ANALYSIS OF AC LINE FLUCTUATION FOR TIMING SYSTEM AT KEK
D. Wang∗ , K. Furukawa, M. Satoh, H. Kaji, H. Sugimura, Y. Enomoto, F. Miyahara, KEK, JapanAbstract
The timing system controls the injection procedure of theaccelerator by performing signal synchronization and trigger
delivery to the devices all over the installations at KEK. The
trigger signals is usually generated at the same phase of an
AC power line to reduce the unwanted variation of the beam
quality. This requirement originates from the power supply
systems. However, the AC line synchronization conflictswith the bucket selection process of SuperKEKB low energy
ring (LER) which stores the positron beam. The positron
beam is firstly injected into a damping ring (DR) to lower
the emittance before entering desired RF bucket in LER. A
long bucket selection cycle for DR and LER makes it difficult to coincide with AC line every injection pulse. This
trouble is solved by grouping several injection pulses into
various of injection sequences and manipulating the lengthof sequences to adjust the AC line arrival timing. Therefore,
the timing system is sensitive to drastically AC line fluctuation. The failure of timing system caused by strong AC line
fluctuation and solutions are introduced in this work.

INTRODUCTION
The electron/positron collider, SuperKEKB, is upgradedfrom the KEKB project since 2010 at KEK, whose goal is
to update the world highest luminosity record and discover
new particle physics by Belle II experiment [1]. The timingsystem at the 700-m long injector linear accelerator (LINAC)
is responsible for the injection of all accelerator complex
which consist of a 7 GeV electron high energy ring (HER),
a 4 GeV positron low energy ring (LER), a 2.5 GeV Photon
Factory (PF) and a 6.5 GeV PF-AR ring (see Fig. 1). During
the phase-2 operation in 2018, a 1.1 GeV positron Damping
Ring (DR) is constructed at the middle of LINAC to lower
the positron beam emittance [2].
e-LERSuperKEKB

HER

7 GeV

LER

4 GeVDR

1.1 GeVPF-AR

6.5 GeVPF

2.5 GeVmodes are defined to perform the pulse-to-pulse modulation.
The master trigger signal of the timing system comes fromthe AC line to follow the fluctuation of power line and keep
the beam energy stable. The bucket selection is implemented
by adding delays to the master trigger signal. However, along bucket selection cycle which caused by the DR makes
it difficult to coincide with the AC line. A scheme called
sequence shift is developed to synchronize with the 50 Hz
AC line [3]. With the growth of the system complexity,
several failures of timing system are observed and analyzed
based on the fault diagnosis system [4].
In this paper, the reason of such failures are identified andsome efforts to improve the reliability and stability of timing
system are introduced.

BUCKET SELECTION FOR LER
The primary task for bucket selection is to provide theability to select an arbitrary RF bucket in the ring for a single
injection pulse. This can be achieved by adding a properdelay time to the gun triggering signal after defining thefiducial bucket in the ring. As the stable phase of RF cavity
coincides based on the common frequency (CF) between
LINAC and two main rings (MRs), the injection is performed
based on the period of the CF [5]. After ℎ times injection, all
RF buckets are filled. The period during which all ring RFbuckets can be filled is defined as a bucket selection cycle(BSC) and the period of BSC can be represented as
𝑇BSC = ℎMR ∗ 𝑇CF(1)

where ℎ is the harmonic number (i.e., the number of RFbuckets), 𝑇𝐶𝐹 is the period of common frequency between
LINAC and MR.
According to the requirements of RF synchronization, several significant frequencies for timing system can be calculated in Table 1. Note that the BSC for DR only is practicallyuseful when performing DR-injection-only mode for beam
study.
Table 1: Bucket Selection Frequencies at KEK LINAC

e+

PF-AR

Frequency

HER
PF e+ Damping Ring

Figure 1: Overview of LINAC, SuperKEKB, and PF/PFAR.
The event-based timing system at LINAC is required toswitch the beam properties at 50 Hz by changing the eventcodes and additional control data. Totally 12 kinds of beam∗ sdcswd@post.kek.jp

PeriodRemarks

2856 MHz350 ps
RF frequency for LINAC
508.89 MHz 1.97 ns RF frequency for DR & LER
114.24 MHz 8.75 ns
Event clock
2.21 MHz452 nsDR revolution frequency
99.39 kHz 10.06 µs
LER revolution frequency
45.15 kHz 22.15 µs
BSC for DR only
2.03 kHz493 µs
BSC for LER only
88.19 Hz11.34 ms
BSC for DR and LER
50 Hz
20 ms
Beam repetition rate

The harmonic number ℎMR at two MRs are 5120, and theinjection opportunity arises at the common frequency of10.385 MHz between LINAC and the SuperKEKB MR (i.e.,every 96.3 ns). According to the Table 1, all RF buckets canbe selected within 493 µs, and the bucket selection process
can be easily accomplished every 20 ms. Nevertheless, if
DR is considered, the complexity of the bucket selection
system grows. The harmonic number at DR is 230 and theleast common multiple between the DR and LER harmonic
numbers is 117760; thus, the BSC for DR and LER becomes
11.34 ms (i.e., 88 Hz).

