BUCKET SELECTION FOR THE SuperKEKB PHASE-3 OPERATION
H. Kaji∗ , KEK, Ibaraki, Japan
Abstract
Bucket Selection is one of the most important components
at SuperKEKB. It manages the timing of injector linac sothat we can select the RF-bucket at main ring to be injectedbeam-pulse. We upgrade the software after the successful
phase-1 operation. Bucket Selection manages the positron
injection with the newly constructed damping ring. The
system works smoothly and robustly in the entire phase-2
period. We discuss the future upgrade which is required forthe phase-3 operation.

INTRODUCTIONThe SuperKEKB accelerator is an electron-positron collider [1, 2] which is built at KEK. The 7 GeV of electronsand 4 GeV of positrons are stored into two main rings (MRs)called HER (High Energy Ring) and LER (Low EnergyRing), respectively.
One of its challenges is the timing management of injector
linac (LINAC) [3]. The difference in the RF frequencies of2856 MHz (LINAC) and 508.89 MHz (MR) makes timing
management complicated. Besides, from phase-2, we must
consider also about the newly constructed damping ring(DR) with the RF frequency of 508.89 MHz.
The Bucket Selection system [4] works for the injection
management as well as the timing management. It decides
lots of operation parameters including the injection timingand provides them to the Event Timing System [5, 6].
The both hardware and software of Bucket Selection work
excellently in the phase-1 operation. However we need theupgrade of software to satisfy the new requirements forphase-2. The new functions must be developed for the DRoperation.
In this report, we explain the general specification ofBucket Selection in the phase-2 operation. Then we describe
the upgrade plan for phase-3.

BUCKET SELECTION
In this section, we introduce the overview of Bucket Selection. Then we describe the logic for the LER injection in
the phase-2 operation [4] since it is quite different from thatin phase-1.

Overview
Bucket Selection controls the LINAC operation timing sothat the beam pulse is coincided with the injection RF-bucketat the injection point. The LINAC timing is changed inevery pulse since we change the injection RF-bucket in every
injection. The hardware of Bucket Selection is described inRef [4].
∗ hiroshi.kaji@kek.jp

The operation phases of LINAC and MR are not always
coincide since their RF frequencies are different (2856 MHzand 508.9 MHz, respectively). The coincidence happens inevery 96.3 ns. It is the common frequency of 10.385 MHz.
In the HER injection case, we adjust the LINAC timing in
range of 0-493 µs. The one of RF-buckets at HER can be selected with this timing range. Note, the harmonic number ofSuperKEKB is 5120 and one cycle of coincidence conditionis 96.3 × 5120 = 492922 ns.
This logic is utilized for both LER and HER in theSuperKEKB phase-1 and the entire KEKB era.

Bucket Selection for LER in Phase-2
The logic to determine the LINAC timing for the LER injection becomes complicated from phase-2 since we operate
DR. Note, the positron-pulses are once stored into DR forradiation damping. Then, they are extracted from DR andtransferred to LER. In this case, we must consider the RF
frequencies and the harmonic numbers of both DR and LER
in the new Bucket Selection logic.
There is no degradation of Bucket Selection in terms ofthe RF frequency. The RF frequency is 508.89 MHz at DR.
Therefore the common frequency among LINAC, DR, andLER is 10.385 MHz.
The problem is in the harmonic number. The harmonic
numbers for DR and LER are 230 and 5120, respectively.
There are 23 kinds of coincidence combinations of RFbuckets. The LINAC timing range to take care of all Bucket
Selection condition becomes longer since we must consider
all combinations of RF-buckets between DR and LER.
The one cycle of coincidence conditions is 96.3 × 5120 ×23 = 11339336 ns (= 11.34 ms). It is longer than the timing
range of 0-2 ms which is allowed from the LINAC hardware.
Therefore, it is impossible to realize all combinations between DR and LER. On the other hand, in every pulse, there
are 4 DR-buckets which can deliver the positron pulse into
the requested LER-bucket.
Bucket Selection choose one of 4 allowed DR-buckets anddetermine the LINAC timing in the phase-2 operation [6].
It is no problem when we operate the only one pulse at DR.

Performance on Phase-2
Here we introduce the performance of Bucket Selection inthe phase-2 operation. The several kinds of MR operations
with different filling patterns including the luminosity run
are carried out in this period.
Figure 1 shows the bunch currents at LER in the operations
with typical two filling patterns. We perform the luminosity
run with 789 bunches. The number of operation bunches
becomes larger and to be 1567 bunches when we performthe vacuum scrubbing with the large beam current.

Figure 2: Schematic view of allowed RF-buckets at DR in
the two pulse operation: it indicates in the case when thetwo bunches spacing 49 RF-buckets are stored. The twopositron bunches in the one LINAC pulse occupy in this way.
The next positrons can be injected into the 80 RF-buckets
in the opposite region. This means only 31 RF-buckets canbe selected as the first bunch of next positron-pulse and thisbecomes the constraint of Bucket Selection.

Figure 1: Bunch Currents at LER: the results from bunch current monitor at LER are shown. The upper and lower plotsshow the results in the 789 and 1576 bunches operations,respectively. The former is the typical filling pattern for theluminosity run. The later pattern is utilized for the vacuum
scrubbing in both the phase-1 and phase-2 operations.
Bucket Selection works excellently and realizes the various requirements of filling patterns in compliance with thebeam commissioning. Even though the injection scheme for
LER becomes complicated, the injections are quite smoothand robust in the entire phase-2 period. Besides the bunch
currents are properly equalized.

