THE FAULT ANALYSIS OF TIMING SYSTEM IN SuperKEKB
D. Wang1,2∗ , K. Furukawa2 , M. Satoh2 , H. Kaji2 , H. Sugimura2
1 SOKENDAI, Kanagawa 240-0193, Japan
2 KEK, Ibaraki 305-0801, Japan
Abstract
The high availability of an accelerator timing system isof great significance for the operation of the SuperKEKB
electron-positron collider. To distribute the high precisionlevel trigger signals for synchronizing all the relevant components in the accelerator complex, event based timing system
is utilized to control the injection procedure in SuperKEKB.
Another critical challenge of the timing system is to control the bucket selection of both electron and positron beam
bunch. The failure in timing system would definitely affectother systems like pulsed magnet, BPM and so on, and as aconsequence leading an unfavourable impact on the effectiveness of the particles collide. Additionally, a newly built
positron damping ring decrease the positron emittance but
meanwhile brings higher complexity of the bucket selection.
In this paper, we will demonstrate the algorithm of the timing system and introduce our fault analysis system whichhelps up to analyze the timing error, improve the accelerator
operation stability.

quirements, there are some restrictions on how the injection
mode can be sequenced.
During the operation, several beam abortion originatingfrom timing system are observed. Based on the further
comprehensive analysis of the timing system, the beam modeis summarized and the stabilizing method is proposed.

TIMING SYSTEM FOR SuperKEKB
The task of the timing system is to synchronize all theaccelerator devices and components like gun, pulsed magnets and BPM in a relatively sequence. A typical injection
procedure works like the Fig. 1. The EPICS IOC at KEKB
Control Room first reads the bunch current in the main ring(MR), calculates the delay value for bucket selection andthen transfers the delay to the IOC locates at Linac Control Room by optic fiber. This delay then is applied at theelectron gun to generator the beam.

INTRODUCTION
The 600-meter long injector linac at KEK provides beams
for 4 different rings. Two synchrotron light source facilities,
2.5-GeV PF and 6.5-Gev PF-AR, and one high luminosity
electron positron collider SuperKEKB with a high energy
ring (HER) and a low energy ring (LER) share the injector
linac. The energy of electron and positron beam is 7.0 GeV
and 4 GeV respectively [1]. The phase-to-phase modulation(PPM) is utilized to achieve the requirement. The timing
system controls the filling process of different beam by distributing control signals to all over the accelerator and any
faults during the filling process will require dumping thebeam [2]. With respect to the pursuit of world’s highest
luminosity, the beam dumping should keep away from beam
dumping as much as possible. The stability of the timing
system is of great significance to the operation.
The bucket selection process is necessary to inject thebeam into its dedicated ring properly. One 1.1 GeV positron
damping ring (DR) is newly constructed to reduce the beamemittance which inevitably increase the complexity of the
LER bucket selection decision. The 135.5-meter long DR
locates at the middle of the linac and separates linac as
upstream linac and downstream linac. To achieve a good
damping effect, the positron beam should cycle inside the
DR at least 40 ms. Apart from that, the beam gate system
is embedded in the timing system to control the injectionand extraction mode at DR as well as avoiding unnecessary
beam firing at electron gun. To meet the some hardware re∗ sdcswd@post.kek.jp

Figure 1: Over of the timing system at SuperKEKB.

Pulse-to-Pulse Modulation
The concept of PPM which originates from CERN is tomodulate beam properties to improve the efficiency of thelinac when different beams are needed [3]. At SuperKEKB,
under 50 Hz beam repetition rate hundreds of accelerator
component parameters will switch swiftly according to theinjection beam mode. As the Fig.2 shows, with the changeof the injection mode, the timing system generates different
event codes pulse by pulse.

