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

Design of Turn-by-Turn BPM in KEKB
Takao IEIRI and Takashi KAWAMOTO
KEK, High Energy Accelarator Research Organization
Oho 1-1, Tsukuba, lbaraki 305, Japan

Abstract
A tum-by-turn beam position monitor (BPM) system
is proceeding in the KEKB. The system employs an 1/Q
demodulator which detects ~ch beam pulse from eachbutton electrode at the accelerating rf frequency of 509
MHz.
Not only a transverse oscillation but also a
logitudinal oscillation. can be detected owing to the 1/Qtechnique. An analog gate would be used for selecting afew bunches from a nulti-bunched beam.

1 Introduction
The KEKB[1] is an asymmetric e+ e- collider with twostorage rings named LER and HER. Bunches are injectedfrom the Linac through transport lines. A positron bunchwith 0.6 nC is injected into the LER and an electron
bunch with 1.2 nC into the HER. About 5000 bunches
will be stored with 2 ns spacing in each ring, where theharmonic number is 5120. Total current is 2.6 A in theLER and 1.1 A in the HER.
Two projects to measure a beam position are now
proceeding in the KEKB. One is the BPM system[2]which measures the closed orbit distortion (COD) by
switching each electrode signal using a common detector.
The COD BPM manages about 900 sets of button
electrodes located around the rings. Though the system
has a high resolution of less than 1 J..Un, it cannot
measure the betatron oscillation and a transient change ofthe orbit.
The other project is the bunch-by-bunch
feedback system.[3] The feedback system has high speed
electronics with great memory capacity and can measure
an individual bunch position with 2 ns spacing.
However, the resolution of measured data is limitted by 8bits. The system would demand a tight phase stability
for the rf signal because of using its 4-th harmonics. Inorder to cover the week points of the above systems, a
tum-by-turn BPM system is required.

2 Technical Consideration
2.1 Required specification
A tum-by-turn BPM system is useful for monitoringtransverse oscillations during an injection. Especiallywhen a first beam is injected into the KEKB, the system
may be indispensable. The system should be sensitive
for a small current and detect a beam position of 0.1 nC
charge or 10 J.lA current to use the system instead ofscreen monitors at commissioning the next year. The
damping rate and the betatron tunes measured frominjection oscillations are useful for tuning the machines.

Detection of charge amounts is also useful for tuning
injection efficiency. The system can be used during astoraged state. When an external kick is applied to abeam, the betatron oscilations are excited. The betatronphase is measured by several monitors located around thering, which helps to estimate an error of the Twiss
parameters.
Resolution of 10 J..1m is required toexchartge the system for the COD BPM.
A beam signal is picked up by a button electrodewhich is common to the COD BPM. Since the rms
bunch length is so short that the pulse width of a pickedup signal is less than 100 ps. A peak value of the beampulse is reduced and the pulse width is widened by passing
through a cable from 50 m to 100 m in length.
Estimated voltage would be about 1 to 2 V with aduration of 1.5 ns when one may observe a bunch currentof 0.5 rnA using a 800 MHz low-pass filter (LPF)
installed at the end of a cable. Since the bunch spacingis 2 ns, the fundamental component of the beam is in 509
MHz. Thus, the fundamental frequency is chosen as thebeam detection.
An estimated signal level was ±7.6 mV at the beam
currenlt of 0.1 rnA using a band-pass filter (BPF) of 509
MHz center frequency and 60 MHz bandwidth. The
bandwidth of 60 MHz widens signal duration to 20 ns.
On the other hand, the thermal noise level depends on thebandwidth. It can be estimated to be -86 dBm assuming
the bandwidth of 60 MHz and the noise figure of 10 dB.
Therefore, the SIN ratio of 54 dB is obtained.
Anestimated position resolution is to be about 50 J..1m at 0.1
rnA.
An averaging technique would improve theresolution. The resolution depends on the beam current
and it would be 0.5 mm when a single bunch of 10 J.lA isdetected.

2.2 Signal processing
There are two approaches to process a beam pulse
signal. One is processed in a time domain, where a peakvalue of a pulse is sampled. A jitter of a sampling pulse
causes an error in the detection. When an acceptablejitter is assumed to be 10 ps, the detected pulse width
should be larger than 5 ns to keep the resolution of lessthan 1 x10-5 at which the jitter error will be neglected.
This method is useful for a single shot measurement.
On the other hand, the signal process in a frequency
domain is useful to reject an external noise.
Thebandwidth of the detection determines the response time.
Thus, this system adopts a hybrid technique of the twomethods to achieve the requirements of both fast detection
and high resolution. Frequency components around 509-412-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

MHz are picked up and demodulated with a 20 MHz LPF.
The pulse width of the demodulated pulse is 40 ns, which
would be free from a jitter error.
An I{In-phase) and Q(Quadrature phse) demodulator
was adopted to detect a beam pulse. The demodulator isalso called an rf vector detector. Principle of the 1/Q
demodulator is shown in Fig. 1. A beam signal is split
into two ways with phase difference of 900. We assume
that the two inputs signals are Vb = Vb sin(mbt + Ob) as abeam signal and vif = vif sin(mift +(}if) as a local signal.
Here, it is not required to fix the phase of both signals.
The output signals of two mixers are orthogonal. The
output signals through a LPF arel't = ~ vb vif sin(Ob- oif)(1)

andV2 = -1

2 vb vif cos(Ob- (}if).

