OPERATIONAL PERFORMANCE OF A NEW BEAM-CHARGE
INTERLOCK SYSTEM FOR RADIATION SAFETY AT THE KEKBINJECTOR LINAC
T. Suwada*, E. Kadokura, M. Satoh, K. Furukawa, KEK, Tsukuba, Ibaraki 305-0801, Japan
Abstract
A new beam-charge interlock system has beendeveloped for radiation safety and machine protection at
the KEKB injector linac. Although the previous softwarebased interlock system was working, it was replaced with
the new hardware-based one to improve its operational
performance and reliability. The new interlock system
restricts the integrated amount of beam charges delivered
to four different storage rings (KEKB e+, KEKB e-, PF,PF-AR) at six locations along the linac and the beamtransport lines. The full-scale operation of the newinterlock system has started in 27 March 2008 afterelaborate beam tests. In this report we describe the beam
tests, the technical improvements, and the operational
performance of the new beam-charge interlock system.

INTRODUCTION
A new injection scheme is under development for
simultaneous and continuous injection delivered from theKEKB injector linac [1] to both the downstream storagerings (the KEKB electron/positron rings and the PF ring).
The technical review was reported elsewhere in detail [2].
The new beam-charge interlock (BCI) system is one of
the R&D subjects pertaining to the new injection scheme.
The BCI system restricts the integrated amount of thebeam intensity passing through the injector linac and thebeam transport line to each storage ring. When it exceeds
a certain threshold level prescribed at each location, theBCI system directly generates a beam-abort request to thesafety control system of the linac for radiation safety andmachine protection. Its technical review was reported indetail elsewhere [3-5].
The new BCI system also supplies a proper
environment to advance the present accelerator complex
towards the future projects. The following four subjects
are underway,(1) to increase of the injection beam intensity towards
the next-generation B-factories,(2) to perform the stable and continuous injection overan hour to the PF-AR,(3) to enable easy beam tunings for the PF injection
with a new beam dump,(4) and to perform the new injection scheme for boththe KEKB and the PF rings.
These subjects are strongly required for improving thestable and fast injection with higher-brightness (orhigher-intensity) electron and positron beams into thedownstream storage rings. In order to perform them, theinstantaneous and integrated beam intensities must be______________________________________________*E-mail address: tsuyoshi.suwada@kek.jp.

controlled pulse-by-pulse at both the linac and eachtransport beam line. Therefore, the hardware-based BCI
system with a higher reliability must have been developed
for these purposes.
The new interlock system enables to relax therestrictions in the beam intensity for each injection mode.
Table 1 shows the comparisons of the old and new beamcharge intensities prescribed at each location along thelinac and each beam transport line.
Table 1: Comparisons of the old and new beam-charge
intensities prescribed at each location.
Location
R0-01
22-44
Linac
KEKBPF
PF-AR

Old[nC/s][nC/h]
1250
625
62.5
5.76x105
0.72x104

New[nC/s][nC/h]
2500
4.50x1061250
2.25x106
2.25x105
5.76x105
7.80x104
2.32x104

Based on this modification, the injection beam intensity
can be controlled every an hour and every a second. In
particular, the instantaneous beam intensity has beenrelaxed by two times. Thus, this modification has enabled
to test high-intensity beam acceleration studies for thenext-generation B-factories. On the other hand, the stableinjection to the PF-AR over an hour has been realized.
For the PF injection, the beam-intensity control has been
available with a new beam dump. Thus, the flexible
beam-intensity control has started and the highly reliable
environment to advance the new injection scheme has
been ready.
The development of the new BCI system has been
finished after elaborate system tunings with beam tests
during two years. The full-scale operation has started
without any significant problems.

FURTHER TECHNICALIMPROVEMENTS
The designs of the new interlock system started in 2006,
and the development of both the hardware and softwaresystem finished in October 2007. During two months
since then, the operational tests were performed to checkthe technical functions, the reliability, and the stability inthe nominal injection operation. We found several faults
during the beam tests. Further the technical improvements
have been performed step-by-step in order to increase thesystem reliability and flexibility. For the hardware system,
metal doors were installed to shield a system rack tofurther reduce electromagnetic noises mainly caused fromhigh-power klystrons. An uninterruptible power supply(UPS) was also installed at each system rack to improve
the operational reliability even for an unexpected power
failure.
On the other hand, for the software system, thefollowing subjects have been performed(1) to add a clock time adjustment function for whole
the system,(2) to improve the stability of a serial communicationline between the AR and the main control room of
the linac,(3) to connect the alarm system,(4) and to improve the operational panels and the datatransfer speed.

