The 9th Symposium on Accelerator &ience and Technology, Tsukuba, Japan 1993

Design and Develop ment of Single-P ass Beam Position Monito rfor TRISTAN Main Ring
M. Arinaga, K. Mori, H. Ishii, T. Naito and M. Tejima
National Laboratory for High Energy Physics (KEK)
Tsukuba, Ibaraki 305, Japan

B.

Abstract

This paper describes the design and development of thesingle-pass beam position monitor for TRISTAN MainRing(MR). This system has been developed in compliancewith the requirement to measure and detect the beam position
every tum-by-turn. At the test run, it caught the decay ofbeam oscillation.

I. INTRODUCTION
Beam positions in the TRISTAN MR have been measured
by the super-hete rodyne method. It has many merits.
However, it has faults in that it cannot measure the beam
position each tum and that it takes a long time to measure
COD. This is the reason why this single-pass beam monitor
system was designed and developed. One set was constructed
and installed in the east symmetric al point section ofTRISTAN MR. This monitor adopted, what we call, theAM/PM method, which is based on the amplitude-to-phaseconversion.

The principle of AM/PM method

Figure 1 shows the block diagram of AM/PM method.
The calculations are made on signals from two pairs ofelectrodes separately, the AC-pair and the BD-pair out of fourelectrode A, B, C, D. As shown in the block diagram, when
the beam passes, a signal is generated in each electrode pair,and from the ratios between two signals, the beam position iscalculated.
Figure 2 shows a vector diagram to convert amplitude tophase. A band pass filter produces transient sinusoidal
ringings from beam signals as shown in Figure 3. The hybrid
IC gives them rc/2 phase shifts. The output signals are asfollows:CA'inP,ut signal

II. THE OUTLINE OF AM/PM METHOD
AL!l-There are many methods to realize .single-pass beam
monitor. We adopted the so-called ratio method which need
not normalize resultant signals by the beam power.[l] Button
electrodes pick up the beam position signals in this system.

A.

2output signal

The characteristics of AM/PM method

The AM/PM has the following characteristics:
a. Electrical circuits treat narrow band signals.
b. Using band pass filters, it is strongly resistant tonoise.
c. The ratio method does not need the normalization
process.
d. It makes the system a little expensive.

Figure 2 : The vector diagram for changing amplitude tophase difference by rt/2 hybrid IC.

Phase
Detect2+b=2.5+
f=IOOMHz

1dB

EX·OR
7C(2Hybrid7C(234dB

Rev.

RFFigure 1: Block diagram of AM/PM method for TRISTAN main ring.
-379-

III. The system description
E= A+ CL;,A.

Construction

F= C+ AL;,
Construction of the system is shown in Figure 1.
The sampled data are recorded in ADC memories at thetiming of each beam, and H80 computer calculates the Bx andBy value.

whereAtan(~)= C ,tan( a)= ;1t~+a= 2, ~-a=B1.

B.

We then havetan( ~1 ) =tan(; -a)Performance is as following.
a. Bands pass filter
fo = lOOMHz, bw = 2.5MHz
insertion loss = 2dB
b. First amplifier
Wide band(-l.lGHz), gain= 22dB
(J.LPC1658C, NEC)NF = 2.5dB
c. Last amplifier
Wide band(-400MHz) , high power
gain= 34dB, NF = 3.5dB(CA2830C,MOTOLORA)
d. ADC/Mem module
1.639mV/bit. (AD9020JZ, ANALOG DEVICES)e. Minimum bunch space timelJ.LSecf. Record length
16384 turns/bunchg. Detectable current range
lbeam = 0.15mA - lOrnA
h. 1t/2 Hybrid
87 degrees - 93 degrees(JH-119, anzac)
insertion loss = 2dB
i. Comparator /PSD
0 degrees- 180 degrees(MC10H107L, MOTOLORA)(AD96687BH, ANALOG DEVICES)
j. Low Pass Filter
fL=4AMHzk. lObits ADC's minimum resolution<50J.Lmtan ( : ) - tan (a)=----=----1 +tan ( :) x tan (a)A-C= A+CI~=tan -t( A- C)
2A+C

Similar relations hold in the BD-pair:

Signals proportional to &2 are available at the outputs of thephase detectors.
Using the following equations, the position from thecenter of the chamber is derived:

Bx = ,. x B1 - B2 ,':.x2
By=~X

yB1 + S2

2Here, ~x and ~y are the coefficients of sensitivity
characteristic of the types of chamber, an example of which
given Figure 4.

C.

I= 1.4mA (Positron)

Signal ASignal C
Signal B

Signal D
Figure 3 :Ringing signals from BPF.
The vertical scale is lOmV/div.
The horizontal scale is 50nS/div.