The Hardware and Software
In addition to the functionality of PPM, the basic role ofa timing system is to deliver the timing signal by dedicatedtiming network. At SuperKEKB, the event based timing
system is deployed at the VME crate with several modules.
The Event Generator (EVG) and Event Receiver (EVR) provided by the MRF company are mainly used to generate anddistribute the event codes to the accelerator devices [4]. The
VME-based time-to-digital converter (TDC) module with asuper-period becomes much longer which is 96.3 × 5120
× 23= 11.34 ms.
Table 1: Key Parameters of SuperKEKB Timing
Parameter
Linac RF frequency
MR RF frequency
MR revolution frequency
MR harmonic number
DR RF frequency
DR revolution frequency
DR harmonic number

Value2856 MHz508.9 MHz
99.39 kHz
5120
508.9 MHz
2.21 MHz
230

Figure 2: Pulse-to-Pulse Modulation.
resolution of 1 nanosecond developed at KEK is used to monitor the timing [5]. Furthermore, the VME-5565 Reflective
Memory (RM) module provides a high-speed, low latency
data sharing among several timing station nodes. The bucket
selection delay value and other necessary information forbeam injection and extraction which are calculated at themain ring is transmitted to the linac timing station throughthe RM.
The EVG receives a RF signal as the event clock and a
trigger signal to send the event codes which are saved at the
hardware sequencer memory. The event clock at linac is114.24 MHz.

Figure 3: The synchronization between DR and LER. The
positron beam is injected into the DR first and cycle for atleast 40 ms. The bunch extracts from DR later and injects
to the LER bucket.

Beam Gate
The beam gate system is an important part of the timing
system which is able to switch the beam mode and stop
the beam delivery if necessary [6]. This system is of greatsignificance during the operation by controlling the injection/extraction hardware and electron gun. Besides, thetop-up injection and beam study also rely on the beam gate
to control the hardware.
The beam gate information is delivered via event timing
network with the help of EVG’s Distributed Bus (Dbus)
function. The Dbus accepts 8 TTL inputs and transmits thedata via event timing network to all downstream EVRs.

The relationship between the injection opportunities andthe RF bucket in DR and MR can be calculated though
the Eq. (1) and Eq. (2), respectively. By means of theseequations, the relationship between the bucket selection candidates and the delay time are listed at Table 2

Bucket Selection

However, some restrictions exist during the real operation.
Firstly, the injection point should close to the AC powerline (AC50) because of the klystron hardware requirement.
Moreover, only 2 ms fluctuation near the AC50 is the allowed for a stable accelerating. For LER injection, the beamstorage time in DR makes it impossible to finish the injection
in 20 ms. Another restriction which comes from the DR
kicker magnets will reduce the injection opportunities when
performing the two-bunches-one-pulse injection.

The beam should be injected into the proper RF bucket tomake sure the bunch always sees an accelerating voltage at
the RF cavities. As a consequence, the injection delay timemust be calculated in advance.
The bunch synchronization relationship between Linac
and MR should be considered since their RF frequencies
are different. According to the Table 1, the coincidence
frequency between them is 10.385 MHz. Every 96.3 ns (49RF buckets) the injection opportunity reoccurs to fill thebunch to the MR RF bucket. We define the "super-period"as the least timing of injecting to all the RF buckets in a ring.
Thus, the super-period for HER injection is 96.3 × 5120 =493 𝜇s. For LER, since there are 230 RF buckets at DR, the𝐵𝐾𝑇𝐷𝑅 = 𝐼 𝑁 𝐽𝑡𝑢𝑟 𝑛 × 49

(mod 230)(1)

𝐵𝐾𝑇𝑀 𝑅 = 𝐼 𝑁 𝐽𝑡𝑢𝑟 𝑛 × 49

(mod 5120)(2)

CONSTRAINTS

AC50 Synchronization
To inject into the LER, the bucket selection delay should
be calculated based on the damping period. Since the solution appears every MR super-period (493 𝜇s), the one whichTable 2: Bucket Selection Calculation
Injection TurnDelay

DR Bucket

MR Bucket

0
1
2
3
...
230
...
5120
...
20,771
20,772...
117,760

0 ns
96.3 ns
192.6 ns288.9 ns...
22.1 𝜇s...
492.9 𝜇s...
1.99 ms
2 ms...
11.34 ms

0
49
98
147
...
0
...
180
...
29
78
...
0

0
49
98
147
...
1030
...
0
...
4019
4068
...
0

is nearest to the AC50 is selected. Within the 2 ms range,
only 4 of 23 MR buckets combinations can be realized. Take
Fig.4 as an example. The DR injection happens at pulse
number 1 and extraction happens at pulse number 3. The
delay value for extraction should be buffered and sent out at
two pulses later. Both delay values are based on the AC50.
The AC50 can be read from the TDC module at every pulse.

