The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Beam Position Monitor System for KEKB
M.Tejima, M.Arinaga, T.leiri, H.Ishii, K.Mori and S.Hiramatsu
KEK, High Energy Accelerator Research Organization, Oho 1-1 Tsukuba, lbaraki 305 JAPANAbstract
The Beam Position Monitor (BPM) System for KEKB
is now under construction. The system was designed to beable to.measure closed orbit with a high accuracy less than
1 f..Lm within a few seconds. In order to fit to various modesof the multi-bunch operation, we chose a pickup frequencyof 1.018 GHz of a beam signal, twice the accelerating RFfrequency.
Since the system must measure closed orbit for a multibunch (5000), high-current beam (low energy ring LER
2.6A, high energy ring HER l.lA), we paid attention toimprove it about the following, the pickup electrode, thetransmission line, the switch, the front-end electronics andthe BPM support. This paper describes the system
requirements, the design result of the system, and thelaboratory test.

power, we adopted a small diameter (!i112mm) of theelectrode. Figure 1 shows two types of pickup electrodes inthe LER and in the HER.
Close attention was paid to both of resonance higher
order mode (TEll) at the button and trapped mode in theceramic seal[4]. To avoid the growth of coupled bunch
instabilities in the HER, we adopted the cross section
shapes of the feed-through and the rod electrode which may
be made non-axially-symmetric. Heating problems of theceramic seal was solved by optimizing size of the one.

1 Introduction
The design of Beam Position Monitor System for
KEKB[l] was based on some precilous experience in theBPM for TRISTAN[2], such as a multipacting discharge
phenomena in the button .electrodes, the increase
phenomenon of contact resistance at the center conductor of
the electrode connector, radiation damage of transmission
cable, trouble of the coaxial switch by the life. Moreover
the measurement time of the Closed Orbit Distortion(COD) was too long (over 90 seconds). The BPM heads(451 BPMs for LER, 442 BPMs for HER) will be installed
in almost all quadrupole magnets. Furthermore, for beam
orbit feedback of the collision point, two BPM heads will
be installed at the point of QCS magnets, too. Since thesystem is required to measure the beam position with a highaccuracy better than 10 flm in a short time within a few
seconds, we adopted basically same slow method as theTRISTAN BPM system which measures the average
position for the several thousand turns with signal
processing circuits as many as possible.
To measure beam position of tum by turn, fast signal
processing circuit will be also installed in each of 20 local
control rooms[3]. The slow system and the fast system
will share a beam signal from the same BPM head.
In addition to the above, there are some development,such as coaxial switches made by semi-conductor with longlife, VXI (VME-bus Extensions for Instrumentation)
standard module for high-speed processing of the data andPoly-Ethere-Ethere-Ketone (PEEK) semi-rigid cable withradiation resistance.

Figure 1: Electrodes for BPM.
The KEKB vacuum chamber is made of copper towithstand a high peak heat load , and to shield radiationfrom the beam. Therefore, the BPM block is also madefrom a solid piece of copper. Four feed-throughs withelectrodes are brazed onto the block. Two stainless steel
frames are brazed to the block, in order to prevent
deformation. Four-hundred-thirty-two BPM blocks for LER
arc sections and fifty-two ones for RF sections have alreadyfinished manufacture. And four-hundred-eight ones for HER
arc sections and twenty-six LER local correction area arebeing manufactured at present. And we are designing twokinds of BPM block for beam injection chamber in others.
Figure 2 shows the BPM blocks for the LERarc and theHERarc.

2 BPM block

Since the pickup electrode of BPM is exposed a highcurrent beam field, we paid special attention. To realize
sufficient mechanical strength and to withstand the expectedtransmission power, an N-type feed-through with a modified
center conductor having a large diameter (!i14mm), together
with a spring contact, is employed. To reduce the pickup

Figure 2: BPM blocks for KEKBThe completed BPM assembly is supported firmly andprecisely at the end of a quadrupole magnet, as shown in-86-

The Jlth Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Figure 3. The flat surface of two frames also serves as areference plane for the measurement of mechanical andelectrical offset of the BPM. After installation, the BPM
positions relative to the quadrupole magnets will bemeasured within several tens of a Jlm, and the data will be
used to correct the measured beam position data.

