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

Beam Diagnoses
Toshio KASUGAHigh Energy Accelerator Research Organization
Science,Structure
MaterialsofInstitute
1-1 Oho, Tsukuba 305 JAPAN

Abstract

2 Detection of beam parameters

Beam diagnostic systems especially for circularelectron accelerators are described. It is important torecord behavior of individual bunches separately as afunction of the tum number in modem accelerators. Not
only several novel methods to observe beam behavior but
also digital data acquisition and processing system arediscussed.

After completion of an accelerator, it is quite
machine design
natural for us to want to measureparameters and observe beam behavior, and it is alsoessential for improvement of the machine. Measurement of
the beam current, the beam positions at various places in
the ring, the transverse and longitudinal beam sizes( distributions of particles in the three directions ifpossible ) and the energy of the beam is quite essential.
Other machine parameters such as betatron/synchrotron
tune, betatron function, dispersion function, chromaticity,
momentum compaction factor and emittance can bedetermined using the measured beam positions, sizes andenergy.

1 Introduction
As accelerators become sophisticated and very small
beam emittances in the three directions including
understanding of
longitudinal one are required,complicated phenomena due to various causes which
deteriorate beam quality and their countermeasures become
important. In the circumstances, beam diagnostics must
play an important role. A beam diagnostic system consistsof two parts, one of them is for sensing of the beam
parameters and the other is for data acquisition Various
novel methods ( devices ) to detect the beam parameters
have been developed, while development of the latter has
also made rapid progress. For example, in an electron
storage ring, the fast beam-ion instability ( FBII ) may
excite the betatron motion of each bunch in spite of theexistence of a gap in the bunch train to prevent theconventional ion trapping effect. The amplitude and thephase of the oscillation of a bunch depends on its positionin the bunch train. In order to observe this kind ofphenomenon, it is essential to record behavior of allbunches independently as a function of the tum number( a lapse of time ) . Recently, this kind of recording
becomes practicable with the progress of digital signal
processing technique including fast AD conversion andlarge memory capability.
Observation of transient beam behavior is often
necessary to understand beam instabilities. Spectrum
analyzers have been used to observe instabilities hitherto,
however the method is incompetent to observetime-varying instabilities. A feedback damper which
suppresses an instability can be used for this purpose.
Transient beam motion is recorded using a digital dataacquisition system mentioned above after the damper isturned on or off.
In section 2, various methods to detect the beam
parameters are discussed. The digital data acquisition andprocessing systems are described in section 3, andfeedback dampers are presented in section 4.

2.1 Beam Intensity, Current

Electrical or magnetic methods have been used todetect the slowly varying ( frequency range ofl0.3Hz-10"Hz) beam intensity. A DCCT is used tomeasure the DC beam current, and a electrostatic
electrodes and loops ( including current transformers
with magnetic cores ) are used as relatively high speedintensity monitors. However, if the bunch length iscomparable or smaller than the size of the electrode, say
3nun, namely the corresponding frequency spectrum
extends up to 1OOGHz, the property of electromagnetic
wave must be taken into account in the designing ofelectrodes and signal transmission lines [ 1 ] . If
synchrotron radiation from stored particle is available, afast PIN photo diode or a streak camera can be used tomeasure the longitudinal distribution of the bunch. These
optical methods can be used with a fast light shutter
mentioned in section 3. Spectral analysis of the signal
from the electrode for the beam intensity monitor gives thesynchrotron tune.
2.2 Beam position

Measurement of the averaged beam positions along
the ring is well-established and its accuracy seems to be
sufficient [ 2 ] . A recent concern is tum-by -tum
This kind of
measurement of a bunch position.
measurement becomes very easy with the progress ofdigital measuring instruments. An example of the injection
o:rbit is shown in Fig.1 [ 3 ] . In the figure, the bottomgraph shows the horizontal beam positions at various
places in the ring at several turns after i~ection and thetop graph corresponds to the intensity ( sum of signals-23-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997...............i...................•• ·j·········· ········:····~~:!~---···:::::>
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200-20;to2

2nd Tum

1st Tum

3rd Tum

34th Tum

4Injection
Fig.

Injection orbit measured with single pass BPM

from four buttons ) . As the orbit was artificiallydeformed, the beam was lost in the 2nd turn. As seen in
the figure, this technique is indispensable to launch a
newly constructed accelerator with a narrow dynamic
aperture.
Spectral analysis of the signal from the electrode for ·the beam position monitor gives the betatron tune.
2.3 Transverse Beam Distribution

Imaging the beam with synchrotron radiation has
been commonly used to measure the transverse distribution
in electron/positron storage ring. There are two main
sources to deteriorate the resolution of the measurement:
the effect of diffraction and the effect of the observationof an extended segment of the trajectory of the beam.
Although the resolution for horizontal emittancemeasurements in the visible region is sufficient forexisting storage rings, the resolution for vertical one isunsatisfactory for rings with small horizontal to vertical
couplings [ 4] . Pin hole imaging of the X-ray is on of
the solutions. An attempt to estimate the vertical beam size
from a diffraction pattern has been tried [ 5 ] . Laserinterferometry is also used to monitor with a nanometer
spot size [ 6 ] .
200
lb = 13.9 rnA

