InternationalInternational
Conference
Conferenceon Accelerator
on Accelerator
and LargeandExperimental
Large Experimental
Physics Control
PhysicsSystems,
Control1999,
SystemsTrieste, Italy

RF MONITORING SYSTEM IN THE INJECTOR LINAC
H. Katagiri, K. Furukawa and S. Anami
High Energy Accelerator Research Organization, KEK
1-1 Oho, Tsukuba, Ibaraki, 305-0801, JapanFirst, a high-speed measurement of pulsed-waveform isrequired. The monitor station must detect any pulse-topulse event, such as a discharge in the high-power RF
circuits, or jitter in the RF pulse timing. The waveform
digitizer has 1M bytes of shared memory, which can beaccessed by the controller via the VXI backplane. Highspeed data transfer by a shared memory protocol has
achieved repetitive measurements at 25Hz.
Second, the adoption of a built-in type VXI controller
has the advantage of more immunity to electro-magnetic
noise interference. Reduction of noise interference wastaken seriously, because the monitor station had to beinstalled beside the pulse modulator for the high-power
klystron.

Abstract
A new microwave monitor system has been installed aspart of the upgrade of the 8.0-GeV KEKB injector Linac.
Thirty monitor stations constantly monitor the waveforms,peak-power levels, and phases of 59 high-power RF
sources.
The monitor stations, based on VXI, make it possible tomeasure pulse-to-pulse events. Data obtained by monitor
stations is accumulated and presented along with beam
position monitor data to the operator for beam adjustment.
Beam diagnosis can also be performed.

1 INTRODUCTION

2.2 Phase detector

The KEK electron/positron linac was upgraded toincrease its energy from 2.5 GeV to 8 GeV for the KEKB
project. A SLED, which is an RF pulse-compression
device, and a 50-MW klystron were added to increaseavailable linac RF power. As a consequence, the beamenergy gain would be susceptible to errors of RF-phasemodulation timing or SLED-cavity tuning. However, theKEKB collider ring requires the injected beam to have
low-energy dispersion to minimize injected beam loss.
We thus completely redesigned the RF-monitor systemof the linac to realize stable operation of the RF system.
[1]

A newly developed phase detector was installed. The
phase-detection technique is based on a heterodyne
method. That is, the front-end of the phase detector
converts both input signals, a test signal and a reference
signal, into IF signals of 180 MHz. The next-stage circuit
then compares digitally both IF-signals. Finally, the phasedifference is converted to a voltage signal.
The principal advantage is that the output voltage is
proportional to the phase difference. However, it has acharacteristic that causes inaccuracies in the detection of aphase difference at around 0 degrees. To overcome thisproblem, a phase shifter which can shift by 180 degrees
was introduced into the reference line.
To measure a beam-induced field, the dynamic range
for the input power is required to be more than 50 dB. The
beam-induced RF power and the accelerating RF power
are measured alternately when the klystron trigger timing
is delayed. The power difference between both signals is
about 50 dB. However, the original dynamic range of thedetector was only 30 dB. Thus, an attenuator, which isexternally controlled, was added to extend dynamic range.
The attenuator control is synchronized with the beamtrigger.

2 THE RF MONITOR STATION
Thirty RF monitor stations were installed to constantly
monitor the 59 high-power RF sources. Currently, theyalso monitor 8 sub-booster systems. Moreover, monitoringof the SHB (Sub-Harmonic Bunchier) will be added to thesystem
Figure 1 shows a simplified diagram of the RF monitor
station, which consists of a VXI-based control system andRF instruments. Any monitor signal can be connected tothe power sensor and phase detector through RF switches.
The VXI-based control system can measure power leveland phase signals that are simultaneously detected bypower-sensor and phase detectors.
Furthermore, some ideas for measuring the phase of
beam-induced field were introduced into the design.

2.3 Power meter
A power sensor having a differential-type detector anda universal power meter was chosen. The sensor reduces
any interference due to common-mode noise.
At present, the waveform digitizer measures thevoltage-signal from the power sensor. Although thevoltage signal output is nonlinear relative to the input
power level, detection of the relative variation is possible.

2.1 VXI-based control system
The VXI-based control system, which consists of adisk-less controller, a waveform digitizer and a digital
I/O, was chosen for the following advantages.

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2.4 RF switches

4 APPLICATIONS FOR THE MONITORSTATIONThe monitor station must constantly measure any
monitor signals of two RF sources. Therefore, a newly
developed PIN-diode switch having 8 input-ports wasinstalled for repetitive signal selection and measurement.
The PIN-diode switch was chosen because of its
durability for frequent switching operations. Incidentally,
the switch is operated at about 10 seconds/cycle in themonitor station.
In addition, a mechanical RF switch having low
insertion-loss is used to measure a low level beaminduced signal.

