BEAM OPERATION FOR PARTICLE PHYSICS AND PHOTON SCIENCEWITH PULSE-TO-PULSE MODULATION AT KEK INJECTOR LINAC
K. Furukawa∗ , M. Satoh, Injector LINAC group,
KEK, Tsukuba, Japan, SOKENDAI, Tsukuba, Japan
Abstract
The electron and positron accelerator complex at KEK
offers unique experimental opportunities in the fields of elementary particle physics with SuperKEKB collider andphoton science with two light sources. In order to maximize
the experimental performances at those facilities the injector
LINAC employs pulse-to-pulse modulation at 50 Hz, injecting beams with diverse properties. The event-based control
system effectively manages different beam configurations.
This injection scheme was initially designed 15 years ago
and has been in full operation since 2019. Over the years,
quite a few enhancements have been implemented. As theevent-based controls are tightly coupled with microwave
systems, machine protection systems and so on, their modifications require meticulous planning. However, the diverse
requirements from particle physics and photon science, stemming from the distinct nature of those experiments, often
necessitate patient negotiation to meet the demands of bothfields. This presentation discusses those operational aspectsof the multidisciplinary facility.

INTRODUCTION
Particle accelerators require large resources with advanced technologies and experienced personnel in order
to construct and to operate them. Therefore, beams fromcertain accelerators have been shared between several different purposes. A long beam switching interval may be
accepted between some accelerators. On the other hand,in order to switch beams frequently the mechanism called
pulse-to-pulse beam modulation (PPM) have been developedand employed for advanced accelerators [1,2]. However, different beam users may request variety of beam properties
as well as conflicting operational concepts. Thus, meticulous arbitration for multidisciplinary operation would benecessary.
The injector LINAC at KEK has been operated for morethan 40 years and has served for several storage ring accelerators in the past [3]. Presently, it makes multidisciplinary
injection operation to support two light sources and a B
factory collider storage rings with pulse-to-pulse beam modulation as in Fig. 1. It is often referred to as simultaneous
top-up injections [4].

INJECTOR LINAC AT KEK
In 1978 a dedicated (the second generation) light source
project was approved, and then a high energy electron
positron collider project was approved as well. A 400-m
electron linear accelerator was constructed to support both∗ kazuro.furukawa@kek.jp

synchrotron radiation research and high energy physics collier experiment [5]. Since then, it has been serving for various projects which required diverse beam properties as inFig. 2.
Additionally, each project required an upgrade of the injector LINAC which might take up to six years in between
projects, however, even during the upgrade the other projectneeded the beam injection. Thus, the equipment tests, fabrications and installations had to be carefully arranged inorder to support a sustainable injector operation.
For example, it took 5 years to upgrade the accelerators torealize a KEKB B-factory machine [6]. While the collider
ring was shutdown for 5 years, the injector LINAC had todeliver the beam every year to the light source. The injector
performed 3-month injection operation and 3-month upgrade
installation repeatedly. The SuperKEKB upgrade required 6
years from 2010 and another year in 2017 [7]. The injector
utilized the last downstream part of LINAC to inject thebeam for the light sources. And the rest of the LINAC wasreconstructed to support high intensity and low emittanceinjection for SuperKEKB as well as to recover from theGreat East Japan Earthquake in 2011. The longest complete
shutdown of the injector LINAC was 5 months in 2017.
During the upgrade period a particular effort was made forthe alignment in order to suppress the wakefield effect in theaccelerating structure.
In such way, the injector LINAC has negotiated with bothcommunities of photon science and particle physics, and they
understood each other to maximize the overall experimental
performance.

Low emittance
RF-gunDamping
Ringe–

Injector Linac

e+

700 m
High efficiency
e+ generator

2x beamcurrent

PF
2.5 GeV

e–

e+ BT

PF-AR
6.5 GeVe – BT

Beam from Injector and Storage Current
SuperKEKB:
7 GeV e2000 mA
4 GeV e+2800 mAPF:
2.5 GeV e450 mA
PF-AR:
6.5 GeV e60 mA

HER
7 GeV

SuperKEKB
3 km

LER
4 GeV

Belle II
30x
Luminosity

Figure 1: Beam injection from electron positron injector
LINAC to SuperKEKB dual ring collider as well as PF and
PF-AR light source storage rings.

1980
1990
2000
2010
2020
JFY 1970
Exp. Energy 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3

Project

Injection Energy

Photon Factory

2.5 GeV

2.5 GeV

TRISTAN

2.5 GeV

32 GeV

Injection Operation

Construct.
Construct.

Injection Operation

Slow Positron 2.5 GeV - 55 MeV 0.1 - 35 keV
KEKB
PF-AR
SuperKEKB

8 / 3.5 GeV

8 / 3.5 GeV

Operation

Operation

Construct.

