THE TIMING SYSTEM OF KEKB 8-GEV LINAC
K. Furukawa∗ , T. Suwada, N. Kamikubota, M. SatohHigh Energy Accelerator Research Organization (KEK),Tsukuba, Ibaraki, 305-0801, JapanAbstract
The timing system of the KEK electron linac has been
restructured for the KEKB project since a higher precision
was required compared with the previous project. It provides precise timing signals for more than 100 accelerator
devices such as guns, rf sources, beam instrumentation, etc.
along the 600-m linac. The signals can be synchronized toone of four rings, KEKB HER/LER, PF and PF-AR. The
clock system consists of five synchronized frequencies todrive different rf systems. The main clock of the timing
system is 571.2 MHz, which is distributed through a coaxial cable overlapped with timing pulses. The delayed signals for over 100 devices are generated at 15 timing stations along the linac. Gate pulses are also distributed to enable intermittent measurement, etc. Timing signals are controlled through VME and CAMAC with the linac standard
control architecture. The delay step of the timing signals is1.75 ns and the precision is better than 5 ps depending on
the location. It enabled stable long-term operation of thelinac and also the recent “two bunch in a pulse” operationwith its precise controls.

12 TIMING SYSTEM ARCHITECTURE
The linac is sub-divided into eight sectors and the total
length is about 600m. Each sector is a basic unit of an
rf power distribution system. The timing signal is generated at the main timing station synchronizing with several
timing sources. It is distributed with a basic clock to 15
remote timing stations, and then delayed timing signals areprovided for accelerator devices.

2.1

Basic Clock

In the previous project TRISTAN, the rf frequencies ofring and linac are asynchronous. However, in the KEKB
project rf frequencies need to have an integer relation as inTable 1 since a single-bunch beam with a small jitter haveto be injected.

INTRODUCTION

The KEK electron/positron linac injects beams into fourstorage rings; 3.5-GeV positron to the low energy ring and8-GeV electron to the high energy ring of KEKB, 3-GeV
electron to PF-AR and 2.5-GeV electron to PF ring [1]. The
beam for KEKB injection is a S-band single bunch with thebunch length of less than 10 ps.
It is required to provide stable and high-quality beams so
as to achieve optimal experiment results. Trigger timings
for accelerator equipment are important to maintain stable
operation, since they directly affect the beam quality andenergy through the single-bunch operation with the rf compression systems (SLED). Timing precision of the beams
should be at least better than 10 ps inside of the linac. For
KEKB injections it should be better than 30 ps (rms), since
their synchrotron motion should be suppressed as small as
possible and they have to pass through the long and winding transport lines [2].
Because of such higher timing precision requirement,
the development of the integrated trigger-timing signal distribution and its control system had become a key issue ofthe KEKB project. Thus, it was re-designed with more than
10-times better precision compared with the previous system [3]. The reliability of the system was considered mostimportant in the design. And it should support the basis of∗<the stable operation of the linac though all of the electron
source, acceleration system and monitoring system. Timing signals are provided for two sets of electron guns, 59 rf
acceleration units, a pulsed positron focusing coil, injection
kickers, rf and beam instrumentation systems.

kazuro.furukawa @ kek.jp >Table 1: rf frequencies of KEKB linac
PurposeFundamental
Linac SHB1
Linac SHB2
Linac Main
KEKB Ring

Ratio−×11×55×275
×49

Frequency
10.38546 MHz
114.24 MHz
571.2 MHz
2856 MHz508.8873 MHz

These rf sources are generated by frequency multiplying
and dividing, and are distributed to rf systems [4]. The
frequencies are adjusted by 0.1-Hz steps in 10.385 MHz inorder to correct the beam orbit in the ring corresponding to
the change of the circumference of KEKB rings.