The results show the linear relations of the beam
charges integrated every a second interval depending on
the beam repetition rate obtained for one-bunch and twobunch acceleration mode. The results indicate a good
linearity within 1% and with a dynamic range of ~20 dB
although the obtained data include tiny beam intensity
fluctuations.
Figure 2 shows the time trends of the beam charges
integrated every a second interval measured for thenominal KEKB operation during a day. We can clearly
see the variations in the differences on the KEKB electron
and positron beam intensity, on the number of theaccelerated bunches, and the repetition rate. The results
also show that the beam charges are stably measured
pulse-by-pulse without any significant signal noises.

These improvements enabled to implement the flexible
functions of the operation panels and to increase the speedof the data transfer and storage. The data transfer and thecommand controls were also stabilized without anyproblems. The function that automatically adjusts theclock time differences between the detector modules andthe PLCs by communicating the UNIX-based hostcomputer was added. It enabled to quickly let the linac
operators know the abnormal system status. Thus, basedon the software improvements, the operators can get
easily and quickly the proper information and status of theBCI system.

BEAM TESTS
The wall-current monitors are utilized for the beamcharge measurements in the new BCI system. They werecalibrated with both bench and beam tests. The calibration
system of the test bench was reported in detail elsewhere[4,6].

Figure 1: Linear relations of the beam charges
integrated every a second interval as a function of thebeam repetition rate obtained for one-bunch (red) andtwo-bunch (blue) acceleration mode.
Here, the results of the beam tests are briefly described.
The typical results with the KEKB positron beam areshown in fig. 1.

Figure 2: Time trends in the beam charges integrated
every a second in the nominal KEKB operation. The
repetition rate, the number of accelerated bunches (1b:1 bunch, 2b: 2 bunches), and the charge sign areindicated at each data point.
Figures 3 (a) and (b) shows the typical time trends ofthe integrated charges integrated every an hour interval
during a week and a day obtained for the nominal KEKB
operation, respectively. As shown in fig. 3(a), the typicalintegrated intensities are ~3% to the prescribed beam
charges (CL=5.76x105 nC/h) at location KEKB while themaximum of the integrated intensity have attained up to~10% (or 6x104 nC/h) level to it. This level (~3%)
corresponds to the integrated beam charges in the stable
continuous injection operation, in which the electron andpositron injection has alternatively changed every a few
minutes. The maximum level (~10%) corresponds to theinjection with a maximum repetition rate. Thus, we cansee that in the nominal operation condition the measured
beam intensity is far enough away from the prescribedbeam charges.
As shown in fig. 3(b), we can also see that the BCI
system is properly reset every clock hour, and it
successively continues the measurement after the reset
without any time delay. It is also confirmed that thevariations in the linear slope of the measured integratedcharges depend on the change of the beam repetition rate.

(a)It should be noted that the unit is indicated by the ratioof the integrated beam charges to the prescribed ones ateach location. The maximum of the beam charges
integrated every a second (hour) at R0-01 was ~35%(~13%) for the primary high-intensity electron beam for
positron generation.
Thus, it was confirmed that the beam intensity levels
were much less than the prescribed beam charges in thenominal operation for each injection mode. The results
clearly also show the good reliability and stability of theBCI system without any dangerous excesses on the beam
intensity for all the beam modes.
The BCI system enabled to store other important data,that is, the beam charges and the shot numbers integrated
during a day and a week at each location. The 1-weekbeam-charge data depending on the injection modes arerequired to radiation-safety records in the linac operation.
We have no such kinds of data logging systems until now.
It is expected that these data give an important operational
history not only for radiation safety but also for the longterm stability and reproducibility of the injector linac
operation.

(b)

SUMMARY

Figure 3: Time trends of the beam charges integrated
every an hour during (a) a week and (b) a day in thenominal KEKB operation.

OPERATIONAL PERFORMANCEThe successive full-scale operation of the BCI system
has been stably implemented during three months without
any significant problems while no abort-signal requests
were generated in this operation. The typical chargeintensity levels measured in the nominal operation aresummarized in Table 2.
Table 2: Integrated amounts of the beam charges
normalized by those prescribed at each location obtainedin the nominal linac operation.
Location

R0-01
22-44
KEKBPF
PF-AR

Beam Charges(1s interval)[%]
8-35
1.5-5-Beam Charges(1h interval)[%]
3.5-132-12
3.5-100.2-0.6
2-5

The full-scale operation of the new beam-chargeinterlock system has started since 27 March 2008. The
new BCI system highly improved its reliability andstability for radiation safety and machine protection for
the KEKB injector linac and the downstream beamtransport lines. It was confirmed that this new system has
stably continued to work during three months until thissummer shutdown (the end of June 2008) without any
significant problems. An environment to advance several
new activities for our future projects is ready now.

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