UPGRADE OF EVENT TIMING SYSTEM AT SUPERKEKB
H. Kaji∗ , K. Furukawa, M. Iwasaki, E. Kikutani, T. Kobayashi,
F. Miyahara, T.T. Nakamura, M. Suetake, M. Tobiyama, KEK, JapanT. Okazaki, East Japan Institute of Technology Co., Ltd, JapanT. Kudo, S. Kusano, Mitsubishi Electric System & Service Co., Ltd, Japan
Abstract

Table 1: Beams provided by Linac

SuperKEKB is the upgrade of KEKB. The designed luminosity is 40 times larger than the KEKB achievement.
One of key items for this luminosity enhancement is twicelarger storage beam current. Injector Linac is required
more efficient injections although the injection control becomes complicated. The Event Timing System at Main
Trigger Station is upgraded. We perform the basic study forthe new configuration of Event Timing System and confirm
it satisfies the SuperKEKB requirement.

SUPERKEKBSuperKEKB[1, 2] is the electron-positron collider atKEK, Japan and will be started the commissioning in early2015. This is the upgrade of KEKB[3, 4] which achieved
the world’s largest luminosity for colliders[5]. The energy of electron (positron) beam is 7.0 GeV (4.0 GeV)
so that the center of mass energy is on the Υ(4S) resonance of 10.58 GeV. The designed luminosity is set to be
8 × 1035 cm−2 s−1 and 40 times larger than the KEKBachievement. This luminosity upgrade project is realized
by two parts. One is 20 times smaller vertical beta function
at the interaction point. The other is, related to this report,
twice larger storage beam current at both main rings (MRs).

INJECTION CONTROL
Injector Linac[6] plays more important roles in theSuperKEKB project. The beam current at two MRs is doubled and it must be kept with the top-up injection. Besides injection scheme becomes complicated for positrons
because of the newly constructed Damping Ring (DR). In
this section, Linac is explained briefly. Then the injection
controls at SuperKEKB are described.

Injector Linac
Linac provides beams into four rings, namely, KEKBHER (High Energy Ring), KEKB LER (Low Energy Ring),
PF[7], and PF-AR[8]. Positrons are injected into KEKBLER while electrons are injected into other three rings. Thebeams provided by Linac are summarized in Table 1.
There are new features at SuperKEKB. Positrons areonce stored into DR for at least 40 ms and injected into
KEKB LER. The top-up injection for four rings1 are per∗ hiroshi.kaji@kek.jp
1 Top-up injection for three rings (KEKB HER, KEKB LER, and PF)
has already been realized in the KEKB era.

Direction
KEKB HERKEKB LERPF
PF-AR

Particle

Eneragy

Charge−7.0 GeV
4.0 GeV
2.5 GeV
6.5 GeV

5.0 nC
4.0 nC
0.2 nC
5.0 nCee+e−e−formed after finishing the construction of new beam transport line for PF-AR.
Injection control is performed by Main Trigger Station2 ,which is described in the later section, and the star-type
optical network. The Event Timing System[9] is used inthis system.

Ring Selection
One of the injection controls is Ring Selection. To realize top-up injection into more than one ring simultaneously,
the injection ring must be changed frequently. However
Linac needs different operations for the individual injection rings.
Linac arbitrates the requests from four rings and schedules operations for a next couple of seconds. Injections
are performed with this schedule and the injection ring ischanged pulse-by-pulse. At SuperKEKB, more than 150of Linac parameters are changed every injection pulse, typically with 50Hz.
Injection scheme of positrons becomes complicated. It
must be processed in a part of long term schedule. Afew examples of schedules for the Linac operations areshown in Figure 1. The injection process of positrons at
SuperKEKB cannot be finished before the next beam pulse
is launched. The positrons are stored into DR for at least
40 ms and the storage time depends on the injection rate.
More than one process is implemented in parallel. Theelectrons are injected while the positrons are damping. Besides, in case of positrons, the first and second halves of
Linac work separately. The first (second) half works forinjecting positrons into DR (MR).
Ring Selection at SuperKEKB requires the upgrade ofMain Trigger Station. The new system should have the capability to control more than one process simultaneously.
The injections must be performed with knowing the longterm injection schedule for positrons.
2 We call it also Main Timing Station.