UPGRADE TOWARDS PHASE-3
In the phase-3 operation, the beam current at MR will
be gradually increased to enlarge the luminosity. The designed beam currents are 3.6 A and 2.6 A for electrons andpositrons, respectively. The upgrade of Bucket Selection isnecessary since the more frequent injections are required.
In this section, we explain the requirements, issues, andupgrade plan of Bucket Selection for phase-3.

Requirements for Phase-3
In phase-3, the beam life time with the large beam current
operation is estimated to be ∼5 minutes at LER. We must
perform the LER injection in > 25 Hz.
On the other hand, there is the other requirement to theDR storage time (damping time). The minimum DR storage

time of 40 ms is required for positrons to guarantee enoughradiation damping.
The storage time becomes shorter than 40 ms with the >25 Hz injections when we carried out the one pulse operation
like phase-2. The damping time is determined as the intervalof the LER injections in the one pulse operation.
In phase-3, the two pulses operation at DR will be performed when the LER injection rate become larger than25 Hz. The positron-pulses are injected and extracted withthe fast-in-fast-out rule. The damping time becomes twiceof the injection interval. We can realize the 40 ms damping
in the 50 Hz LER injection.

Constraint to Two Pulse Operation
There is the constraint when we select the DR-bucket in
the two pulse operation. It is demonstrated in Fig. 2. Bucket
Selection can select only 31 DR-buckets when the previous
positron pulse has already occupied some of DR-buckets.
We cannot inject the new pulse into those DR-buckets. Besides, the ±50 buckets from them must be vacated sincethose are the timing for the rising and falling edge of kicker
magnets.
There is the other constraint in the selection of DR-buckets.
Only 4 of 23 combinations can be realized in the 2 ms rangeof LINAC timing adjustment, as we described in the previoussection.
As the result of above two constraints, Bucket Selection
is deteriorated hardly. We cannot choose more than half ofLER-buckets in every pulse. The bunch current equalizationis quite difficult in this situation.

RF phase shifting at LINAC
We plan to modulate the RF phase in the LINAC section
between DR and LER (2nd LINAC). By controlling the RFphase in pulse-by-pulse, we can realize the coincidence ofoperational phases among LINAC, DR and LER in more 11times [4]. The entire time range to select all combinations
between DR-buckets and LER-buckets becomes shorter than

2 ms which is allowed in terms of the LINAC hardware. Thepossible RF phase is listed on Table 1.
Bucket Selection needs to provide the RF phase at 2nd
LINAC as well as the operation timing. The RF phase isdelivered on the data buffer transfer of Event Timing System
while the operation timing is instructed on the timing ofEvent delivery.
One of concerns in this new scheme is the timing difference between the trigger timing from Event Timing Systemand the RF phase. They are also summarized on Table 1.
In principle, the trigger timing also is coincided with RFphases at LINAC, DR, and LER on the normal operation
timing. However, in new scheme, we modulate the RF phaseat 2nd LINAC to make different kinds of coincidences withthose at DR and LER. The coincidence with the clock of
Event Timing System is broken in those cases. The timingof trigger is slightly detuned with respect to the beam pulse
and the RF phase. It becomes at most 900 ps as shown in
Table 1.
If some hardware at 2nd LINAC does not accept above
difference, we must assign the fine delay output channel ofEvent Receiver (EVR). This channel has 20 times finer delay
function based on the GTX technology. However, number offine delay channels are limited. The replacement of EVR canbe considered if necessary. The EVR developed at SINAP[7] has fine delay functions on all 8 output channels.
We must distinguish the LINAC components which require the precise timing trigger and check number of suchkinds of components. We develop and install the software
to control the fine delay function on EVR.
Table 1: The normal and additional RF phase at 2nd LINAC:
the additional phases are realized by modulating 2856 MHzRF phase with listed values. It makes the synchronization
with the different RF-buckets at LER. On the other hand, thesynchronization with the clock of Event Timing System isbroken. It makes the timing difference between beam andtrigger.
RF Phase (degree)

Timing Difference (ps)±0+161.6+183.7+345.3+7.3+169.0+191.0+352.7+14.7+176.3+198.4±0−893+179−715+357−536+536−357+715−179+893

CONCLUSION
The Bucket Selection manages the injection timing at theSuperKEKB collider. Its hardware and software work excellently in the phase-1 operation. The software is upgraded tosatisfy the new requirement in the phase-2 operation.
The upgraded Bucket Selection realizes the LER injection
with DR. It is operated smoothly and robustly in the entire
phase-2 period.
The two pulse operation at DR is planned in phase-3. It
needs further upgrade of Bucket Selection. The RF phasemodulation is necessary and it also is managed by Bucket
Selection.
There is the concern about the trigger timing. We must
consider it before the phase-3 operation.

REFERENCES[1] Y. Ohnishi et al., “Accelerator Design at SuperKEKB”, Prog.
Theor. Exp. Phys., vol. 2013, no. 3, p. 03A011, 2013.
[2] K. Abe et al., “Letter of Intent for KEK Super B Factory”,
KEK Report 20014-4.
[3] M. Akemoto et al., “The KEKB Injector Linac”, Prog. Theor.
Exp. Phys., vol. 2013, no. 3, p. 03A002, 2013.
[4] H. Kaji et al., “Bucket Selection System for SuperKEKB”, inProc. of 12th Annual Meeting of PASJ, Fukui, Japan.
[5] H. Kaji et al.. “Injection Control System for the SuperKEKB
Phase-1 Operation”, in Proc. of 13th Annual Meeting of PASJ,Chiba, Japan.
[6] H. Kaji et al., “Injection Control System for the SuperKEKB
Phase-2 Operation”, in Proc. of 15th Annual Meeting of PASJ,Nagaoka, Japan.
[7] H. Kaji et al., “New Timing System for Damping Ring at
SuperKEKB”, in Proc. of ICALEPCS2015, Melbourne, Australia, 2015.