MICROWAVE MONITOR SYSTEM FOR THE KEKBINJECTOR LINAC
H. Katagiri, K. Furukawa, H. Hanaki and S. Anami
High Energy Accelerator Research Organization, KEK
1-1 Oho, Tsukuba, Ibaraki, 305-, JapanAbstract
A microwave monitor system was newly developed formaintaining stable operation of the linac. Thirty monitor
stations are now being installed in order to monitor 58
high-power RF sources.
The monitor station consists of microwavemeasurement equipment and a VXI system containing a
diskless controller, a wave-form digitizer, etc. Thecontroller has an Ethernet connection to servers. Most ofcontrol programs on the controller and the server havebeen developed by means of the visual environment VEE.
The completed stations are automatically watching andmeasuring the wave forms and peak power levels of highpower RF pulses. Any wave form can be selected anddisplayed on the servers like an oscilloscope. An
additional function, automatic phase control for example,
will be serviced this summer.
1 INTRODUCTION
The KEK electron/positron linac had been upgraded inorder to increase its energy from 2.5 GeV to 8 GeV for
the KEKB project. A SLED, which is an RF pulsecompression device, and a 50-MW klystron wereemployed for a reinforcement of the linac RF-powersource. As a consequence, the beam-energy gain would
be susceptible to errors of an RF-phase-modulation timingor the SLED-cavity tuning. However, the KEKB collider
ring requires the injection beam to have low energy
dispersion in order to minimize the injection beam loss.
We thus completely reformed the RF-monitor systemof the linac in order to realize stable operation of the RFsystem.[1]2 DESIGN POLICY
We attached greater importance to detecting the relativevariation of the monitor signals than to measuring theabsolute values.
The 30 monitor stations were installed for constantly
monitoring the 58 high-power RF sources. The monitor
station must detect any momentary variation of the wave
form, such as a discharge in the high-power RF circuits, orjitters in the RF pulse timing. Therefore, the VXI system
was chosen in order to achieve high-speed data transfer
between a computer and a measurement instrument.
In our case, the RF monitor system comprises a few
server computers and 30 client VXI controllers. A

diskless configuration of the VXI controller is
indispensable for eliminating any system failure caused bya hard-disk crash, and saving labor for file-systemmanagement. Thus, a cluster architecture by HP-UX wasadopted for our server-and-diskless-clients construction.
3 CONSTRUCTION OF THE RF MONITORSTATION
Figure 1 shows a simplified diagram of the RF monitor
station, which comprises a VXI system and RF
instruments. Each station monitors two high-power RF
sources. The VXI system comprises a diskless controller,a wave-form digitizer and a digital I/O module. An A/D
converter will be added to the system for measuring
monitor signals converted to DC.
The wave-form digitizer has 1M bytes of sharedmemory, which can be accessed by the controller via aVXI backplane for high-speed data transfer to thecontroller. The resolution of wave-form digitizer is only 8bits. This means that the effective resolution might be2%. However, a measurement with a DC offset and at alower range gives a resolution better than 0.1% or so.
The controller operates the following RF instruments
via the VXI digital I/O module or the built-in GP-IB.
3.1 Peak-power meter
The power meter and the peak-power sensor, made by
Giga-tronics, were chosen by reason of the following two
advantages:
First, the peak-power sensor has a differential-type
detector, which reduces any interference due to commonmode noise. Second, the power meter also has an
oscillator for calibrating the power sensor. The system
can thus execute automatic calibration by feeding thecalibration signal to the sensor via an RF switch.
The peak-power sensor measures the power level atthe desired timing, which is determined by an external
trigger pulse with an internal variable delay. The VXI
controller reads the measured power level via GP-IB,
while the detected envelope of an RF pulse is digitized
and measured by the VXI wave-form digitizer.
3.2 Phase detector
As mentioned before, the monitor station will be equippedwith a phase detector, and will perform automatic phaseadjustment (phasing) of high-power RF sources. The
principle of phasing is summarized as follows:

The phase detector measures the phase of RF signals
monitored at the output port of the accelerator guide. Thebeam bunch passing through the accelerator guide induces
microwaves in it. The phase of the induced wave and of
the accelerating microwaves can be measured when theklystron RF timing is delayed in order to separate both
microwaves. Then, a phase shifter of the high-power
klystron is adjusted so that the accelerating RF phase is
set to be 180 degrees apart from the induced wave phase.
The phase-detection technique is based on theheterodyne method. That is, a front-end part of the phasedetector converts both input signals, a test signal and areference signal into IF signals of 100 - 200 MHz. The
next-stage circuits then compare both IF signals andconverts the phase difference proportionally to a voltage
signal.
3.3 RF switches
The monitor station is equipped with a solid-state switch
and a mechanical switch in order to measure some RF
signals of two high-power RF sources.
A solid-state switch was adopted for frequent
measurements, because it has a higher switching speed
and long lifetime independent of the number ofswitchings. It, however, has the following disadvantage