(a) KEKB e-:25Hz, e+:25Hz50Hz
Trigger㼑㻗㼑㻙20ms(b) SuperKEKB e-:25Hz, e+:25Hz50HzTrigger
㼑㻗㼑㻙Hэ'5H'5э05Hэ05H'DPSLQJ(c) SuperKEKB e-:40Hz, e+:10Hz50HzTrigger
㼑㻗㼑㻙Figure 1: Examples of injection schedule. In cases of 25 Hz(e− ) : 25 Hz (e+ ) at KEKB, 25 Hz : 25 Hz at SuperKEKB,
and 40 Hz : 10 Hz at SuperKEKB are shown. The brownand green arrows indicate electron and positron injections,
respectively.

Bucket Selection
The other important control is Bucket Selection[10].
This is implemented only when a beam is provided into
the KEKB MRs. There are 5120 RF-buckets in each MR.
The operator send a request to Linac about the fill pattern
into the RF-buckets. The Bucket Selection system controls
the injection RF-bucket to realize the requested fill pattern.
The injection RF-bucket is controlled with changing thedelay time of Linac operation. In the individual 20 ms periods, the Linac operation is synchronized with the reference
signal which is made from the revolution of injection ring.
In case of MRs, Linac is operated with adding delay timeto this reference signal. In this method, the injection RFbucket is determined with the delay time. For example, if
Linac is operated after 96.3 ns longer delay time, the beampulse is injected into the 96.3 ns later RF-bucket.
The delay time is set to be 0–493 µs with 96.3 ns step.
For considering the RF frequencies of Linac (2856 MHz)and MRs (508.9 MHz), an opportunity of injection comes
every 96.3ns. After 5120 opportunities, it takes 493 µs,
all RF-buckets have their own injection opportunities. The
493 µs cycle of reference signal is used at KEKB.
In case of the positron injection at SuperKEKB, the delay time for Bucket Selection becomes longer and to be 0–
11.34 ms with 96.3 ns step since the DR-bucket also mustbe considered. All of MR-buckets can be selected in the0–493 µs delay time. However, the DR-bucket coupledwith injection MR-bucket changes every 493 µs period. We
choose the harmonic number of 230 for DR so that there
are 23 kinds of combination between DR-bucket and MRbucket. Then, the delay time for selecting both DR-bucketand MR-bucket becomes at most 11.34 ms (= 493 µs×23).
Main Trigger Station needs the upgrade also for Bucket

Selection. All triggers must be synchronized with the reference signal for selecting the RF-bucket with the above
mentioned delay time method. At KEKB, the analog coincidence of injection trigger and reference signal is performed in 50 Hz. However, it becomes difficult since thecycle of reference signal is expanded to be 11.34 ms.
We choose the different approach to synchronize triggers
with the reference signal. The new configuration of Event
Timing System for SuperKEKB is developed.

EVENT TIMING SYSTEM AT
SUPERKEKB
We use the Event Timing System at Main Trigger Station. The timing trigger and the instruction for actionsin the next injection are delivered from this system to theLinac local devices. In this section, the Event Timing System and its new configuration at SuperKEKB is introduced.

Event Timing System
The Event Timing System is composed of optical network connections between an Event Generator (EVG) andan Event Receiver (EVR). The EVG sends 1 byte datanamed Event-Code. It is encoded with 8B10B encoding
so that the delivered Event-Code indicates precise timing.
One optical cable can be considered as 256 cables of triggerline since the Event-Code is distinguished into 256 types.
The EVR receives the Event-Code and activates the programmed action, such as the NIM/TTL signal production,on the Event-Code timing. The action can be programmed
for the individual Event-Code types and the CPU interruption also is possible.
At the SuperKEKB project, we use the MRF products, VME-EVG-230 and VME-EVR-230RF[9]. We haveknowledge and experience to operate these modules fromthe KEKB project. The EVG is installed at Main Trigger Station. The operation is synchronized with the114.24 MHz of RF clock at Linac. Therefore, the EventCode is delivered with the timing in units of 8.8 ns (=96.3 ns/11). The EVRs are installed with the local devices
along with the Linac beamline. They interrupt the CPU
to change the configuration of local devices. The preparation for the injections to the individual rings has been programmed at the local EVRs in advance.