HP VEE (Visual Engineering Environment), which is a
visual programming language for instrument control, wasemployed for developing the application programs of theRF monitor station. It brings higher productivity, although
we had expected that the execution speed would beinsufficient for practical applications. However, a compilemode, which was appended to VEE version 4, improved
the execution speed. As a consequence, the greater part of
the application is still written in VEE. Parts of theapplication were written in the C language and linked tothe VEE application to obtain higher throughput.

VXI-bus SystemDigital
I/O

Waveform
Digitizer

PhaseDetector

PowerSensor

Disk-lessController

LAN

4.1 Automatic measurement

GPIB

The RF monitor station normally performs anautomatic measurement for any monitor signal.
The process acquires and accumulates waveform dataof 200 pulses at 40-millisecond cycle. A routine foraccessing the VXI shared memory is written in Clanguage. After the routine reads 1000 words of raw data,
the power level and phase are computed. Thus, the dataobtained for 200 pulses is averaged to improve theaccuracy. Moreover, maximum value, minimum value
and standard deviation are computed. Those parameters
are stored on the server’s storage disk.
Raw data of the waveform for 200 pulses is temporarily
maintained in the shared memory on the VXI controller.
This continuous waveform data can be utilized in thefuture for the diagnosis of RF sources and accelerating
structures.
At first, we expected that the effective resolution of thewaveform digitizer might be 2%, because the digitizer has
only an 8 bit A/D converter. But the effective resolution
was improved by the following technique. The
measurement is made at a higher sensitivity with an added
DC offset. That is, the digitizer measures some expanded
part of the waveform. With this technique, a phaseresolution of 0.2 degree was achieved.

PowerMeter

RF Switches

Sub-booster High-power High-power
SystemRF Unit 1
RF Unit 2
Figure 1: Construction of the RF monitor station.

3 SYSTEM ADMINISTRATIONThe RF monitor system consists of 30 VXI systems andtwo server computers. A disk-less configuration of theVXI controller is indispensable for eliminating any system
failure caused by a hard-disk crash. The clusterarchitecture of HP-UX was employed to achieve ourserver and disk-less client configuration. Currently, theNFS disk-less architecture of the HP-UX version 10.20 isoperating stably.
The network for the linac control consists of a main
network and several segmented device-group networks.
The RF monitor system is in an RF control network,
which is part of a device-group. Cluster servers andclients for the monitor system utilize some network
services from the linac control system, such as NFS, DNS,NIS and NTP. Those services are useful for cluster systemadministration. [2]4.2 Remote control service
To provide remote control of the monitor stations forthe linac operator, a remote-control service has been
added. The service suspends automatic measurement, andpermits remote control upon a request from the operator.
A UDP based communication routine, which was written
in C language, communicates with the remote-control
program.

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5 APPLICATIONS FOR LINACOPERATION
5.1 Remote controller similer
The linac operator can control any monitor stations via
the remote-control program without having to log-in to aremote host. The program, which is produced by VEE,
communicates with each monitor station for instrument
control. It can select any monitor signal, and control awaveform digitizer in a manner similer to an oscilloscope.
The program is utilized for the diagnosis andadjustment of RF sources such as SLED cavity tuning.
Figure 2 shows the SLED-monitor signal.

Figure 3: RF trend graph.
variation trend of the injector affects. The phase-stabilityof the injector directly beam stability. The phase variation
trend is also helpful for reconfirming the phase setting
when a beam parameter has been changed.
A phase measurement of the beam-induced field causedby single bunch beam was achieved. Thus, a phaseadjustment by a beam-induced method is being studied.

7 ACKNOWLEDGMENT
The authors thank the linac staff for their valuable
support and help.

REFERENCES[1] Design Report on PF Injector Linac Upgrade for
KEKB (in Japanese), KEK Report 95-18, 1996.
[2] K. Furukawa, et al., “Microwave Control andMeasurement System at the KEKB Linac”,
Proceedings of the International 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”, Proceedings of the First AsianParticle Accelerator Conference, KEK, Tsukuba,Japan, 1998.
[4] H. Katagiri, et al., “Microwave Monitor System for
the KEKB Injector Linac”, Proceedings of the First
Asian Particle Accelerator Conference, KEK,Tsukuba, Japan, 1998.

Figure 2: Remote control program.

5.2 Data viewers
A viewer program, which is written in Tcl/TK
language, is available for searches and displays of storeddata. Tcl/TK is utilized for many linac operation
applications.
The RF trend program shows graphically the power
level and the phase history of RF sources. Figure 3 shows
the RF trend program.

6 CONCLUSION
The installation of monitor stations has almost been
completed. Several application programs are in routine
use for linac operation. Especially, consider a phase-71