Correction of errors and sensitivity

C-1. Errors
There are the following error sources.
a. Difference in cable lengths
b. Difference in gain of amplifiersc. Difference in phase characteristic and insertion loss ofHybrid ICs
d. Difference in phase characteristic of comparators
Correction and calibration[2]The errors caused by parts can be minimized byadjustment, if a set with similar performance is available.
However, the error caused by the amplifiers cannot beovercome, so the best choice of the amplifier IC is essential.
The last, remaining errors can be corrected by software basedon calibration of the system.
The calibration of the circuit was performed at the benchtest system. Moving a signal source inside a test chamber, weobtained mapping from the real signal positions into thepositions given by the present AM/PM circuit. An exampleof the mapping is given in Figure 4. Comparing the resultant
mapping with that given by the conventional super-heterodyne
circuit, we identified individual error sources and corrected
them. Some attenuators are inserted in order to make
impedance matching. This is useful also to reduce reflectionand oscillation in the signal. After trial and error, 3dBs'
attenuator is inserted at the front of BPF and 1t/2 Hybrid IC at
the present. The cable is all semi-rigid cable to avoid theinterference oscillation of signal and error. Figure 4, which is
C-2.

'93/05/24

Performance-380-an example of mapping and the sensitivity coefficients ~x and~y. mentioned section II, respectively gives the conversion
coefficient from phase to beam position as 0.235 rom/degree
in x direction and 0.239 rom/degree y direction.
Though the old system utilities 12bit-ADCs, the present
system adopts lObit-ADC. In order to examine the effect of
this low resolution, a -50dBm test signal is fed to the test
chamber and the resultant ADC outputs are comparedexchanging the input electrode pairs.

..

MAPPING of BPM
VOAT2¥MAPAP015. OAT/93/05/20...

!

/y=6mm~Figure. 5 shows the beam oscillation at the injection.
According to the FFT analysis made at the injection, both ·betatron and synchrotron oscillations appear at the injection.
The betatron oscillation decays rapidly while the synchrotron
oscillation persists. The observation shown in Figure 5 isconsistent with the FFT analysis shown in Figure 6. Verticaland horizontal tunes agreed with values measured by ananother tune-meter.

~~~

x=20mmx=-20mm

IV. PERFORMANCE~:;:~' :::•1'""'"'00 .

~. f-"'"~''"";-::::.~......: .......: ......
>::: -·fVYVI(yVlff-i~f v~rn~Y.fY

vi11Ffi41v~rnn~PnIIHIN··

Rovolution~ f::±::J

Figure 5 : An example of oscillation of electron beam.
This oscillation occurred due to a vertical kicker,
when electron beam is circulating at theinjection stage.

"'--r=-6mm

DM•a~o.., ... 69"'"'.,.

...

""

1

Degreet"UI...t"\..DI/./~y=0.239_,.,at antenna position y=O.

1'9~"" """•• '•••·

Figure 4 : An example of mapping by AM/PM method.
~x • ~y are coefficients of sensitivity.
Dx, Dy are values of offset from geometricalcenter.
x, y are antenna position

0Sensitivity
C-3.
After the correction and calibration, this system's gain
became 46 dB at the maximum, and the detectable range ofbeam current was from 0.15mA to lOrnA.
Improvement
We are trying to increase the amplifiers gain toapproximately 50dB in order to lower the limit of detectablecurrent range down to O.lmA.
Output of the PSD, driven by outputs of the comparators
in ECL levels, is uni-directional, while the ADC is fabricatedso as to mate with bi-directional signals. We are going toinsert a level shifter between the PSD and the ADC so thatthe full range of ADC is efficiently used. This has anotheradvantage, that is, to simplify the data processing because theADC output becomes zero when the beam is in the normalposition.
The present system memorizes only the ADC output tosave the time of data storage. It is desirable to save the data in
the form of beam position.
C-4.

••.,~" •··;••,.

·~:!lot.

Y.Position

1R~~~~~;a~~:J...

at antenna position x=O.

1020

JO40

KHz50Figure 6 : FFT analysis of electron beam
1. Vx = 35.30KHz, 2. Vy = 21.04KHz, 3. Vs = 11.67KHz

V. CONCLUSION
Developmen t of the single-pass-m onitor is very
successful. The ability to catch the beam oscillations must
extend the application of the beam position monitor.
Four more systems will be installed in the near future, inwhich improvements suggested in this paper will be made.

VI. REFERENCES[1] J. Borer, "Instrumentation and diagnostics used in LEP
commissioni ng, with accent on the LEP beam orbit
measuremen t system", Workshop on Accelerator
Instrumentation, F.N.A.L.,Batavia, Il.,USA, (Oct. 1990), pp.
1-34[2] H. Ishii et al, "Beam position monitor system ofTRISTAN Main Ring", Proc. Accelerator Science andTechnology, Tokyo, Japan, (Oct. 1987), pp.207-209-381-