UPGRADE OF KEK ELECTRON/POSITRON LINAC CONTROL SYSTEMFOR THE BOTH SuperKEKB AND LIGHT SOURCES
K. Furukawa∗ , Y. Enomoto, H. Kaji, H. Katagiri, M. Kurashina, K. Mikawa, T. Miura,F. Miyahara, T. Natsui, I. Satake, M. Satoh, Y. Seimiya, H. Sugimura, T. Suwada
High Energy Accelerator Research Organization (KEK), Oho, Tsukuba, 305-0801, Japan, andSOKENDAI (The Graduate University for Advanced Studies), Oho, Tsukuba, 305-0801, Japan
Abstract
KEK injector linac has delivered electrons and positrons
for particle physics and photon science experiments for morethan 30 years. It is being upgraded for the SuperKEKB
project, which aims at a 40-fold increase in luminosity overthe previous project of KEKB, in order to increase our understanding of flavor physics. This project requires ten-times
smaller emittance and five-times larger current in injection
beam from the injector. And many hardware components
are being tested and installed. Even during the 6-year upgrade, it was requested to inject beams into light sources
storage rings of PF and PF-AR. Furthermore, the beam demanding approaches from those storage rings are different.
SuperKEKB would demand highest performance, and unscheduled interruption may be acceptable if the performance
would be improved. However, light sources expect a stable
operation without any unscheduled break, mainly because
most users run experiments for a short period. In order todeal with the both requirements several measures are taken
for operation, construction and maintenance strategy including simultaneous top-up injections.

INTRODUCTION
SuperKEKB, an asymmetric-energy electron-positron
double-ring collider, is being commissioned with beams.
The construction has been performed since 2010 till the endof FY 2017. It is expected to increase our understanding of
new physics beyond the standard model of elementary particle physics with a 40-times higher collision rate comparedto the world’s highest luminosity at the previous project
KEKB, by doubling the stored beam current and employing
the nano-beam collision scheme [1, 2].
SuperKEKB consists of electron positron full-energy injector linac, 7-GeV electron ring (HER), 4-GeV positronring (LER), and positron damping ring (DR), that is being
constructed. The electron positron injector linac has delivered electrons and positrons for particle physics and photon
science experiments since 1982. It has been rejuvenatedsince 2010 towards SuperKEKB collider for its extremely
high luminosity, that requires injection beams with highcurrent and low emittance in transverse and longitudinal
directions [3, 4].
However, the injector linac had to continue the injection
into two light-source storage rings of PF and PF-AR evenduring the SuperKEKB construction period, and then it is∗ kazuro.furukawa@kek.jp

Figure 1: The configuration of electron/positron accelerator complex at KEK with linac and four storage rings ofSuperKEKB-HER, LER, PF and PF-AR.
required to deliver beams into four multi-purpose storagerings with different qualities realizing the simultaneous topup injections (Fig. 1).

BEAM INJECTIONS FOR PARTICLEPHYSICS AND PHOTON SCIENCE
Natures of beam demanding approaches from those experiments of particle physics at SuperKEKB and photon
science at PF/PF-AR are so different that the operation becomes tough to be planned, especially for the construction
and maintenance period.
The particle physics experiment at SuperKEKB always
pursues the limit of performance. They expect highest possible experimental data with long-term integrated performance(between every large international conference). It may continue for more than ten years. They often take risks of ashort-term shutdown if the integrated performance would
increase. Such a particle physics experiment at SuperKEKB
may be characterized as below.
• Experiment by long-term and fixed user group.
• Performance oriented.
• Pursuing yearly integrated performance.
• Tendency to avoid preventive maintenance.
• May have enthusiasm for improvements.
• Can develop common understanding between experiment and accelerator groups.
• Operators are trained through everyday operation.
On the other hand, the photon science experiments at PFand PF-AR are performed for only a few days at a time, andthe stability is most required over the performance improve-ment. Especially, a non-scheduled outage should be avoided
and a preventive maintenance is preferred. Such photon
science experiments at PF and PF-AR may be characterized
as below.
• Experiments by short-term and many groups.
• Stability oriented.
• Avoiding risks of unexpected outage.
• Prefer routine preventive maintenance.
• Peaceful operation.
• Bit of distance between experiment and acceleratorgroups.
• Operators learn through document.
Short-term and many groups
Because the injector should support the both experiments,
the time and budget allocation optimization should be balanced between those very different experiments.