Figure 3: Support of the BPM block.

3 Calibration
Before welding the BPM-head to the beam pipe, allBPMs will be calibrated at a test bench (Figure 4) where theelectrical center of the BPM is identified with a high degreeof accuracy by using a movable antenna (1.018GHz CWsignal). The measurement is made at the 273 mesh points
in the central area of ±lOmm (horizontal) x ±6mm(vertical) with 1 mm step. Setting of a BPM block,movement of antenna probe and data taking areautomatically controlled by a micro-computer.

the geometrical one, and kx(l,Q) and ky(O,l) are reciprocals
of sensitivity of BPM in the x and y direction, respectively.
As another method of calibration, a practical model for
output signals of BPM electrodes was proposed to define ageometric monitor center by assuming that each electrodehas its ideal position-response function. We can give aprecise definition of geometrical center of KEKB BPM[5]. ·4 Signal processing method
The following performance requirements have been
given to the BPM read-out system design, such as theposition resolution better than lOJ.Lm, the COD measuring
time within a short time of -1 second and beam current
range of 10mA-2.6A.
In order to satisfy these requirements, we have adopted
basically the same signal processing method as that of theTRISTAN Main Ring BPM read-out system. Four beam
signals from each pickup unit are detected with a common
detection circuit, which detects a higher harmoniccomponent of the revolution frequency.
The signal detection circuit consists of a superheterodyne circuit, a 16 bits ADC and Digital Signal
Processor (DSP). Figure 5 shows a block diagram of thefront-end electronics. In order to measure beams with ahy
multi-bunch configurations, a pickup frequency of
1.018GHz has been chosen. This corresponds to twice theaccelerating RF frequency, and the 10240'th harmonic of therevolution frequency (-99.9KHz).

Figure 5: Block diagram of the front-end signal processor.

Figure 4: Test bench for BPM calibration.
We set an antenna at the position (X, Y) and measure thefour electrode outputs (A,B,C,D), and then obtain thenormalized quantities (H,V) by
H=(A-B-C+D)/(A+B+C+D),V=(A-B-C+D)/(A+B+C+D).
Next, the mapping data are fitted by least-square method tothird order polynominals of

where 0 :::;; i,j :::;; 3, 0 :::;; i+j :::;; 3. In these expression, kx(O,O)
and ky(O,O) gives the deviation of the electrical center fromThe super-heterodyne circuit converts a pickup frequency
into a intermediate frequency (IF=20KHz). To obtain good
linearity in a wide signal dynamic range, the rectifier stage
such as a synchronous detector is removed out. The IF
signal is digitized directly by the ADC with 1OOkHz
sampling rate, and the frequency spectrum is calculated by a
DSP with the 2048 points FFT ( Fast Fourier
Transformation ) to obtain the required signal which appears
at the spectrum peak. The DSP has a data aveqtge function
to increase the SIN ratio. The effective SIN ratio will reach
92dB which corresponds to the resolution of less thanlJ.Lm(rms) in the position measurement for the lOrnA beamat the HER.
We tested the position resolution by using prototype ofthe signal processing module. We inputted to the module
four signals that the source signal (1.018 GHz) wasdistributed by a RF divider circuit and measured the test data
repeatedly many times. Then the inputted signal level was-60 dBm, the sampling conditions was changed as follows:-87-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

the FFT measurements of 128, 512, 2048 points and theaverage of 128, 32, 8 measurements. As a result of the test,
we confirmed the performance of SIN ratio within 92dB andmeasuring time less than one second per a BPM as shownin Table 1.
Table l: Resolution & Measuring time
FFT points* .
Average.
.128*1512*12048*1
128*128
512*322048*8

