Bucket Selection of KEKB and tits Possible Reconstruction for SuperKEKB
Eiji Kikutani, Kazuro Furukaw, Makoto Tobiyama,High Energy Accelerator Research Organization, Oho 1-1, Tsukuba, Ibaraki , Japan
Abstract
Last year, the KEKB bucket selection system, which
generates appropriate bunch-ﬁlling patterns in the KEKBrings, was reconstructed to accomplish the simultaneous
injection to the three rings, the low-energy and the highenergy rings of KEKB, and the PF ring. In this paper, we
describe the main point of the reconstruction. In addition,
we discuss the possible extension to this system to the SuperKEKB rings.

ning in this IOC under Vx-Works, and is determines which
bucket should be ﬁlled in the next shot of LINAC beam andset the delay value into the delay modules. The programruns synchronizing with injection, i.e., the gun-trigger signal is accepted by Vx-Works interrupt mechanism.
The decision, which bucket should be ﬁlled, is done
based on the following information:
1. the ﬁlling pattern which the operator gave to the system,
2. bunch current information of whole ring,

INTRODUCTION
In storage rings for colliding experiments or lightsources, often a very large number of bunches are stored
to obtain high luminosity or high-brilliance photon beams.
One example of these cases is the KEKB collider at KEK
in Japan. It is an energy-asymmetric double-ring collider
for B-physics. KEKB consists of an 8 GeV electron ring(HER), a 3.5 GeV positron ring (LER) with an injector
linac (LINAC) which provides these rings with electronand positron beams.
It is expected that coupled bunch instabilities will occur
in rings, when a large number of bunches are stored withvery short spacings. In principle, strengths of these instabilities can be evaluated based on an estimated impedanceof the rings. However, from practical point of view, thegrowth rate of the instabilities varies depending on conditions, even with the same total beam-current. One of theimportant conditions which determines the strength is a ﬁlling pattern, that is, how the RF buckets are occupied bybeam bunches.
We designed and constructed a system for realizing any
desired ﬁlling pattern in the HER and LER, as a part of theKEKB timing system. Essentially, the central hardware ofthe system is a variable delay which manipulates the trigger
timing the electron gun of the injector. The delay time isdetermined by an intelligence implemented in the system.
We call this system ”Bucket Selection System”[1].

ARCHITECTURE OF THE BUCKETSELECTION SYSTEM
As explained above, the bucket selection system is a subsystem of the KEKB-LINAC timing system. Like all thecomponents of KEKB, this system is working under theEPICS environment. One VME Input/Output controller(IOC) is assigned to this function. We call this IOC ”Delay IOC”. In this subrack, VME digital programable delay
modules (named TD4V) are installed and they determine
the rigger-timing in LINAC. A software program is run-3. the bucket address last ﬁlled.
Brief explanations for ﬁrst two items of information will begiven below. The ﬁll pattern information is loaded in thesystem prior to the operation. The loading is done by anoperator manually and the man-machine communication isorganized by an IOC named OP-IOC. Bunch current information is used for equalizing bunch-charge per bunch andit is delivered by the Bunch Current Monitor IOC (BCMIOC). This IOC controls a bunch-by-bunch current monitor, which is made of a wide-band ﬂash ADC (509MHz,8-bit) and a series of memory. This a by-product of theKEKB bunch-by-bunch feedback systems and closely described in reference[2]. With this monitor, digitized bunchcurrent information of each bunch is obtained in one revolution time (10µs).
To summarize, three IOCs, namely Delay IOC, OP-IOC
and BCM-IOC are taking part in this system. They are connected with ﬁber cables which are independent of standardTCP/IP communication system. The speed of this networking system is 250 Mbit/s. A fast and stable transfer of thebunch current information from BCM IOC to Delay IOC
is essential. The connection of these IOCs is schematicallydepicted in Fig. 1.

A BRIEF HISTORY OF THE KEKBINJECTION SCHEME
Traditional operation and continuous injection
At earlier stage of the operation of KEKB, beam-current
plots was like a saw-tooth, that is, experiment run with decaying currents and at the moment when the currents went
down to some critical values, the beam was added. During
the injection, data taking was stopped to avoid the damageof detector components by injected beam particles. Since
2004, the injection scheme was converted to ”continuous
injection scheme1 , in which the detector continued datataking except just a few milliseconds after the injection.
1 At PEP-II, they call this scheme the ”trickle injection”

Delay IOC
from Event Systemdelaymodules

are injected to several rings without any pause due tochanges of energy, beam species etc.
After many efforts[4], in 2008, we succeeded to start thesimultaneous injection.

to linac gun

BCM IOC• LINAC gun is ﬁred every 20 milli seconds (i.e. 50Hz)
shared memory optical cable

OP IOCcontrol network

bunch current monitors

Figure 1: Connection of the IOCs. The delay IOC plays
the central role in the bucket selection. The BCM IOC
sends the bunch-current information to Delay IOC through”Shared Memory optical cable.

• each shot of the LINAC beam is delivered to one of
the rings listed above• the event system[3] supervises which operation modeis selected

RECONSTRUCTION IN THE BUCKETSELECTION SYSTEM
For the realization of the simultaneous injection, a triggering scheme of the bucket selection system was changed.
Before the start of the simultaneous injection, electron injection and positron injection were done in two different
operation modes of LINAC. It means, although the common resource was used for the both beams, the injection ofeach beam were independent operation, conceptually.
Figure 2 shows a simpliﬁed ﬂow-chart of the software.
It runs in synchronizing with the beam injection timing.
When one action ended it stops to run and waits for thenext injection. By the semTake function provided by VxWorks, the software restarts the action.
initialization

This scheme enabled us to increase the integrated luminosity per day. In addition, decreased dynamic range of thecurrents enabled us to tune the machine easily.