Injection Operation

2.5 GeV - 6.5 GeV 5 - 6.5 GeV
7 / 4 GeV

Injection Operation

7 / 4 GeV

Construction

Injection

Figure 2: Electron positron accelerator projects operated by the injector LINAC for past 40 years. The injector has continued
the beam delivery through several upgrades since 1982 without a long shutdown.

PULSE-TO-PULSE BEAM MODULATIONAT KEK INJECTOR LINAC

location can behave independently under different beammodes [10]. Such a behavior of the single injector can berecognized as multiple virtual accelerators as in Fig. 3 [11].
As such virtual accelerators were realized, a conflict ofbeam requests from particle physics and photon science
experiments with diverse beam qualities has been muchrelaxed.

At the beginning of KEKB project, the injector LINAC
filled the storage rings once in about an hour [8]. In thelater phase of KEKB it became important to perform top-up
injections into both KEKB and PF experiments to maintainthe stable experimental conditions. The stable stored beam
currents in the collider rings were essential to keep the collision with crab crossing by crab cavities. At the same time
the photon experiments requested top-up injection for higher
precision data acquisition.
Thus, a concept has been introduced of which a single
injector behaving as multiple virtual accelerators for corresponding storage rings.

While technical issues of multi-purpose beam sharing
were mostly resolved with simultaneous top-up injections,operational demanding approaches including maintenanceplan and upgrade strategy must be negotiated between injector LINAC, SuperKEKB and light source divisions.

Simultaneous Top-up Injections

Operation and Maintenance Consideration

The injector LINAC can accelerate beam bunches 50 times
a second. If device parameters for microwave generators,
magnets and guns are modulated every 20 milliseconds, asingle injector can perform top-up injections into multiple
storage rings as if it injects beams simultaneously. Suchsimultaneous injections have been realized since 2009 [4].
The system is controlled globally and synchronously byan event-based PPM control system [9]. In the system ten
virtual beam modes are defined, and four of them were corresponding to injections into storage rings of SuperKEKBHER, SuperKEKB-LER, PF and PF-AR. Each beam mode isaccompanied with about ten fast event codes, and each eventcode can distribute to whole accelerator complex with a single byte information and precise timing of approximately 10
picoseconds. More than 200 device parameters along theaccelerator are modulated in response to one of those eventcodes.
Typical injections per second are presently 5∼10 times forSuperKEKB electron, 10∼25 times for SuperKEKB positronand 0.5 times for PF and PF-AR. They are frequently adjusted
by software or human operators depending on the injection
status.

The injection concepts and beam property requirements
are quite different between downstream accelerators. Because the injector LINAC is expected to support such very
diverse experimental requests, the time and budget allocation
optimization should be balanced between them.

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 completely independent. For example,
a beam stabilization feedback loop at the same physical

MULTIDISCIPLINARY BEAM INJECTION

Event-based
Control System
Every
20 ms

F.B
e− GunF.B

PF InjectionARC

e– (2.5GeV, 0.2nC)

F.B
e− GunARC

Damping ring

SuperKEKB-LER Injection

F.B

e– (3.5GeV, 10nC)

F.B

e+ Targete+ (4GeV, 4nC)

e− GunARC

SuperKEKB-HER Injection

F.B

e– (7GeV, 4nC)
e− GunARC

F.B

PF-AR Injection

F.B
e– (6.5GeV, 0.2nC)

Figure 3: Single LINAC behaves as four virtual accelerators(VAs) to inject their beams into four separate storage rings.
Each VA would be associated with several independent beam
feedback loops.

3000 photon science users per year come to light source
facilities in small groups for only a few days at a time. Thoseshort-term users require the beam operation stability overthe peak performance. If an unexpected beam shutdown
occurs during the allocated experimental time slot, they may
lose the opportunity to acquire experimental data.
Thus, the risk of beam interruption must be avoided, anda four-hour routine preventive maintenance period is scheduled every other week1 . Light source users don’t interact
much with accelerator operations as far as the beam availability is satisfied.
As the beam lifetime is more than a day, the injection
beam charge per pulse is low and the injection frequency isalso low as described above. A lower value is preferable tomaintain the injection radiation suppressed.
However, from the viewpoint of operator skills, a stable
operation means that operators rarely face unusual situations
if they operate only on light sources. Then, they have tolearn through training documents.