Configuration at Main Trigger Station
Main Trigger Station is placed the vertex of optical network and connected with the local devices. The EVG delivers two kinds of Event-Code. One is used for the timing
trigger and the other is for the instruction to the local devices about the actions in the next pulse.
We upgrade the Event Timing System at Main Trigger Station to satisfy the complicated requirements ofSuperKEKB injection. Figure 2 is the new configuration.
The two-layers configuration of EVGs is developed.
The upper-layer EVG is operated with the long term sequence. This is triggered once a few second by the analog coincidence of injection triggers (20 ms period, 50 Hz)

Coincidence of 50Hz and 11.34ms cycle
TTL (<50Hz)

EVG

VME sub-rack50Hz pulse

Upper-layer EVG:

TTL
E
VGE
VR

TTL

EVGE
VR

TTLTrig

Oscilloscope

operate with a few seconds of sequence

Generate Events with 50Hz
as injection trigger

EVR
TTL (50Hz)

TTL (50Hz)EVG

EVG

Lower-layer EVGs:
operate with 50Hz
Generate Events with114.24MHzRF clock

Figure 3: Test setup. The two-layers EVGs configuration isproduced. The timing accuracy of output trigger is examined as a reference of RF clock. The transmission of Eventis shown by green arrow.

adding delay time for Bucket Selection
EVRs for

EVRs for1st half of Linac

2nd half of Linac

Figure 2: Configuration of Event Timing System at Main
Trigger Station. Two layers of EVGs are composed. The
upper-layer EVG is operated with a few seconds of sequence and generate Event-Codes in 50Hz. The lowerlayer EVGs are operated with the 50Hz trigger and generate
Event-Codes with adding delay time for Bucket Selection.
The transmission of Event-Code is shown by green arrow.
The signal lines for PF and PF-AR are omitted on this figure. Subsequently, they will be described in the separate
paper.
and the reference signal (11.34 ms). The injection schedule
is programmed on this EVG and the Event-Codes for timing trigger are delivered to the lower-level in 50 Hz. Thepositron injection via DR is possible since its longer injection process can be programmed on this EVG as a part oflong term schedule.
There are two lower-layer EVGs. The first and second
halves of Linac are separately controlled by them. They
are operated in 50 Hz and delivered Event-Codes to the local devices with adding the delay time for Bucket Selection. The calculation of delay time is possible since thetime relation between every input trigger and the reference
signal is precisely controlled by the upper-layer EVG.

is inserted in the same VME bus and controlled from thesame CPU as other Event modules for simpleness. We usethe sampling oscilloscope and the accuracy of timing measurement is better than 1 ps. The EVGs are synchronizedwith the 114.24 MHz of RF clock. The timing of same RF
clock is measured with triggering by the Event System.
Figure 4 is an example of measurements. The mean andstandard deviation of measurements are checked and they
are defined as trigger-timing and jitter, respectively.
The effect to the accuracy of trigger-timing is studied
when the upper-level EVG is operated with the long term
sequence. The results are shown in Figure 5. The upperlayer EVG is operated with different length of sequence
from 8.8 ns (one RF clock) to ∼2 s (2 × 108 RF clocks)
and generates an Event-Code at the end of sequence. Weconfirm the two-layers configuration works well. The trigger provided by this setup is accurate and its jitter is to bealways ∼10 ps. There is no significant difference in the resultant jitters when we change the length of sequence up to∼2 s.
We study also the long term stability. Figure 6 is the re-PERFORMANCE STUDY
The performance study for the new configuration ofEvent Timing System is carried out. Although we continue
to use the same model of Event Timing modules since theKEKB project, there is no experience for us about two key
elements in the new configuration. One is the two-layers
configuration of EVGs. The other is the EVG operation
with long term sequence, such as a few seconds, by one
input trigger. We study whether the new system satisfies
timing accuracy of O(100) ps, which is the requirement for
the timing trigger in the SuperKEKB injection.
The setup of performance test is shown in Figure 3. The
two-layers configuration of EVGs is produced. The mostdownstream EVR should be placed distantly with the Linac
local devices in the real operation. However, in this test, it

Figure 4: An example of timing measurement. The timing
on phase zero of RF clock is measured by the oscilloscope.
The mean and standard deviation is determined statisticallyafter collecting 1000 samples. They are determined as thetrigger-timing and jitter, respectively.

