BEAM FEEDBACK SYSTEM CHALLENGESAT SuperKEKB INJECTOR LINAC
K. Furukawa∗ , R. Ichimiya, M. Iwasaki, H. Kaji, F. Miyahara,
T.T. Nakamura, M. Satoh, T. Suwada, KEK, Tsukuba, 305-0801, Japan
Abstract
The SuperKEKB electron/positron asymmetric collider,currently under construction, is designed to elucidate newphysics beyond the standard model of elementary particle
physics. This goal will only be achieved by precise measurements with a luminosity that is 40 times as high as thatof the KEKB. The injector linac is to be upgraded to enable a beam size of 50 nm at the collision point, which is
20 times smaller than that of the KEKB, and a doubling of
the stored beam current with a short lifetime of 10 min. At
the same time, two light-source rings, the PF and PF-AR,
will be filled in top-up injection mode. To this end, the linac
will need to be operated with precise beam controls. Duallayer controls with EPICS and MRF event systems are being enhanced to support precise pulse-to-pulse beam modulation (PPM) at 50 Hz. A virtual accelerator (VA) concept
is introduced here to enable a single linac to be modeled
as four VAs switched by PPM, where each VA corresponds
to one of the four top-up injections into the storage rings.
Each VA is associated with independent beam orbit and energy feedback loops to maintain the required beam qualities. The requirements of the SuperKEKB HER and LER
for the beam emittance, energy spread, and charge are especially challenging.

INTRODUCTION
During a decade of successful operation, the data obtained from the KEKB asymmetric electron/positron collider has provided important insight into the flavor structure of elementary particles [1]. The KEKB for B-physics
is being upgraded towards SuperKEKB to achieve a target
luminosity of 8 × 1035 cm−2 s−1 that is 40 times higher
than the current luminosity in an effort to elucidate newphysics beyond the standard model of elementary particle
physics. The injector linac will be upgraded to enable abeam size of 50 nm at the collision point and a doublingof the stored beam currents of the 3.6-A positrons in the 4GeV low-energy ring (LER) and the 2.6-A electrons in the
7-GeV high-energy ring (HER), both with short expected
lifetimes of 10 min [2]. The linac will require full-energy
injection with an energy spread of 0.1%, beam emittancesof 20 and 10 mm·mrad at the end of the linac, and electron and positron bunch charges of 5 and 4 nC, respectively. Two bunches in a pulse are expected at a pulse rateof 50 Hz.
Low-emittance, high-current electrons will be deliveredby employing a photocathode RF gun, and high-current
positrons will be generated using a flux concentrator (FC)∗ < kazuro.furukawa @ kek.jp >and large-aperture accelerating structures, before being
damped to a low emittance through a damping ring (DR).
The injector will inject electrons into two light-source
rings: the Photon Factory (PF) and the PF Advanced Ring
(PF-AR). All four storage rings of the SuperKEKB (theHER, LER, PF and PF-AR) will be filled in top-up injection mode (Fig. 1) using pulse-to-pulse modulation (PPM)
at 50 Hz, effectively allowing the injector to perform virtually simultaneous injections [3].
A number of beam orbit and energy feedback loops were
installed to maintain the beam stability for KEKB injections, particularly when the stability of the machine was
not well understood [4]. For SuperKEKB injections, these
feedback systems will need to be improved to meet theadditional requirements of low-emittance, high-intensity
beams and virtually simultaneous top-up injections into thefour rings. We describe an upgrade plan for these feedback
systems here by introducing the concept of virtual accelerators (VAs).

PULSE-TO-PULSE MODULATIONCONTROL
The control system at KEKB is based on the Experimental Physics and Industrial Control System (EPICS) [5] andscripting languages. EPICS realizes the abstraction of theaccelerator equipment layer, and the scripting languages,
including SADscript [6], successfully achieve the integration of innovative ideas into the accelerator operation [7].
Following advances and developments in hardware devices
and EPICS, the “channel access (CA) everywhere” concept
was adopted to enable new subsystem deployment.
Later in the KEKB project, for a higher experimental
performance and light sources that share the same injector,
it became favorable to inject beams in top-up mode into all
the storage rings. In the PF, a stable stored beam current
is necessary for precise experimental results, and for the
KEKB, stability was also desired for sensitive beam collision tuning to increase the luminosity. To that end, simultaneous top-up injection was established for three storagerings (HER, LER, and PF) in 2009.

Figure 1: Layout of the SuperKEKB injector linac andbeam delivery system to the four storage rings of the experimental facilities.

Figure 3: Dual-tier controls with EPICS CA at the top andfast event synchronized control at the bottom.
Figure 2: Overall configuration of the event-based control
system at the injector linac. A total of 18 event-receiver
stations are spread across the 1-km-diameter facility.