Operation for Collider Experiment
On the other hand, the particle physics experiment at
SuperKEKB is essentially performed by a large single usergroup, Belle II. It always pursues the limit of performance.
They expect a highest possible volume of experimental collision data within a year for higher statistics.
They often take risks of a short-term beam interruption
if the integrated performance might increase. They are reluctant to allocate time for preventive maintenance period.
They hope to continue their experiment until an accelerator
device or the beam fails. They are sometimes involved inmachine improvement and machine-detector development.
They aren’t afraid to take a chance to change the machine
and to improve the performances.
The stored beam current has been gradually increased
more than an ampere, and the beam lifetime is typically lessthan 10 minutes. Even though the beam charge per injection
bunch is more than 2 nC, beam injection frequencies forelectron and positron are more than 10 Hz respectively as described above. As the detector data acquisition is presently
forced to suspend for about a millisecond during the injection, they prefer larger beam charge. There exist many
challenging issues related to such large beam charges, thatmust be solved one by one.
1 As the availability improved, the maintenance period will be allocatedevery four weeks from the autumn of 2023.

Continuous Improvements and Protection Systems
During the discussion between accelerator divisions, additional beam control requirements may arise, that would belater implemented. As the injection system has become complicated, such modification needs careful considerations.
The safety systems especially require thorough investigations to make them work without any failures [12]. For
example, the interlock threshold of beam charge for radiation
limit is hundred times different depending on the beam injection modes or the beam dumps [13]. The beam repetition is
50 times a second and the beam charge need to be integrated
for each beam mode. The measurement and beam shutdown
have to be performed precisely.

OPERATIONAL ACHIEVEMENTS
Before the construction of the injector, there was growing
expectation for the world-class Japanese domestic collider
in the experimental particle physics research after the successes in the world-level theoretical research. On the other
hand, in the field of synchrotron radiation science, there
were demands for a dedicated accelerator for synchrotron
radiation research.
In order to realize both of those accelerator projects, anelectron linear accelerator was constructed in 1982. Since
then, the injector LINAC has been in operation for more than
40 years, supporting the consecutive accelerator projectsof Photon Factory (PF), TRISTAN, KEKB, PF-AR, andSuperKEKB as depicted in Fig. 2. The injector facility also
runs the slow positron facility for long that also contributes
to the material science field [14].
The brief operation history is given in Fig. 4, that shows
long-term yearly operation hours without a long break. The
beam availability has been mostly kept more than 99% depending on the control and operational changes in the year.
Statistics of the Injector Linac Operation
8,000

8TRISTAN

KEKB

7,000

SuperKEKB
Operation hourPartial failure

6,000

7

6

Injection stop
5,000

5

4,000

4

3,000

3

2,000

2

1,000

1

0

Rates (%)

Operation for Photon Science

When the operators deal with the beam for particle physics,
they often face small failures because of such improvement
challenges. Thus, they continue to receive on-the-job training.

Operation (hour)

The overall schedule for the electron and positron accelerator complex is discussed between divisions three times ayear. Dates of maintenance and weekly operational details
are discussed between operational leaders. Long-term upgrade plans are developed balancing the possible outcome
and risk at each accelerator. During such discussion eachdivision tends to focus on different aspects.

01982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020 2022

Fiscal year

Figure 4: Yearly operation hours and failure rates of theinjector LINAC. The blue line indicates device failure rate,that could have been covered by redundancy and so on. The
red line indicates the complete beam interruption.

Improvementof Integral
Luminosity[6] M. Akemoto et al., “The KEKB injector linac”, Prog. Theor.
Exp. Phys, vol. 2013, p. 03A002, 2013. doi:10.1093/
ptep/ptt011

41.6 /pbfor 5.15 hours
on May.16.2019
with
Top-up injections

Luminosity [1032]

Luminosity [1032]

17.5 /pbfor 5.15 hours(in 5 fills)
on May.6.2019

Beam Current [A][3] K. Furukawa et al., “Achievement of 200,000 hours ofoperation at KEK 7-GeV electron 4-GeV positron injector linac”, in Proc. IPAC’22, Bangkok, Thailand, Jun.
2022, paper THPOST012, pp. 2465–2468. doi:10.18429/
JACoW-IPAC2022-THPOST012

Beam Current [A]The continuous operation of the electron-positron injector
LINAC has supported the history of advanced research at
KEK by providing beams for various projects as shown in
Fig. 2.
Recent introduction of event-based PPM controls that enabled simultaneous top-up injections for multiple purposes
was most significant since the construction of the injector.
That demonstrates how the controls contribute to the performance of accelerators.
One of the example is shown in Fig. 5. Because of the severe operational condition in SuperKEKB the simultaneous
injections improved the collision performance (luminosity)
by 237%.

237% improvement

Figure 5: Comparison of integrated luminosity before andafter the introduction of simultaneous top-up injection. Redlines indicate stored beam currents at storage rings, andthe yellow line is the resultant luminosity that improved by
237%.

CONCLUSION
KEK injector LINAC continues multidisciplinary beam
delivery with simultaneous top-up injections to support bothphoton science and particle physics experiments. It often
carries administrative and operational negotiations to a successful conclusion to enable short-term and long-term optimizations and to enhance performances for both disciplines.

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