PLC-BASED BEAM CHARGE INTERLOCK SYSTEM FOR RADIATIONSAFETY IN THE KEKB INJECTOR LINAC
E. Kadokura*, T. Suwada, M. Satoh, and K. Furukawa, KEK, Tsukuba, Japan
Abstract
A new PLC-based beam charge interlock system isunder development for radiation safety and machine
protection in the KEKB injector linac. This system
restricts integrated amounts of beam charges passing
through several locations along the linac before they are
injected into four different storage rings (KEKB e+e storage rings, PF, PF-AR). The beam charges aremeasured using wall current monitors and detection
electronics controlled by PLCs. This system issues beamabort requests directly to the radiation safety control
system through twisted hardwired cables when theintegrated amounts of beam charges exceed the thresholdlevel prescribed at each location. In this report, we
describe the new design of the PLC-based beam chargeinterlock system, with particular focus on thedevelopment and performance of the PLC controls.

INTRODUCTION
The KEK B-Factory (KEKB) injector linac [1] delivers
electron (or positron) beams to four different storage ringsin daily operations. One of the purposes is to directly
supply 3.5-GeV positron and 8-GeV electron beams to theKEKB storage rings, where a high-energy e+e- colliderexperiment (KEKB project [2]) is in progress. For theKEKB operation, the continuous injection scheme [3] isimplemented in order to maintain the stability of beamcollision and peak luminosity as high as possible. The
other purpose is to supply 2.5 GeV and 3 GeV electron
beams to two different photon factories (PF and PF-AR,respectively) [4], where extensive studied on materials
science is in progress. The frequency of injection is twice
per day for each ring under the nominal user time.
A new sophisticated injection scheme has been
investigated in order not to disturb the beam injections tothe four storage rings with each other and to keep thestabilization of the storage rings independently. The new
injection scheme requires a simultaneous and continuous
injection [5, 6] to all the downstream storage rings.
Rresearch and development (R&D) studies on the new
injection scheme have been conducted since 2004.
The programmable logic controller (PLC)-based beam
charge interlock (BCI) system is one of the R&D systems
required for the new injection scheme. This system
restricts the integrated amount of beam charges passing
through the linac, and also that delivered to the beamtransport line of each storage ring without any conditionsin terms of the beam mode. When the integrated amountexceeds a certain threshold level prescribed at eachlocation along the linac, the BCI system issues a beamabort request directly to the safety control system of thelinac for radiation safety and machine protection.

Although the present software-based BCI system isimplemented on a UNIX-based host computer by utilizing
the beam position monitor (BPM) system [6], it is not a
dedicated system for this purpose. It reads beam charges
measured by the BPM system and calculates theintegrated amount of beam charges. Based on similarinterlock conditions, the beam abort request can be sent tothe safety control system through the computer network.
However, the software-based system has several
drawbacks: strong dependence on the BPM system, slow
data-acquisition speed, low reliability for the new
injection scheme, etc. We have developed the new BCIsystem, overcome these drawbacks, and increased thereliability of radiation safety and machine protection,even in the new injection scheme.

BCI SYSTEMThe PLC-based BCI system has been newly installed
along the linac beam line. Maximal integrated amounts ofbeam charges are restricted at six locations (Locat.) along
the beam line in the tunnel, while the detection systems
are installed at three stations (St.) in the klystron gallery.
The maximum allowable integrated amounts of beam
charges are prescribed at each location and they aresummarized in Table 1.
Table 1: Maximal integrated amounts of beam chargesprescribed at each location along the injector linac.
Q [nC]
Q [nC/pulse] x Hz TimeSt.
Locat.
B-8

B-8

2-2

2-2

6-A

LinacKEKBPF
PF-AR(integrated)(per pulse)

int.

2500
4.5 x 1061250
2.25 x 106
2.25 x 105
5.76 x 105
7.83 x 104
2.88 x 104

50 x 50
25 x 50
25 x 50
12.5 x 50
1.25 x 50
3.2 x 50
0.87 x 25
0.32 x 25shshhhhh

Here, the restricted beam charges are defined as theintegrated amounts of beam charges measured per second(and/or per hour) depending on the location while thebeam charge per pulse refers to the average charges atmaximum repetition rates.
The block diagram of the BCI system is shown in Fig.
1. Pulsed beam signals from a wall current monitor(WCM) are transmitted and received pulse-by-pulse by
the detection electronics (or BCI module) through a 35m-long coaxial cable at a detection speed of more than 50
Hz. It measures the integrated amount of signal charges
based on a charge integration circuit and transforms thesignal charges into beam charges using calibration
coefficients. The hardware system of the WCMs anddetection electronics are described elsewhere [7]. Here,the PLC-based control system is described in detail.

every clock hour, while that with an interval of 1 s is reset
every second.
This system can automatically restart the measurementof the beam charges, even during a power failure, since
the data obtained before the power failure is temporally
stored in the EPROM of the detection electronics. These
data are reloaded in memories after recovery from thepower failure. It should be noted that these functions
maximize the reliability of radiation safety in the injector
linac.

PLC-BASED CONTROL SYSTEMThe PLC-based control system comprises a main PLC,
three slave PLCs, and a sub-PLC, as shown in Fig. 3.