Figure 5: Schematic view of allowed RF buckets at DR in
the two bunches injection.
MR bucket in every pulse, a possible solution is to modulate
the RF phase in the Linac between DR and LER. By shifting
the Linac RF phase pulse by pulse, the coincidence opportunities increase and much more MR buckets are available.
Now the RF phase shift is still under experiment.

Figure 6: If DR bucket number 0 and 49 are already occupied,
only bucket 100 to 130 are available for the new bunches.
This constraint also decrease the available buckets at LER.

8/9 Pulses Sequence Shift

Figure 4: Bucket Selection calculation of LER.

Damping Ring Kicker
Owing to the 100-ns kicker magnets field rising time,
if the distance between newly injected bunches and stored
bunches is smaller than 100 ns, the stored bunches would be
affected by the magnetic field. Under this circumstance, only
31 DR buckets can be selected. This process is demonstrated
in Fig.5.

RF Phase Shift
Considering both restrictions, only 2887 MR buckets canbe selected during the two bunches LER injection. Figure 6
shows the allowed buckets at DR and LER. To select all the

The LER super-period is 11.34 ms which can not coincide with AC50 every pulse. To overcome this, a method
called 8/9 pulses sequence shift is utilized. As shown in
Fig.7, inside the 8/9 pulses sequence the LER delay value
can be acquired based on the fiducial point and current pulse
number. After 8 pulses sequence, next sequence can synchronize with super-period again if next pulse starts 1.25
ms early. Similarly, the next sequence is launched 1.4 ms
later than usual 20-ms period [7]. By means of this method,
every pulse is able to coincide with the super-period. The
determination of next sequence length is based on the AC50
value. The AC50 should come at the middle of the 20 ms
pulse to keep a stable operation.

FAILURE MODE ANALYSISFor continuous injection, the injection sequence includes
many beam pulses. The operator can change the injection
sequence based on the operation situation. The sequencevaries from 2 (40 ms) to 500 (10 s). Owing to the bufferedbucket selection of DR extraction, the bunch stored at DR
might not be extracted if the buffered delay value is lost

SUMMARY

Figure 7: Timing relation between 8 pulses injection, 9pulses injection and LER super-period.
during the sequence shift. To avoid this situation, currently
the beam gate must be closed during the sequence shift by
the operator. But this process is not robustness enough.
Another failure is caused by the AC50 drift. Since theinjection needs to synchronize with AC50 every pulse, normally the 8/9 pulses sequence shift is able to handle theslightly drift and always keeps the AC50 comes at the center of every pulse. However, sometimes the AC50 drifts
severely and the program processing time might be limited
if the AC50 comes too late [8].

Timing System Upgrade Plan
To solve these failures, we plan to refactor the bucket
selection software and add critical data monitoring part todiagnose and eliminate the error as the Fig.8 shows. Considering the high VME CPU load, a new EPICS IOC will
be added to monitor these data shared by reflective memory.
The original event codes are important to help us identifying the failure source. A high-speed event codes logging
system has already been developed. Furthermore, many critical data monitoring and aggregation need to be done. The
commissioning will be performed on the end of 2020.

With the upgrade of the timing system at the SuperKEKB,
both hardware and software has encountered several failures.
One of the reason is the high complexity of the whole system.
To diagnose and improve the stability and maintainabilityof the timing system, the structure and algorithm need to be
fully analyzed. Several requirements are also demonstrated.
Then, the failure modes based on the event code log and data
analysis are briefly explained.
After the distinction of different failure modes, several
solutions are proposed which include dedicated monitoring
IOC, high-speed event code logging and bucket selection
program refactoring.

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Figure 8: The failure mode monitoring and analysis systemof the Event Timing System.

[8] D. Wang et al., “The Fault Diagnosis of Event Timing System in SuperKEKB”, presented at the 17th Int. Conf. on Accelerator and Large Experimental Physics Control Systems(ICALEPCS’19), New York, NY, USA, Oct. 2019, paper
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