Global and fast controls were then established for pulseto-pulse beam modulation (PPM), or virtual accelerators(VAs) [8]. The control system, based on 10-year-old hardware and conventional EPICS software, was inadequate forcontrolling the beam within 20 ms, and so a new control
system with an event notification mechanism that was capable of regulating ∼150 parameters at 50 Hz was installed.
This event-based control system managed the low-level RF,
high-power RF, pulsed magnets, electron gun, injection
systems, and beam instrumentation, that were spread overa distance of 1 km. While the event-based control system
was supervised by EPICS software, it had a dedicated communication link for fast, global, and robust control [9].
The event generator sent timing signals and control
data to 18 event-receiver stations arranged in a star-like
topology (Fig. 2). Each link between the event generator and a receiver consisted of a single optical fiber andprovided both synchronized timing signals (with a precision of approximately 10 ps) and synchronized control
through a realtime software mechanism (with a precisionof about 10 µs). Recent technological advances in fieldprogrammable gate arrays (FPGAs) and small form-factor
pluggable (SFP) transceivers have enabled reliable control
in this configuration.
VME-based event control modules in the generator(EVG230) and the receiver (EVR230RF) from MRF [10]were utilized in the system. The event generator provides
several events per pulse depending on the injection ring,and device controls are synchronized to the linac RF clock.
The same “dual-tier” control system with the conventional EPICS and event-based control, depicted in Fig. 3,
will also be essential for the SuperKEKB. Simultaneous
injection will be maintained, as the beam lifetime will be
more limited at the SuperKEKB HER and LER. Several
more event-receiver stations will be installed to cover additional pulsed accelerator equipment and beam instrumentation. Event generators and receivers from SINAP are also
being evaluated, especially for the DR integration [11]. A
number of other parameters will have to be managed precisely in a pulse-to-pulse manner to realize lower emittancebeams for a higher luminosity [12].
The event-based control tier manages global and fast

controls over a pico- to microsecond range, while the
EPICS control tier covers slower parameter controls for the
event-based controls, as well as existing conventional controls. The EPICS tier arbitrates operation requests of average beam-repetition rates from the three rings, and schedules different beams pulse by pulse.1 Such requests occur
every several seconds, and the beam mode schedule is reprogrammed at the event-control tier through the EPICS
CA upon each request. This dual-tier control system isalso an optimal configuration for next-generation accelerator systems.

EMITTANCE PRESERVATION
Low-emittance, high-current electron and positron
beams will be generated by upgrading the linac with facilities such as an RF gun, an FC, and a DR. If the accelerator equipment is aligned perfectly and the machine is stable, then beams are injected to the rings without emittancedegradation. Based on beam dynamics simulations with anadvanced bunch compressor, it is found that the local andglobal alignment tolerances are 0.1 and 0.3 mm, respectively, depending on the bunch length. However, alignmentof the 600-m linac itself is a challenge, and furthermore, theGreat East Japan Earthquake in 2011 disturbed the alignment by several millimeters.
If the machine is not aligned well, then the beam iskicked transversely by the focusing magnets and passes
through the accelerating structures at an off-center position, inducing a transverse wakefield. This wakefield thenkicks the tail of the beam bunch causing it to form a banana
shape, which generates projected emittance blow-up [13].
If the distortion is small and non-linear effects are ignored,
then we should be able to determine an initial beam orbitthat will cancel the distortions in both the horizontal andvertical planes [14].
It is important to adopt this emittance preservation mechanism along the linac, and we must also be prepared to handle beam orbit and other instabilities by employing feedback loops.
1 Under typical operating conditions, the average injection rates withKEKB were 25 Hz for the LER, 12.5 Hz for the HER, and 0.5 Hz for thePF, which were frequently changed to maintain the stored beam current
automatically.

VAs, the optimization process will need to be iterated.
Depending on the location in the injector linac, the beamenergies in the PPM VAs are very different, which makes
the iteration optimization somewhat complicated. To simplify the process, we subdivide the linac into several segments as listed in Table 1.
Table 1: Possible segment energies (GeV)
Segment
HER LER PF PF-AR
Figure 4: Single injector linac modeled as four PPM VAs
managed by an event-based control system.