compared with a mechanical switch. First, it has a higher
insertion loss. Besides, the nonlinearity of the insertion
loss distorts the RF output signal when the input power
exceeds a certain level. Second, the amplitude and phaseof the output RF signal depend on its temperature. After
those factors were seriously taken into account, a PIN
diode made by HP was selected.
On the other hand, a mechanical switch is employed
for measuring the faint beam-induced signal accurately,
and feeding the calibration signal to a peak-power sensor,
because it has the advantage of low insertion loss.
However, a phase adjustment or the sensor calibration is
performed only once a week at most. Therefore, a shortswitching lifetime of the mechanical switch will not be aproblem.
3.4 VSWR meter
A VSWR meter was installed in every high-power RF
source in order to protect the klystron from reflected RFpower. It monitors the forward-going power level and thereflected power level, and then computes the VSWR
constantly. When the VSWR exceeds a limit level, analarm signal is detected by a safety-interlock circuit of themodulator.

Figure 1 Diagram of the RF monitor station
4 RF CONTROL NETWORKThe computer-network system of the KEKB injector linac
comprises a main network and several segmented devicegroup networks as presented in Figure 2. The network
form is based on a star-like topology in order to ease any

trouble-shooting. The reasons for adopting the separated
device-group networks were to localize any problems andto ease traffic. An optical Ethernet is employed for thedevice-group networks in order to reduce any noiseinterference caused by the high-power klystron
modulators.[2]An RF control network, which is one of the devicegroup networks, handles the VXI systems, servercomputers, and PLC-based modulator controllers.[3] The
server computers provide some services, such as NFS,
BOOTP and TFTP for the client VXI controllers.
Currently, two server computers manage 29 diskless VXIcontrollers. Besides, we are planning to prepare anotherserver computer for backup.
Main Control RoomConsole
SystemMain Computer
SystemMain Network
SwitchSwitchSwitchOther Groupe
Networks(magnet, vacum etc.)

ServerComputers

RF Control Network-------Repeater
Repeater
PLC - - VXI - - PLC

PLC - - VXI - - PLCFigure 2 Network construction
5 CONTROL PROGRAMS FOR THE RF
MONITOR SYSTEMHP VEE (Visual Engineering Environment), which is a
visual programming language for instrument control, wasemployed for developing the application programs for theRF monitor system.
The RF monitor system is controlled by a combinationof an instrument control program on each VXI controller
and a total management program on the server computers.

will also be performed when the program accepts arequest from the server this summer.
Thus, the VXI controller performs automatic
measurements as a routine task, and must simultaneously
accept any request, such as instrument control, phaseadjustment and sensor calibration from the server
computers.
Incidentally, X Window achieves a remote control ofthe monitor station. However, the instrument control by X
Window will conflict with the automatic measurement,
because both processes run independently. That is,compatibility between both the processes is required.
First, the VEE's communication routine based on TCP
was tried, but could not satisfy the required throughput.
Finally, optimized UDP communication routines, which
can be called by the VEE application, were newly writtenin C language.
5-2 Management program
The management program on the servers has the following
functions as the linac operator's interface:
1) Instrument control panel
It can select any monitor signal, and control a wave-form
digitizer like an oscilloscope. The raw data of the wave
form is traced at about a 100 ms interval. Figure 3 shows
the traced SLED-monitor signal.
2) Display logged data
It reads the measured data from a storage disk, anddisplays them as a trend graph.
A phase-adjustment control and a sensor-calibration
control are to be added this summer.

5-1 Instrument control program
The instrument control program has the following
functions:1) Automatic measurement
This is a routine task on the VXI controllers. Currently, itmeasures the peak-power level, pulse height, pulse width,etc. of the RF monitor signals; a phase measurement will
be added. The measured data will be stored periodically,so that any long-term variation of the data can bedisplayed as a trend graph. Moreover, raw data of thewave form are stored when a momentary variation isdetected. The raw data will be utilized for wave-formdiagnosis in the future.
2) Instrument remote-control service
When the instrument control program accepts a remotecontrol request from the servers, this program suspends
any automatic measurement, and permits the server tocontrol the instruments.
3) Further functions
The phase adjustment and peak-power sensor calibration

Figure 3 Example of instrument control panel
6 REFERENCES[1] Design Report on PF Injector Linac Upgrade for KEKB(inJapanese), KEK Report 95-18, 1996.
[2] K. Furukawa, et al., "Microwave Control and Measurement
System at the KEKB Linac", Proceedings of theInternational Conference on Accelerator and Large
Experimental Physics Control System ‘97, IHEP, Beijing,
China, 1997.
[3] H. Hanaki, et al., "Low-Power RF Systems for KEKB
Injector Linac", in this proceedings.