AC LINE SYNCHRONIZATION
It is usually necessary to maintain the beam intensity andquality on the same level for consecutive triggers. Some
devices are sensitive to the AC line phase and might notsatisfy the stability requirement when working at different
AC line phases. The klystron is a good illustration of this.
Some klystrons use the AC-powered filament or heater toheat the electron cathode, and the changing AC phase mightgenerate a magnetic field that eventually affects the velocity
modulation process of the klystron. The frequency of the ACline is not ideal 50 Hz. The Tokyo Electric Power Company,
which supplies AC power for KEK, adjusts the frequency of
the AC power line within 50±0.2 Hz to balance the supply
and demand requirement of the electricity market [6]. Thetiming system is required to follow the drift of AC50.
For HER injection, the BSC is 493 µs and the gun timing
can be calculated based on the coincidence between AC50
and BSC. However, for LER injection with DR, the BSC islonger and to be 11.34 ms, so it cannot coincide with AC50
every 20 ms. Therefore, a method called sequence shift isutilized to deliver the triggers at the same phase of AC50,
while it is also synchronized with the BSC.

SEQUENCE SHIFTA set of injection pulses are grouped as an injection sequence. By shifting the length of the sequence, the synchronization between BSC and AC line becomes possible.

8/9 Pulses Sequence Shift
For HER injection, the BSC is 493 µs and the gun timing
can be calculated based on the coincidence between AC50
and BSC every 20 ms. However, for the positron injection
process, the bunch is first injected from LINAC into DR
and then extracted from DR into LINAC after a pre-definedstorage time. Hence, the BSC for the combination of DR
and MR is 22.68 ms; therefore, synchronization with AC50
at every pulse is impossible. Consequently, an 8/9-pulse
injection sequence was developed.
As shown in Fig. 2, the core ideology of the 8/9-pulse
sequence is to define the 50 Hz fiducial point and record theAC50 arrival time and BSC timing. Inside the 8/9-pulse
injection sequence, the bucket selection delay value for thepositron can be acquired based on the fiducial point and current pulse number. After an 8-pulse sequence (i.e., 160 ms),Figure 2: By shifting the injection sequence, the BSC for DR
and MR injection can synchronize with the 50-Hz AC50.
the next sequence can synchronize with the BSC again if thenext pulse starts 1.25 ms early. Similarly, launching the next
pulse 1.4 ms after 9 pulses (180 ms) also meets the synchronization requirement (see Fig. 3). Using this method, thefiducial value for the BSC in every pulse can be calculatedusing the pulse number, and it is possible to operate theinjection into DR and the extraction from DR at differentpulses. The determination of the next sequence length isbased on the AC50 value, which is measured by TDC forevery pulse.

Figure 3: AC50 position in a pulse is manipulated by thesequence shift.

16/18 Pulses Sequence Shift
The DR operation is carried in 2018. To achieve a satisfactory damping effect, a DR storage time of at least 40 msis required. There also exists a restriction of the maximum
storage time in the DR. The injection and extraction delay
are calculated together, and both follow the AC line phaseat the injection pulse. Thus, the extraction timing trigger
has a larger discrepancy at the downstream LINAC for along storage time. The difference between the extraction
timing and AC50 arrival timing at extraction pulse becomeslarger when the storage time increases. Consequently, themaximum storage time is chosen to be 200 ms, while theminimum storage time is 40 ms. To supply the requirementof the maximum DR storage time (200 ms), the sequencelength is doubled to apply the 16/18-pulse sequence shift in
2018, and the sequence shift values are enlarged to 2.5 and2.8 ms [2].

Sequence Shift Algorithm
The algorithm of the sequence shift is based on the AC50
timing of the previous pulse and uses it as an estimation
the AC50 value in the near future. A reference value is also
required to act as a comparison point. Obviously, there are
only two sequence types, i.e., 16-pulse sequence and 18pulse sequence. Therefore, the AC50 shift value, i.e., 2.5

and -2.8, can be used to represent the sequence types 16 and18, respectively. Because the program operation requires
some time, the sequence shift decision should be done at
the start of the current sequence. It is then easy to calculate
the next sequence type based on the current sequence type
and the AC50 timing of the starting pulse. The detail of thesequence shift algorithm can be referred from [7].

robustness of the sequence shift algorithm. Switching to the8/9-pulse injection sequence is a possible method to solve
the timing failure because the AC50 adjustment speed isfaster.