”Simultaneous” injection
However, functions of LINAC is not only providing theKEKB rings, but also providing PF ring and PF-AR, which
are light-source rings at KEK. Table 1 summarises the requirements of beams, beam species, energy, intensity[3]from each ring. Naturally, during the injection to PF andPF-AR, the KEKB rings were not able to inject beams andcurrents went down for a few to ten minutes.
Table 1: The beams which required to deliver to the rings.
The positrons are produced by bombarding a target withhigh-intensity electron beam.
op. mode particle
energycharge # of bunchs
HERe−
8.0 GeV 1.2 nC
1 or 2LERe+
3.5 GeV 0.6 nC
2LER∗e−
4.0 GeV 10 nC
2PFe−
3.0 GeV 0.1nC
1PF-ARe−
3.0 GeV 0.1nC
1In order to compromise the requirements of these rings,
we pursued the possibility of ”simultaneous” injection
sechme. By simultaneous injection we mean that the beams

semTakeinput bunch-current
data of every bucket

decide which bucket tobe filled and calculatesdelay time

Figure 2: Simpliﬁed ﬂow chart of the software. The program run in synchronizing with the beam injection and it isdone with the Vx-Works real-time functions.
In the old system, the interrupt signal was delivered by
the bunch-current monitor, which measures the charge ineach bucket just before the injection. As shown in Fig. 1,
the Delay IOC and BCM IOC was connected with a highspeed optical cables, and the interrupt signal was transferedthrough this cable. Two bunch current monitors are prepared for the two rings and each of them are motivated
to measure changes by the trigger signal of the injection
kicker in each ring.
After the reconstruction, the interrupt signal is deliveredby the Event System. This system sends the trigger signal to the bucket selection system, and additionally it provides the information showing in which mode LINAC will
be operated, just 20 milli seconds before the actual injection. The bucket selection system read this information andif it is ”HER” or ”LER”, the bucket selection action is done.
For this purpose, new interrupt access port was prepared. (ared module in Delay subrack in Fig. 1.)
In parallel with the change of the hardware, the ﬂow of
the bucket selector was changed. Before the change, theﬂow was branched when the particle species (i.e. triggersource) was detected.

from the bucket last ﬁlled. It will avoid a pile-up of the betatron oscillation due to the injection. The second method
is to ﬁll the beam into the bucket with the least bunchcharge. This method can regulate the bunch charges among
the all occupied buckets. We call this function Bunch Current Equalizer (BCE). Through experiences we learned that
this scheme is very powerful for practical operation.
At present, the program in Delay IOC, which is coded in
the C-language, is executing the BCE task. In SuperKEKB,
the Event System will be used and the intelligence implemented in the system can do this task. In this case, thebunch current monitor should be connected to the Event
System more tightly than the present case. This should not
be straightforward and if the ﬂexibility with the C-language
program can be preserved or not is not clear at present. We
will continue discussing what architecture is hopeful forthe new bucket selection system.

SUMMARYPERSPECTIVE TO SUPERKEKB
Fill-pattern editing
Through the ten-years history of KEKB operation a large
number of ﬁll patterns have been tried and some of themwere used for actual operation for physics experiments.
Since the start of the operation in 1998, the number ofbunches has been increased step by step, to get higher luminosities. On the other hand, from a view point of studiesof accelerator physics, other types of ﬁll patterns have been
used, some of them were similar to that for physics experiments and others were not.
The bucket selection system provides operators with afunction of generating a standard ”bunch-train pattern”, inwhich operators can specify, (1) the number of trains, (2)the number of bunches in a train, (3) bunch spacing in atrain. The system can generate also an arbitrary ﬁll pattern.
In this case, operator should specify on or off for each RFbucket. In KEKB rings there are 5120 RF buckets and a5120-line ASCII ﬁles (we call it ”pattern ﬁle”) is used toinform the system of the ﬁll pattern. Operators can edit thepattern ﬁle with standard editors like Emacs. It is sufﬁcientenough for present operation, but in future a more convenient GUI might be necessary. It can manipulate standard
ﬁlling patten components, such as the abort gap, or the pilotbunches.

A Dynamic decision-making based on bunchcharge information
The mission of the bucket selection system is to realize
the ﬁll pattern speciﬁed by an operator. This means thatthe system is informed of only static information. Then, in
what order the buckets are fed by bunches from the injector
should be decided by the system itself.
In the present system the order is decided in two different
methods. One is to alter the target bucket with some algorithm, with which a bunch will be injected to a bucket far

The operation scheme of LINAC was upgraded to realize
the ”simultaneous” injection, in which KEKB HER, LER
and the PF ring are fed continuously. For the timing manipulation, the Event System was introduced for this purpose.
The bucket selection system was reconstructed to meet therequirement of this new injection scheme. In future SuperKEKB, the timing system will be based on this EventSystem. We will continue the design works for the newbucket selection system for SuperKEKB.

REFERENCES[1] E. Kikutani et al.,, “Bucket Selection System of the KEKBRings”, Proc. APAC98, Tsukuba, July 1998, p. ***.
[2] M. Tobiyama and E. Kikutani, ”Development of a high-speed
digital signal precess system for bunch-by-bunch feedbacksystems”, Phys. Rev. STAB 3, 012801, (2000).
[3] K. Furukawa et al., Proc. IPAC’10, Kyoto, May 23-26, p.
1271.
[4] N. Iida et al., ”Pulse to pulse switching injection to three ringsof diffeenret eneries form a single electron linac at KEK”,Proc. PAC09, .