Gun to positron target
Positron target to DR
DR to the end of linac

3.5
4.6
7.0

3.5
1.1
4.0

3.5
4.6
2.5

3.5
4.6
6.5

VIRTUAL ACCELERATOR MODELS
There are two types of VAs that are both important tothis project. The first is a VA constructed virtually within acomputer simulation to represent a real accelerator. This“simulation VA” is often utilized to improve the performance by comparing beams between virtual and real accelerators or to design the accelerator before construction
is completed [15]. As EPICS has a switch to change process variables to simulation mode, several institutes employ
such a mechanism.
The second VA type is one that corresponds to PPM
beam modes. Each of the “PPM VAs” maintains independent variable parameters, and machine operators treat those
PPM VAs as separate machines that exist in a single realmachine. For the injector linac, we may need to consider agroup of PPM VAs consisting of (Fig. 4)• a 7-GeV electron injector for the SuperKEKB HER,
• a 4-GeV positron injector for the SuperKEKB LER,
• a 2.5-GeV electron injector for the PF, and• a 6.5-GeV electron injector for the PF-AR.
Positrons are kept in the DR for more than 40 ms to reduce the beam emittance, and the same beam spans a temporal distance of more than two pulses. Most of the beamcharacteristics are lost in the DR, but the beam charge andtiming against MR are maintained. Furthermore, a single
pulse may contain dual bunches separated by 96 ns, andthese two bunches should be handled separately because
the second bunch is affected by the wakefield of the firstbunch.
While all the PPM VAs share about 1000 common parameters, each PPM VA maintains an independent set ofaround 200 synchronous parameters. Common parameters
correspond to equipment that does not vary from pulse topulse. As the synchronous parameters can change in PPM,
each parameter can have multiple values (currently up toten), and one set of these values are associated with one
PPM VA. Both the common and independent parameters
will need to be optimized to achieve the beam propertiesof the respective PPM VA. Thus, a simulation VA is required for each PPM VA. Since our present simulation environment based on SAD/SADscript is not designed to handle such common parameter optimization between several

BEAM EMITTANCE CONTROL
Depending on the alignment result and the stability of
the injector, the management strategy of the beam emittance may vary. Initially, we will only be able to monitor
the beam characteristics as much as our resources will allow. We are in the process of enhancing the synchronizedbeam instrumentation such as the strip-line beam position
monitors (BPMs) [16], wire scanners for the beam emittance [17], and a beam deflector for single-shot sliced emittance [3], as well as equipment monitors [18].
Simple beam stabilization feedback loops will first be
installed for the beam orbit and energy with a proportionalintegral (PI) control as in KEKB, where monitors are BPMs
and actuators are steering magnets or low-level RF system. Pairs of point-to-point orbit stabilizers of 90-degreeapart in the phase space, as well as energy stabilizers withenergy-spread compensation will be employed [19]. We
have found that these instruments are indispensable in locating the origin of the instability and for continuing beamstudies under unusual beam conditions.
As noted, a certain beam orbit can be employed to cancelout the beam bunch distortions introduced by the wakefield.
This orbit can be determined by scanning the initial pointin the beam phase space and applying a specialized optimization feedback algorithm such as the downhill simplex
to maintain the orbit. However, it is only after the beam has
been commissioned that we are able to confirm how the orbit may be affected by the ground/girder alignment stability
and equipment stability and what time constants these may
have. We thus may need to apply different feedback techniques for the stabilization.
Each PPM VA will be associated with several feedback
loops that will be applied to a set of process variables that
correspond to the respective PPM VA. The process variable
sets and feedback loops of each PPM VA are independent,
as shown in Fig. 5.
If the time constant of the beam drift is sufficiently slow,
then the feedback application programs will continue tobe based on scripting languages, possibly accompanying
a graphical user interface including the SADscript environment, which can solve linear (and nonlinear) beam optics
online. This scheme is preferable for rapid development.

autumn 2014 and winter 2015 for the injector and ring, respectively.

REFERENCES

Figure 5: Each PPM VA would be associated with several
beam feedback loops independent of the other PPM VAs.
This figure indicates only the minimal beam energy feedback locations.
However, we will need to consider alternatives for routine
operations depending on how often the beam optics change.
If the time constant is fast up to (but not including) pulse
to pulse, then we can initially employ the EPICS ePID
record. The beam optics parameters in the associated simulation VA should be calculated by a relevant helper application, and the feedback parameters should be tuned throughCA. Eventually, we would need multi-monitor to multiactuator loops, and a dedicated custom record may be necessary. A surveillance system of these feedback loops isalso necessary for a routine robust operation.
In parallel to the feedback loop implementation, the associated simulation VAs would be improved by correcting the beam monitors and actuators. Such improvements
should be shared between processes online.

CONCLUSION AND FUTURE
The advanced SuperKEKB design necessitates demanding requirements for the SuperKEKB injector linac. To
achieve the expected beam parameters, PPM and emittancepreservation controls were discussed. Two types of VAs,
PPM VAs and simulation VAs, were introduced, and an implementation process for beam stabilization feedback loops
were proposed based on these VAs. We note that the stabilization process may be affected by the device stability
and the quality of alignment. The design and performanceof the temporal manipulation of a laser pulse and the beam
bunch compressors will also be important. Thus, we willneed to provide a flexible environment in which to construct the beam operation system, and such an environment
should be adapted to handle multiple VAs.
The commissioning of the injector will start in autumn
2013 at a limited power. While the mechanism that wastested in the KEKB project will be applied to the new instrumentation and equipment [19], we will need to determine the best direction for implementing the stabilization.
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