Here, coif =cob is assumed.
obtain an amplitude as(2)From eqs. (1) and (2), welVI =~l't2 + vf
(3)Equation (3) gives the amplitude of the beam signal when
the amplitude Vif is constant. A beam position can beobtained from the difference-over-sum ( !l I I:) algorithm
using four amplitudes corresponding to the amplitude
from each electrode signal named A, B, C and D. Thus,
the horizontal position is given by

H=k~-~-~+~.
VA +VB + Vc + VD~

where k is the sensitivity of the electrodes. On the otherhand, the phase between the beam signal and the rf can begiven from the ratio l't I V2 of an electrode signal by(Jb -{}if= tan-1(- l'} ).

v2

(5)

We could obtain a change of the beam phase assumingthat (}if is constant. The detected phase represents therelative synchronous phase since the detection is carriedout at the accelerating frequency.

3 System
3.1 Overview
Block diagram of the tum-by-turn BPM system isshown in Fig. 2, where only one channel of four channels
is drawn. Button electrodes and cables to a control room
are common to the COD BPM system. A switch selects
a beam signal between the HER and the LER. The
feature of the system is that each signal from eachelectrode is detected by the corresponding detector. Thismethod was adopted in the single-passage BPM of theTRISTAN beam transport lines.[4] In the transport
BPM, variations of the position offset were investigated
for three years. The result indicates the variations areless than ±50 J.1 m. Thus, the variations of this system
would be estimated to be within ±0.1 mm, but they are
not investagated yet.
A bench test was performed to measure thecharacteristics of the 1/Q demodulator. A short pulse wasgenerated and passed through a BPF with the center
frequency of509 MHz. A ringing wave was divided into
two ways and rectified by two IIQ demodulators. A peakvalue of a rectified pulse with a 20 MHz LPF wassampled by an ADC described next. First, the dynamic
response of the demodulator was measured by changing aninput amplitude. Figure 3 · shows the intensity as thesum of two amplitudes and the normalized position
represented by the !l I I: value. The lineasity insures arange of more than 40 dB. The normalized position
varies with ±2 xl0-3 , which corresponds to the position
variations of ±60 J.lm, where k=30 mm is assumed.
The rms noise of the demodulator was about 3 counts for
2k sampling data in the ADC, where the ADC has 14bits. When a signal level is half of the fun· scale, theSIN is estimated to be 62 dB.
3.2 Components
An ADC module using a model ADS-945 (DATEL)
was developed in VXI C-size. The VXI bus is used for ahigh-speed transfer of obtained data. The specifications
are listed Table 1. The ADC has the memory capacity of64 kW per channel with 14 bits. Total 8 ADCs areneeded for one BPM. The rms noise of the ADC is 2
counts in 2k samplings.
Table 1 Specifications of VXI-ADC.
Resolution 14 Bits
Bandwidth 20 MHz
Memory64 kWich, 2chClock
5 MHz max.
Input Range ± 1.25 V

Figure: 1 Principle of 1/Q demodulator.

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

ExL

- 1 channet.of 4 drawn -~Cali-Start Pulse

By-pu~

ButtonElectrodes@: ~+-30MHz
G=30dB

10 ns

14 Bits
64kW/ch~Clock

Cable Delay

MultipleClock

DelayedStart

ModuleExL
Revolution

100kHz to SMHz

1attenuation with 4 dB step to 28dB---------------delay
2ns step
withVXI

13 bits

L - _ _ _ _ _ _ _ _ _ _ _ _ _ __._ _ _ _Dl/0

L _dividerratio..{!f5121!::._1/160j_ ~ _ _ _ _ _ _ _ _ _Figure: 2 Turn-by-Turn BPM system.
The author (TI) wishs to thank the staff of the KEKB
beam monitor group, especially ProfessorS. Hiramatsu
for useful comments,.,.
01000

PeriodRF Clock••51201111111111111 <fi IIIII 111111111111
Start Pulse
Delayed Pulse with D=3 clocks
Multiple Delayed Pulse with M=810 L..J...J...JU...U...L..L.L..J...J....I...J..J...J....I..J...J....L.J...J....I...J..J....U...L..L...L..L..L..U..J..J 10-35 -30 -25 -20 -15 -10 -5
00 0 0 0 0 u 0 0 0 0

Attenuation(dB)Figure: 4 Multiple delayed module.

Figure: 3 Dynamic response of 1/Q demodulator. The
attenuation of OdB corresponds to the bunch current of0.28 mA.
An analog gate[5] which has a switching time of 0.2
ns was developed to cope with a multi-bunched beam of 2ns spacing. However, the gate width will be expanded to
10 ns to reduce switching noise. Thus, five successive
bunches (a batch) will be gated.
A delay module TD2[6] was widely used in theTRISTAN. The module outputs a delayed pulse for arevolution period with a 2 ns step. In order to sample
several batches within one revolution, the TD2 module
was modified. The multiple delayed module outputs
equally spaced pulses for a delayed revolution pulse in one
revolution as illustrated in Fig. 4. When the multiple
factor M is 32, the module outputs 32 pulses in arevolution. Thus, 2000 turns data of 32 batches will besampled and S10red.

Rererences[1] KEKB B-Factory Design Report, KEK Report 95-7

(1995).
[2] M. Tejima et. al., in these proceedings.
[3] E. Kikutani and M. Tobiyama, "Strategy for
Developing Fast Bunch Feedback Systems", KEK
Preprint 97-14 submitted to the Particle Accelerator
Conf., Vancouver, BC, Canada (1997).
[4] T. Ieiri and M. Arinaga, KEK 93-2 (1993).
[5] T. Ieiri et.al., "Development of an Analog Switch
Gating a Single Bunch", Proc. of lOth Symposiumon Accelerator Science and Technology, Hitachitaka,
Japan (1995) 142.
[6] K. Ishii, KEK Report, KEK83-14 (1983).

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