Operation and MaintenanceThe difference between those experimental requirements
affects the modes of operation and maintenance. In the pastat the beginning of the TRISTAN project, the injector wasoperated continuously as long as possible without any maintenance. However, it was understood that such operationand maintenance strategy doesn’t provide maximum averageperformance.
When the KEKB project was started, the injector rejuvenation had to be performed even continuing the light-source
injections. Thus, the scheduled maintenance was preferable
for the construction. As photon science experiments preferred preventive maintenance, later it was decided to have
8-hour maintenance every other week.
For particle physics experiment at SLAC it seems that theaccelerator was operated continuously until ”Repair Opportunity Day” was declared when major failures were accumulated. Such strategy was possible if the users were notmulti-purpose.
During the KEKB upgrade it was possible to allocate
9-month shutdown in 1997, and major injector devices wereinstalled during that long shutdown [5]. For the SuperKEKB
upgrade it was difficult to schedule a long shutdown time
until 5-month shutdown was allocated in 2017. During thattime light-source injection was performed.

Comparison to Other Facilities
Injectors at other facilities were often required to support multi-purpose experiments as well. For example, theSLAC/SLC linear collider initially injected electrons into theNSRL light source. However, it became difficult to allocate
the beam for the light source as SLC took the higher priority.
Thus, it was decided to construct a separate injector, and theboth experiments could be performed independently.
The electron injector at DESY/HERA also required todeliver the beam into the light source. Contrary to SLC,HERA didn’t require continuous injection and the both rings
shared the same injector without any difficulties.

SIMULTANEOUS INJECTION AND
VIRTUAL ACCELERATOR CONCEPTAt the beginning of KEKB project, the injector filled
the storage rings once in about an hour [6]. The situation
resembled to DESY/HERA, and four rings shared a single
injector. In the later phase of KEKB it became importantto perform top-up injections into the both KEKB and PF
experiments to maintain the stable experimental conditions.
The stable beam currents in the collider rings were essential
to keep the collision condition with crab cavities. At thesame time the photon experiments requested top-up injection
for higher precision data. As the SuperKEKB storage rings
will have shorter beam lifetime, the situation becomes closer
to SLC, and a solution is needed.
It is not easy to meet the requests from both of the particle
physics and photon science experiments. Since the later
phase of KEKB project, a concept has been introduced ofwhich a single injector behaving as multiple virtual accelerators for corresponding storage rings.

Simultaneous Injections
The injector linac can accelerate beams 50 times a second.
If device parameters for microwave generators, magnets andguns are modulated every 20 milliseconds, a single injector
can perform top-up injections into multiple storage rings
as if it injects beams simultaneously. Such simultaneous
injections have been realized since 2009 [7, 8].
The system was controlled globally and synchronously
by an event-based pulse-to-pulse modulation (PPM) controlsystem [9]. In the system ten software beam modes are defined, and four of them were corresponding to injections intostorage rings of KEKB-HER, KEKB-LER, PF and PF-AR.
Each beam mode is accompanied with several fast eventcodes, and each event code can distribute to whole accelerator complex with a single byte information and precise
timing information of approximately 10 picoseconds. About
200 device parameters along the accelerator are modulated
in response to one of those event codes.
At the end of the KEKB project typical injections per
second were 12 times for KEKB-HER, 25∼37 times for
KEKB-LER and 0.2 times for PF. PF-AR will be included
in simultaneous injections as a new beam transport line wasconstructed in 2017 [10].

Virtual Accelerator Concept
The event-based PPM controls enables not only pulse-topulse device controls but also pulse-to-pulse beam diagnosis
readouts. Device controls and beam diagnosis under different beam modes are totally independent. For example,
a beam stabilization feedback loop at the same physicallocation can behave independently under different beammodes [11]. Such a behavior of the single injector can berecognized as multiple virtual accelerators as in Fig. 2 [12].
As such virtual accelerators were realized, a conflict ofbeam requests from particle physics and photon science

Event-based
Control System
Every
20 ms
F.B
e− GunARC

F.B

PF Injectione– (2.5GeV, 0.2nC)

F.B
e− GunARC

Damping ring

F.B

SuperKEKB-LER Injectione– (3.5GeV, 10nC)

F.B

e+ Targete+ (4GeV, 4nC)

e− GunARC

SuperKEKB-HER Injection

F.B

F.B

e– (7GeV, 5nC)
e− GunARC

PF-AR Injection

F.B
e– (6.5GeV, 0.2nC)

Figure 2: Single linac behaves as four virtual accelerators(VAs) to inject their beams into four separate storage rings.
Each VA would be associated with several beam feedback
loops, independent of loops in other VAs.
experiments with diverse beam qualities has been muchrelaxed.