Figure 1: Block diagram of the beam charge interlock
system.
The PLCs control the detection electronics and receive
the integrated beam charge data every second. When theintegrated amount of beam charges exceeds a certain
threshold level prescribed at each location (see Table 1),
the beam abort request issued by the detection electronics
is transmitted directly to the safety PLC installed in themain control room. After receiving the beam abort request,
the safety PLC immediately refrains from transmitting atrigger signal to the electron gun. A block diagram of thegeneration scheme for the beam abort requests is shownin Fig. 2.

Figure 2: Block diagram of the scheme for issuing
beam abort requests.
When the 1-sec (and/or 1-hour) interval of theintegrated charge data exceeds the threshold level, adriver circuit of the safety PLC supplying a 24 V DC
level switches off a no-voltage contact Simultaneously,
the detection electronics issues a beam abort request witha long pulse of 100 ms (and/or a zero-voltage level)
directly to the safety PLC through a twisted hardwired
cable. The abort request with an interval of 1 h is reset

Figure 3: Block diagram of the PLC-based control
configuration for the BCI system.
Five BCI modules are controlled by the main PLC(YOKOGAWA, FA-M3) with a CPU module(YOKOGAWA, F3SP66-4S) through the slave PLCs,
which are connected in a loop with optical fibers (FA-bus(type2), 10 Mbps) over a total length of 1.3 km. The main
PLC controls the digital input/output (DI/O) modules inthe slave PLCs. The DI/O modules are connected to theBCI modules with 32-bit parallel communication cables.
The main PLC performs several functions for a set ofparameters and data acquisitions of the BCI modules. The
sub-PLC controls one BCI module. The communication
between the main PLC and the sub-PLC is achieved
through a 1.1-km-long pair cable with a 128-bit serial
two-way communication protocol because such a long
distance inhibits the nominal communication protocol
using optical fibers.
The TP is connected to the main PC through RS422with a communication speed of 115 kbps. All functions of
the BCI modules are set through the TP. The TP is also
connected to the linac control network through ethernet. Italso facilitates the development and maintenance ofsoftware programming. The main PLC is also connected
to a Linux-based archiver, where EPICS Input-Output
Controller (IOC) is running, through the linac control
network. A data logging system based on the EPICS
archiver stores the data from the main PLC.
The power supply of the main PLC is switched on
through an on-delay timer in order to communicate withhigher reliability to the slave PLCs at the beginning of thecontrol system. Power is supplied to the entire control
system through uninterruptible power systems (UPSs)
using batteries in order to avoid any problems due to an
unexpected power failure.

DATA COMMUNICATION
Communication overview
The PLCs read the data, integrated amounts of beam
charges for the 1-sec interval, detected beam-shot number,alarm status, etc., through the DI modules from the BCI
modules. They also store the data along with integrated
amounts of beam charges for a 1-hour-interval, 1-dayinterval, and 1-week-interval, after the calculations in thememories of the PLCs. These implementations can beprocessed within 1 ms, where the sampling time intervalof the DI module is set to 250 μs. On the other hand, thetime interval for data acquisition of the DO module is lessthan 1 ms.
The alarm histories and time traces of the integrated
amounts of beam charges obtained at each location aredisplayed on the TP in real time.

Communication between PLC and BCI module
The word-type data with 24 bits and bit-type data,
respectively, are mutually transferred between the PLCs
and the BCI modules with a handshake procedure byusing “Write” and “Read” commands defined by the sideof the PLCs. A series of these commands is summarizedin Table 2.
Table 2: Protocols for data communication between thePLC and the BCI module using “Write” and “Read”
commands.

These commands are implemented with the handshakeprocedure as shown in Fig. 4.
Set Commandfrom PLC to BCI

Set Writeｆrom BCI to PLC Get Write(ACK)
Read Commandfrom PLC to BCI

Set Command

W_Command
W_Data2 ms20 ms

W_Command

Read command

Set Write
Get Read (ACK)
R_Command
R_Data2 ms
20 ms
Read Data

from BCI to PLC
Set ReadGat Write(ACK)

2 ms
20 ms

Figure 4: Handshake procedures for data communication
between the PLC and the BCI module.

For example, the data are set from the PLC to the BCI
module by sending a series of commands, a command(Write Command) to be set (W_Command), data
(W_Data) to be written along with a bit command to beset (Write Bit Command). One cycle of the data set basedon the “Write Command” is completed by receiving anacknowledge signal (Get Write (ACK)) from the PLC.
For reading data, a similar procedure is implemented byusing “Read” commands. The TP facilitates theimplementation of these commands.

Communication between PLC and EPICS
archiverThe main PLC is connected to the Linux-based EPICS
archiver via the linac control network. This archiver logs
the data stored in the main PLC through an EPICS IOC.
A database is created from these logged data. The
application software displays time traces of the integrated
amounts of beam charges, beam abort status, etc., on any
X-terminal by utilizing the database for achieving stablelinac operation. Such simple command-based controlarchitectures strongly support the stable operation of theBCI system.

SUMMARY
We have successfully installed a PLC-based BCI
system for radiation safety and machine protection in theKEKB injector linac. This control system has good
performance and high reliability along with the detection
system based on the WCMs. It enables us to reinforce theincrease in beam intensities and repetition rates toward
the new injection scheme of the linac operation.

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