SIN ratio[dBJ
71.6±0.177.7±0.1
83.4±0.1
92.2±2.0
91.9±0.8
91.7+0.2

Resolution[l:!:mJrms
4.5
2.2
1.2
0.4
0.50.5Measuring
time[secJ
0.007
0.027
0.110
0.820
0.839
0.867

The digital data calculated by signal front end processor
is sent to a VXI bus of mainframe in each local controlroom, where they are transformed into beam position data.
Each VXI station is linked directly to VME-bus ofIOC(Input I Output Controller) of EPICS which is thecontrol system for the KEKB [6], by the high-speed
Multisystem eXtension Interface bus(MXI). The IOC andUNIX work station for the operator's consoles are connected
through a high~speed network, such as FDDI.

20 local control rooms

In the early stage of KEKB operation, we will install
240 front-end signal processing modules arid 480 RF
switching modules. Therefore four BPMs of the LER andthe HER will be multiplexed to a common circuit and we
can measure the closed orbits of two rings within a few
seconds.

VXI,. ... -----

VXI·------.-~~~~
1------l_____ _~-----­'~------

5 Layout of the BPM system
A schematic layout of the Beam Position Monitorsystem is shown in Figure 6. The electronics units are
distributed in 20 local control buildings around the ring.
The beam signal from the four pickup electrodes for each
BPM are transmitted through independent coaxial cables (theaverage length is 80 m long ).
A 20 em long radiation-resistantPEEK insulation cable
is used between the pickup electrode and the signaltransmission cable to overcome possible damage due toradiation. The expected radiation dose at the BPM connector
is 7.3x1o5 (9.1x107) rad/year for the LER(HER). This
radiation level is manageable by the radiation resistant cable.
The results of the irradiation experiment proved that PEEKinsulation cable withstand 1 Grad of y-ray irradiation.
At the input of the detection circuits the signals are
selected by RF switches. The effects of the cable impedance
variation during a period of several days have been evaluatedfor signal cables of the TRISTAN BPMs. It has been foundthat the equivalent shift of the measured beam position due
to such effects is less than 10Jlm. Since the frequent
measurement of the beam orbit is expected at KEKB, theuse of mechanical RF switch was abandoned at the earlystage of the design studies. We considered two types of RFswitches: a PIN diode switch and a mercury switch. The
result of the on/off switching test, the measured fluctuationof the insertion loss was less than 1x1o-4 for both types ofswitches. This contact fluctuation translates into ameasurement error of <10Jlm. We chose a PIN diode switch
after all, as it was good characteristic as compared with. amercury switch.
After installation of the BPM system, we will also
calibrate each gain ratio of all BPMS about signaltransmitting from a BPM head to a front-end circuit, such asa loss factor of a long cable and a attenuation of constant oflow pass filter and RF switch, using a specially designed 4fold reference signal distributor.

r-----t-------

1------L _____ _I __ ------

6 Main-frame for VXI

Figure 6: Schematic of the BPM system.

[1]

[2]

[3]
[4]
[5]

[6]

-88-

REFERENCES
N.Akasaka et al., "Design of Beam Position Monitor
System for KEKB", Proc. 1996 Europ. Part. Accel.
Conf., Sitges (Barcelona), Spain, June 10-14. /KEKPreprint 96-48.
H.Ishii et al., "BEAM POSITION MONITORSYSTEM OF TRISTAN MAIN RING", Proc. 1987
the 6th sympo. on Accel. Sci. and Tech., Tokyo,Japan.
T.leiri et al., "Design of Turn-by-Turn BPM in
KEKB", Proc. the same conference.
T.Obina et al., "Damped Button Electrode forB-Factory
BPM System", Proc. 1995 Part. Accel. Conf., Dallas,Texas, U.S.A./KEK Preprint 95-41.
K.Satoh and M.Tejima, "Calibration of KEKB Beam
Position Monitor", Proc. 1997 Part. Accel. Conf.,
Vancouver, BC, Canada, May 12-16. /KEK Preprint
97-33.
K.Katoh et al., "Status of the KEKB accelerator control
system development", Proc. the same conference.