2.2Beam Timing Signal

The maximum beam repetition rate is 50 Hz, which islimited by modulators of high-power rf systems. The beamtiming is generated by synchronizing to both the revolution
frequency of each ring and the power line frequency for thenoise elimination. Then a delay may be added to select abucket in the ring. For KEKB the timing is chosen to selectone of 5120 buckets with synchronizing to the fundamental clock, 10.385 MHz [5]. Figure 1 shows the simplified
timing chart.

50Hz
0 ~ 0.5ms

Table 2: Timing signal transmission and generation

Bucket Selection

StationBeam Station

Location
NumberClock

Electron Gun
1
TD4R

Delay
Field Bus
Purpose

TD4R
RS232C
Beam

2856MHz571.2MHz114.24MHz
508.88MHz
Gun Grid Pulser
Master Trigger
Ring Kicker/Septum

Primary
SectorStationSub-booster
9
TriggerReceiver
TD4
CAMAC
Low-level rf
Beam Monitor

Secondary
SectorStationControl Room
5
From Primary
Station
TD4V
VME
High Power
SupplyFigure 1: Simplified relation between clock and timing signals.
ules are called TD4V, TD4 and TD4R depending on thecontrol interfaces of VME, CAMAC and RS232C.
The 16-bit ECL counter in the module can delay a signal up to 114 µs with a timing jitter about 3 ps (rms).
Since each accelerator device requires different timing, delay modules are installed separately, and the number ofmodules is about 160.

2.5

Figure 2: 50-Hz mono-cycle pulse (above) and the combined 571.2-MHz clock and 50-Hz timing signals transmitted to remote stations.

2.3

Distribution of Clock and Timing Signal

Since many of the timing signals require high precisionand they are synchronized to 571.2 MHz in the linac, it isdecided to distribute the timing signal with the clock signal.
The clock is used to generate delays appropriate for thecorresponding purposes and to re-synchronize the timing
for the precision.
A 50-Hz mono-cycle pulse with a width of 1 ns andthe 571.2-MHz clock of about 36 dBm are combined ina trigger transmitter module at the main timing station asin Fig. 2. It is transmitted on a single 30-D coaxial cablewith low loss and good phase stability. Then it is split withdirectional couplers at remote sector stations, where timing
and clock signals are regenerated separately with trigger
receiver modules.

2.4

Generation of Delayed Timing Signals

Various delayed timing signals are generated dependingon the purposes at the remote timing stations . They aregenerated by counting the 571.2-MHz clock starting fromthe beam-timing signal with delay modules. Those mod-Pulsed rf Modulation

Timing system is used both for low-level microwave
generation and high-voltage pulse generation in the rf system. Timing signals for low-level rf are generated at theprimary sector stations located at the sub-booster klystrons,
and define the rf pulse envelope and the phase-flip timing
for rf compressors (SLED). Pulsed rf from a sub-booster
klystron is distributed to high-power klystrons (up to 8) inside the sector. While SLED timing is important to maintain stable high-power rf, the system provides sufficient accuracy. The low-level rf timing is optimized to acquire thehighest beam energy and to equalize the beam energies of
two bunches in a pulse [7].
Timing signals for high-voltage pulses are generated at
the secondary sector stations located at the sub-control
rooms. They are delayed separately and transmitted tohigh-power klystron modulators.
There are 59 high-power klystrons in the linac and someof them are put into stand-by mode depending on the operation mode. Two low-level pulse envelopes are generated. One of them synchronized to the beam and another is
57 µs apart which corresponds to the stand-by mode. Oneof them is selected with the high-voltage pulse timing.

2.6

Beam and rf Instrumentations

Timing signals for beam and rf instrumentations are generated at the primary sector stations almost the same way
as for rf systems. However, beam repetition is sometimes
lower than 50 Hz. Thus, four kinds of gate signals are generated at the main timing station, and are distributed using
twisted-pair cables to select the appropriate beam pulses
depending on the devices. The TD4 module is equippedwith a inhibit input, with which one of the gate signals isconnected to inhibit the output signal.
Delayed timing signals from TD4 are transmitted to 30
rf measurement stations and 18 beam measurement stations which covers streak cameras, beam position monitors,
timing-shutter cameras, wire scanners as well as injection
septa and kickers.