TIMING SYSTEM FAILURES
The timing system, which satisfies the requirements tooperate the DR for the positron injection, was installed andcommissioned in 2018 [2]. As the growth of the systemcomplexity comes from the bucket selection and 16/18-pulse
sequence shift, several failure modes of the timing system
are observed. An event code log system, which monitorsand saves all event codes received in EVR, is developed
to diagnose the failure modes in 2019 [4]. The severity ofsome timing system failures is minimal because the trigger
signals for devices are masked and inhibited by beam gatesystem [8] when some abnormal events are transmitted. On
the other hand, a timing system failure that stops the deliveryof event codes for a few minutes is severe for the physics run
because it usually triggers the beam abort system to dump
all the beams at SuperKEKB. It normally takes 10 minutes
for the operator to check the status of the devices and restart
the injection. The occurrence of such kind of failure is rare(i.e., 9 times) in 2020. However, the number of failuresincreases to 18 times in a month since the 2021 spring, andthe beam operation is frequently interrupted. Thus, it isurgent and significant to understand the failure cause andstabilize the operation. By analyzing the event code log
system and auxiliary timing data, such malfunction of thetiming system is caused by strong AC50 drift.

Figure 5: Comparison of AC50 arrival timing between 16/18pulse sequence and 8/9-pulse sequence.
The restriction of the sequence length originates from theDR storage time and the root reason is the coupling of DR
injection and extraction calculation. Shifting the RF phaseat downstream LINAC is an efficient way to increase the DR
extraction opportunity and relieve the stress of long sequence
management. The DR extraction delay is calculated basedon the shifted RF phase at downstream LINAC. Under thesecircumstances, the dependency between DR and MR injections can be removed. The operation of 8/9-pulse sequence
shift is possible as the DR injection and extraction delay
calculations can be separated. More descriptions about RFphase shifting can be found in [9].
Apart from the 8/9-pulse sequence shift, the sequence
shift algorithm can be further improved by considering theAC50 drift value. Currently, the estimated AC50 arrival time
is used to determine the next sequence type. If strong AC50
drift happens, the estimated value has a large deviation fromthe real value. The timing system could enter race conditions
when an inappropriate sequence type is selected. To avoid
such a situation, the sequence shift algorithm is modified
and the average AC50 drift value in recent pulses is utilized
to help the sequence shift algorithm to estimate the AC50arrival time. The new estimation becomes closer to the realvalue even when AC50 fluctuates strongly.

STABILIZATION OF TIMING SYSTEM

According to our simulation, the new algorithm can handle an AC50 drift ranging from −156 µs to 158 µs. Figure 6 shows a timing system failure which is caused by thestrongest AC50 drift situation that we have recorded during
the past two years. The data was recorded by TDC in 2019.
The AC50 drift value reached 120 µs and lasted for several
seconds. Figure 7 compares three sequence shift schemes,
16/18-pulse sequence shift which is currently using, 8/9pulse sequence shift, and 8/9 pulse sequence shift with thefunctionality of using the past AC drift value. The results
show that the new algorithm can handle the most significant
AC50 drift situation.

As Fig. 5 shows, compared with the 16/18-pulse injection
sequence, the 8/9-pulse injection sequence increases theThe 8/9-pulse sequence operation with AC50 drift compensation has been planned from the 2021 summer.

Figure 4: If AC50 arrives too early or too late in the 20 mspulse, the logic of timing system may fail
As Fig. 4 shows, every pulse the timing system needs sometime to process the data. If AC50 drifts continuously, theAC50 arrival time may comes too early or too late. Therefore,
the timing system logic may fail.

system with a long bucket selection cycle can be referred tofor the design of future circular accelerators.

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Figure 6: Extremely strong AC50 drift recorded by TDC.

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Figure 7: Comparison of different sequence shift schemes.

SUMMARY
The timing system failures that happened at LINAC frequently interrupt the operation of SuperKEKB. We analyzed
the algorithm of the injection sequence shift, the timing system failures caused by strong AC50 fluctuation are solved after upgrading the sequence shift by applying short sequencelength and performing a reliable AC50 estimation. The simulation results show that the new method can handle themost severe AC drift situation we have ever met in the last
two years. The analysis procedure and solution for a timing[7] D. Wang et al., “Analysis and stabilization of AC line synchronized timing system for superKEKB,” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,
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