PULSED MAGNETSIn the previous KEKB project, injection beams were pulseto-pulse modulated with moderate beam optics, and optics
matchings were performed at corresponding beam transport lines [11]. However, for SuperKEKB injection the precise beam orbit and optics management is necessary within
0.1 mm in order to suppress the emittance blow-up [13].
Small form-factor (3U) pulsed power supplies were developed for quadrupole magnets with advanced design ofspecifications of 1 mH, 330 A, 340 V and 2-ms pulse widthwith energy recovery from the magnetic coils up to 75%.
Pulsed power supplies for steering magnets were designed aswell for 3 mH and 10 A. Girders were also designed in-house
which is ready for 0.1-mm alignment (Fig. 3).
Those power supplies were designed with 0.5-ms flat top,
and need pre-trigger before 3 ms before the beam. Suchtiming signals can be directly generated for SuperKEKB
beams because linac and rings are tightly synchronized. It is,however, complicated to generate timing signals with a long
delay for light sources, because those rings are injected on
accidental timing coincidence and those rings compensate
the circumferences by changing RF frequencies. The timinggeneration algorithm was developed to prepare pre-trigger
and was confirmed.
Pulsed magnets of 30 quads and 36 steerings were massproduced and installed during 5-month shutdown in 2017.
They were confirmed to satisfy the specification of 0.1%
(rms) stability at 50 Hz. And they will support the sophisticated multi-purpose operation scheme.

CONCLUSION
Prospective issues and solutions for a single injector linac
at KEK are discussed to support the both particle physics
and photon science experiments. While demands from thoseexperiments have become much advanced, many of technical difficulties can be resolved by employing the virtual
accelerator concept. It is expected to achieve the collisionexperiments in SuperKEKB soon as well as light source
experiments at PF and PF-AR.

REFERENCES[1] H. Koiso et al., “Commissioning Status of High Luminosity
Collider Rings for SuperKEKB”, in Proc. IPAC’17, Copenhagen, Denmark, May 2017 paper TUZB2, pp. 1275–1280.
[2] Y. Ohnishi et al., “Accelerator design at SuperKEKB”, Prog.
Theor. Exp. Phys., 2013, 2013, 03A011.
[3] M. Satoh et al., “Commissioning Status of SuperKEKB Injector Linac”, in Proc. IPAC’16, Busan, Korea, May 2016,
paper THPOY027, pp. 4152–4154.
[4] K. Furukawa et al., “Progress of 7-GeV SuperKEKB Injector
Upgrade and Commissioning”, in Proc. IPAC’17, Copenhagen, Denmark, May 2017, paper TUPAB004, pp. 1300–1303.
[5] A. Enomoto et al., “Construction Status of the KEKB 8-GeV
Injector Linac”, in Proc. APAC’98, Tsukuba, Japan, Mar
1998, paper 4E202, pp. 292–294.
[6] K. Furukawa et al., “Present Performance and Future Upgradeof the KEKB Electron Linac”, in Proc. LINAC’02, Gyeongju,Korea, August 2002, paper TU431, pp. 382–384.
[7] K. Furukawa et al., “Event-based Timing and Control System for Fast Beam-mode Switching at KEK 8-GeV Linac”,
in Proc. PAC’09, Vancouver, Canada, May 2009, paper
FR5REP010, pp. 4797–4799.

Figure 3: Girder design for pulsed magnets with precisealignment movers.

[8] M. Satoh et al., “First Simultaneous Top-up Operation ofThree Different Rings in KEK Injector Linac”, in Proc.
LINAC’10, Tsukuba, Japan, September 2010, paper WE203,pp. 703–707.

[9] PPM controls, http://www-linac.kek.jp/cont/epics/event/.
[10] N. Higashi et al., “Construction and Commissioning of Direct
Beam Transport Line for PF-AR”, in Proc. IPAC’17, Copenhagen, Denmark, May 2017, paper WEPAB044, pp. 2678–2680.
[11] K. Furukawa et al., “Pulse-to-pulse Beam Modulation andEvent-based Beam Feedback Systems at KEKB Linac”, inProc. IPAC’10, Kyoto, Japan, May 2010, paper TUOCMH01,pp. 1271–1273.
[12] A. Akiyama et al., “Accelerator Control System at KEKB andthe Linac”, Prog. Theor. Exp. Phys., 2013, 2013, 03A008.
[13] Y. Seimiya et al., “Investigation of Beam Variation and Emittance Growth Simulation with Misalignments and Beam Jitterfor SuperKEKB Injector Linac”, in Proc. IPAC’17, Copenhagen, Denmark, May 2017, paper TUPAB005, pp. 1304–1307.