3CONTROL SOFTWARE

The control of the timing system is a part of the integrated linac control system. It is based on the multi-layerarchitecture, in which lower layer servers support different
pieces of hardware, and upper layer servers are designed toprovide hardware-independent services to the application
layer [6]. The timing control is named as “trig” and upper layer server is called “rtrigd”. Rtrigd serves various
timing related functions to the application layer throughTCP-based remote procedure calls (RPC), while it communicates with several different lower-layer servers, such as
for CAMAC-based TD4 and VME-based TD4V, via UDPbased RPC.
The common application software is used for the routine operation and for the optimization of beams, which includes a beam mode switch panel, a parameter save-restore
panel, active and passive correlation plots, beam and device feedback systems. Device specific application panels
are also used by operators. They all provide the basis of thestable linac operation. Furthermore they are essential to operate on the positron injection to KEKB with two bunches
in a pulse since the two-bunch beam is sensitive to its timing [7].

4with a result from beam simulation codes. Thus, the timing
jitters for the electron gun and the streak camera are considered to be much better than 9 ps. The stability of theresultant beam is satisfactory.

PERFORMANCE CONSIDERATION

Timing switch and adjustment are carried routinely fromoperation software such as automatic beam stabilizers, abeam mode switch panel, etc. They work sufficiently well.
The system is also utilized efficiently to support the twobunch injection mode, which was adopted recently.
During the commissioning of KEKB, it was found thatsome TD4 modules stopped the outputs for just 200 ms andthe event occured about once a month. Because of suchrare events, it took more than a year to ascribe the issue toa comparator circuit. After all of the comparators in TD4s
were replaced, we have never experienced the trouble.
An evaluation of the timing jitter of this system was carried using a 20-GHz sampling scope. The timing signals
regenerated at remote sector stations were transmitted backto the main timing station through a 20-D coaxial cable andcompared the original signal. The typical jitter value was
4 ps (rms) and the maximum measured jitter was 5 ps forthe farthest station. They were found to have sufficient precision.
Figure 3 shows the beam bunch structure measured bya streak camera. The measured bunch length was 9 ps(FWHM) for a 10-nC single-bunch beam, and it agrees wellFigure 3: Beam bunch structure measured by a streak camera. The bunch length is 9 ps for a 10-nC bunch.

5 CONCLUSIONSThe timing system of the KEKB linac has been designed
and operated stably and it performs well beyond its requirements. It also works well for the two-bunch acceleration ofthe linac, which was not planned originally.

6 ACKNOWLEDGMENTS
The authors are grateful to the previous group leader
Dr. T. Urano who originally designed and constructed mostpart of the system.

7REFERENCES[1] K. Furukawa et al., “Present Performance and Future Upgrade of the KEKB Electron Linac”, Proc. of LINAC2002,
Gyeongju, Korea, 2002, p.382.
[2] “KEKB B-Factory Design Report”, KEK Report 95-7, 1995,
Chap.12.
[3] T. Urano and K. Nakahara, Nucl. Instr. and Meth. A247,1986, p226.
[4] H. Hanaki et al., “Low-Power rf Systems for the KEKB Injector Linac”, Proc. APAC’98, Tsukuba, Japan, 1998, p.139.
[5] E. Kikutani et al., “The KEKB Bucket Selection System”,
Proc. APAC2001, Beijing, China, 2001, p.669.
[6] K. Furukawa et al., “Accelerator Controls in KEKB Linac
Commissioning”, Proc. of ICALEPCS’99, Trieste, Italy,1999, p.98.
[7] K. Furukawa et al., “Beam Feedback and BPM Read-out Systems for the Two-bunch Acceleration at the KEKB Linac”,
Proc. ICALEPCS2001, San Jose, USA., 2001, p.266.