5.05

16

Trigger-timing (ns)Jitter (ps)

20300

5.04

120.88s
8.8µs

8.8ns

85.03

8.8ms

250

5.02

4200
5.01

01

10102

103

104

105

106

107108Length of sequence (clock)109

Figure 5: Results of the two-layer setup test. The lengthof sequence at the upper-layer EVG is shown in RF clock
unit. That at the lower-layer EVG is fixed to be 8.8 ns, one
RF clock, during the measurements.

5CONCLUSION
We upgrade the Event Timing System at Main Trigger
Station to satisfy the requirements of SuperKEKB injection. The positrons are injected via DR and it makes theentire operation of Injector Linac complicated. The twolayers of EVGs is configured at Main Trigger Station.

100

4.98
4.97

50
4.96
4.95
20.5

sults of five days of measurements. The measurements for
1 min-period are carried out continuously with monitoring
the room temperature. We clearly observe the timing dependence in temperature.
We determine the magnitude of drift of trigger-timingfrom the 2D histogram in Figure 7. It is evaluated from theslope of distribution to be 18 ps/degree. This is reasonably
small and no problem when we operate the Event Timing
System with keeping the temperature within ±1 degree by
the air conditioning.

150

4.99

2121.52222.523

23.5 24 24.5 25
Temperature (degree)0Figure 7: Timing vs temperature. The 2D plot made withresults in Figure 6. The slope is evaluated by fitting with
1D polynomial function to be −18.00 ± 0.16 ps/degree.
We perform the basic studies for the new configuration.
The accuracy of timing with the new configuration is determined to be 10 ps and there is no significant difference inthe length of sequence at the upper-layer EVG. The accuracy satisfies the requirement for the SuperKEKB injection.
We observed the drift of trigger-timing by the changeof room temperature. The magnitude is determined by thestudy to be 18 ps/degree. The influence on accuracy is reasonably small when we operate the Event Timing Systemwith air conditioning.
We conclude there is no problem in the new configuration of Event Timing System for SuperKEKB.

Trigger-timing (ns) Temperature(degree)

REFERENCES[1] Y. Ohnishi et al., “Accelerator design at SuperKEKB”,
Prog. Theor. Exp. Phys., 2013, 03A011.

28262422201816[2] K. Abe et al., “Letter of Intent for KEK Super B Factory”,
KEK Report 2004-4.
[3] K. Oide, “KEKB B-Factory”,
Prog. Theor. Phys. 122, 2009, pp.69-80.

Jitter (ps)

5.08
5.06
5.04
5.02
54.98
4.96
4.94
4.92

[4] S. Kurokawa et al., “Overview of the KEKB accelerators”,
Nucl. Instrum. Meth. A 499, 2003, pp.1-7.
[5] T. Abe et al., “Achievements of KEKB”,
Prog. Theor. Exp. Phys., 2013, 03A001.

141312111098766/13[6] M. Akemoto et al., “The KEKB injector linac”,
Prog. Theor. Exp. Phys., 2013, 03A002.
[7] http://www.kek.jp/ja/Facility/IMSS/PF/PFRing/.
[8] http://www.kek.jp/ja/Facility/IMSS/PF/PFAR/.
6/14

6/15

6/16

6/17

6/18

DateFigure 6: The five days of measurements during Jun13–Jun17 in 2013. The timing is measured every one minute
together with room temperature.

[9] http://www.mrf.fi/index.php/vme-products.
[10] E. Kikutani et al., “KEKB Bucket Selection System, Presentand Future”,
Proceedings of the 7th Annual Meeting of PASJ, August 4-6
2010, Himeji, Japan.