
Emittance preservation is an essential issue for electron
beams because a damping ring is not available for theelectron beam. The emittance measured at the beamline
near the RF gun is good, as shown in Fig. 3. At the linac
end, the emittance is also good immediately after orbit
adjustment. However, a long period with no adjustment
causes gradual emittance growth; the latter part of thegraph in Fig. 3 illustrates this. The cause of this emittance
growth is suspected to be the long-term RF phase drift.
However, the reason for this is not clearly understood, andfurther investigation is required.

Figure 2: History of electron beam charge for the HER.

Figure 3: Long-term emittance value history of the electron beam. Emittance of RF gun beam (upper) and end of linac(lower).

Sometimes, a two-bunch operation as shown in Fig. 4
has been attempted. However, the injection rate of thesecond bunch tended to be low, and we had to perform theone-bunch operation. As shown in Fig. 4, there appears to
be no problem with the beam orbit. The emittance orenergy spread of the second bunch would have a problem.
Two-bunch operation is one of the challenges to be solvedin the next operational term.

Figure 4: Two-bunch operation of the electron beam.
The stabilization of high-power laser systems also increases beam charge. We want to establish a method foremittance preservation inside the linac for high-charge

operation for the next SuperKEKB operation. The issue ofincreased emittance in the beam transport (BT) line between the linac and ring also must be resolved. Studies on
two-bunch operation and emittance preservation are essential.

Positron beam
Figure 5 shows a one-year history of the positron
beam charge, including the primary electron beam charge
for the LER. The beam charge was 3 nC, which is close tothe final target. The thermionic gun generated a primaryelectron beam with a charge of 12 nC. The length of thebunch extracted from the cathode is 1 ns. We used two
sub-harmonic bunchers (SHB) to compress the bunch
length. The frequencies of SHB1 and SHB2 were 114
MHz and 572 MHz, respectively, and these were used tomake the bunch length sufficiently shorter than the Sband RF wavelength. We used a streak camera to adjust
the RF phase of the SHBs and an S-band buncher for
beam bunching. In addition, we succeeded in a two-bunch
operation. The second bunch charge was almost the sameas the first bunch charge. We maintained a stable twobunch injection to the LER.

Figure 5: History of the positron beam charge for the LER.
The 2022ab SuperKEKB operation achieved a stable 3.0
nC two-bunch positron beam operation. Many improvements were made to achieve this stable positron beamoperation. Figure 6 shows the long-term positron beam
charge history. Before 2020, the beam charge was low,
resulting in FC breakdown problem. This problem wassolved by adopting a new robust FC and refining theapplied voltage waveform [7]. Modifications were madein 2020 to increase the positron beam transportation in theLAS structure immediately after the FC. The LAS structures are in a long solenoid magnet with a transverse kickowing to their asymmetric structure. Steering magnets
were added inside the solenoid magnet to compensate forthe transverse kick. In 2021, positron beam transportation
remarkably increased, as shown in Fig. 6. Since then, theamount of charge has been gradually increased by increasing the primary beam and improving beam transport.

Figure 6: Long-term (a few years) charge history of thepositron beam.

LINAC UPGRADE PLANCurrently, we are working on various linac upgrades [8].
KEK injectors have a long history, and various measuresagainst aging and performance improvements are required.
One important issue is the replacement of accelerating
structures. Old accelerating structures are operated atlower voltages owing to field emission or breakdown
problems. In addition, the cooling channels are often
broken. Therefore, the development of new accelerating
structures is an urgent issue. We tested the newly developed structure and a new pulse compression system. Figure 7 shows the new accelerating structure, and Fig. 8
shows the spherical-cavity-type pulse compressor (SCPC)[9]. The new accelerating structure and SCPC have provided high accelerating voltages in the conditioning system, and we are testing them online. The introduction ofthese new structures will increase the overall injector
acceleration voltage in the future.

Figure 7: New accelerating structure.

Figure 8: SCPC.
Pulse magnet upgrades are also important. Pulse magnets have already been installed in sector 3 to 5. However,DC magnets are used in sector A to 2. We must transport
positron primary and electron beams in J-arc with common optics. Beam adjustment can be complicated andsometimes causes beam loss. Therefore, pulsed magnets
should be installed near the J-arc. Magnets with large
apertures are required in the J-arc. However, the pulse
quadrupole magnets currently in use only have narrow
apertures for low emittance. Developing a new magnet
with a large aperture and a high-current magnet driver isnecessary. The new pulse-magnet diameter was designed
to be 44 mm, whereas the existing pulse-magnet aperturewas 20 mm. Accordingly, the maximum current of thenew pulse-magnet driver had to be increased from 300 A
to 600 A. This high-power pulse-magnet driver is currently being tested.
A new device, the fast kicker, was also introduced. Thiskicks only on the second bunch. Sometimes, the orbit of
the first and second bunches become separated owing totheir energy deviation. The fast kicker was introduced todirectly compensate for this orbit misalignment. The time
separation between the first and second bunches was only
96 ns. The fast kicker will be tested in the next operational term.
We plan to introduce an energy compression system(ECS) in the electron beam line. Currently, the ECS isused only for the positron beam line. The 4 nC electron
beam is affected significantly by its longitudinal wakefields, and it makes a large energy spread. Therefore, anelectron ECS is necessary in the future.
Currently, we have to save the electric power consumed
in the linac operation. In accelerators, particularly linear
accelerators, the klystron efficiency dominates a large
portion of the power loss. In the KEK linac, the efficiencyof the conventional S-band klystrons is only 45%, which
can be significantly improved. The conventional klystron
output is 50 MW. We aim for an 80 MW of RF power
output with the same modulator to increase the efficiencyof the klystrons. We have begun designing a multibeam
klystron (MBK) with an efficiency greater than 70%. In
the future, installing a high-power klystron and a new
accelerating structure are expected to significantly improve the performance of the KEK linac.
We will continue to make various upgrades forSuperKEKB.

SUMMARY
In the first half of 2022, the KEK injector linac provided
electron/positron beams in the SuperKEKB phase 3 operation. Sufficient charge of electron/positron beams could
be injected into the HER and LER while achieving simultaneous four-ring injection. During this period, various
issues were identified. Emittance preservation and establishment of a two-bunch operation are required for theelectron beam. The beam loss must be reduced in both theprimary and positron beams. All systems of the injector
are required to operate with many controlled long-termdrifts. Various upgrades are planned to improve the injector performance. Replacement with new acceleratingstructures is gradually progressing. A pulse magnet upgrade is planned for flexible beam operation. We are currently developing high-efficiency klystrons to save power.
To further improve the performance of SuperKEKB, wewill continue to perform beam studies and various upgrades in the KEK injector linac.

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