2. 4 Longitudinal Beam Distribution

Streak cameras are widely used to measure thelongitudinal distribution of the beam. The photon counting
method is also used for this purpose [ 7 ] . Longitudinal
distributions of probe bunches that follow a main bunchwith a large current are measured in order to estimate
the longitudinal wake field generated by the main bunch.
A large dynamic range is required for this purpose because
the distributions of the probe bunches, whose currents
have to be small not to interact with each other, have to bemeasured precisely. The photon counting system, which
has large dynamic range, has been used to measure tlie
wake field [ 8 ] . An example of the longitudinal
distribution of a bunch measured with a photon counting
system is shown in Fig.2 [ 8] .
2.5 Beam Energy

The measured magnetic fields of bends are analyzed
to determine the energy of the circulating beam. Since
accuracy of the absolute measurement of the field is of theorder of 10·', the typical error of the energy measurement
has the same order. The resonant spin depolarization
method was used to determine the absolute energy of thebeam precisely [ 9 ] . This method can be applied tomeasure the momentum compaction factor which isdifficult to be measured with usual methods.

3 Data acquisition and processing system

150c0+=l:I-.c·;:::

100( /)'650-200-100

0100time(ps)Fig. 2 Longitudinal distribution of bunch

200

Various methods to detect the beam parameters havebeen developed as mentioned above, while development ofdigital data acquisition and processing systems has also
made rapid progress. For example, in an electron storage
ring, FBII may excite the betatron motion of each bunchin spite of the existence of a gap in the bunch train toprevent the conventional ion trapping effect. The
amplitude and the phase of the oscillation of a bunch
depend on its position in the bunch train. In order toobserve this kind of phenomenon, it is essential to record
behavior of all bunches independently as a function of the-24-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997
Growth of the vertical instability 3ms after turning off V-FB

X 10°

Growth of the horizontal instability 13ms after turning off H-FB

4320250Time (ms)Bunch Number

Fig.3

Fig.5

Time (ms)Vertical betatron oscillation amplitudes ofindividual bunches

turn number ( a lapse of time ) . Recently, this kind ofrecording becomes practicable with the progress of digital
signal processing technique including fast AD conversion
and large memory capability [ 10 ] , [ 11 ] .
Recent progress of bunch by bunch feedbacksystems which suppress the longitudinal and/or transverseinstabilities using digital technique is very rapid as
mentioned in the next section. The systems consist of fast
AID converters, digital memory boards with large capacityand digital signal processors. These are used as a transient
analyzer which records behavior such as betatron,synchrotron oscillations or particle number ( beamcurrent) of each bunch in the bunch train. Fig.3 shows
growth of the betatron amplitudes of individual bunches
after turning off the vertical damper of the TRISTAN AR.
The phase spaces in the transverse and longitudinalplane has been measured using a digital transient analyzer[ 12] .
Usually an 8-bit ADC is used in the high speed
digital data acquisition and processing system. Resolution
may be insufficient in this case. As mentioned in thepreceding section, optical beam monitors have several
advantages compared with electrical ones: large dynamic

00

Frequency(MHz)Growth of instability after turning off damper

range, high speed, high noise-immunity etc. A high speed
light shutter which can be opened or closed within abunch spacing is useful to pursue behavior of a specificbunch in a bunch train [ 13 ] . This kind of shutter using aPockels cell has been developed recently [ 14 ] .
4 Feedback system

Observation of transient beam behavior is often
necessary to understand beam instabilities. A spectrum
analyzer has been used to observe instabilities hitherto,
however the method is incompetent to observetime-varying instabilities. A feedback damper which
suppresses an instability can be used for this purpose.
Transient beam motion is recorded using the digital dataacquisition system mentioned above after the damper isturned on or off. Figure 5 shows growth of instability after
turning off the vertical damper of the TRISTAN AR.
Bunch by bunch feedback dampers has been used todamp the longitudinal and/or transverse instabilities. Delaylines were used in the timing circuits of the dampers up tonow, however, as machines become huge and revolution
periods on which delay time depends become large, delaylines must be long and become unsuited to this pmpose.
Digital signal processors are introduced instead of these _analogue delay lines. Progress of digital signal processing
technique enables us to introduce a small number ofdigital filters in place of a large number of ftlters as manyas the bunch number of the ring. Real-time digitaltechnique seems to be indispensable to modem
accelerators.
The transverse feedback system designed for theKEKB factory is shown in Fig.6.
Acknowledgement

Pockets cell

Polarization filter(polarizer)

Fig.4

Polarization filter(analyzer)

High speed light shutter

The author thanks Dr. M. Tobiyarna who willinglyoffered a block diagram ( Fig.6 ) of the KEKB feedbacksystem and data of the instabilities ( Fig. 3,5 ) .

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'The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997
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Fig. 6 Vertical damper for KEKB

References[ 1 ] F.Zimmennann, et al., SLAC-PUB-7456.
· [ 2] F.Caspers, Frontier of Accelerator Technology(World Scientific, Singapole,1996) P.460.
[ 3] T.Honda et al., Proc. 1997 Particle Accelerator
Conference (Vancouver, 1997) (to be published).
[ 4] P.Kuske, Synchrotron Radiation Sources ( WorldScientific, Singapole,l994) P.244.
[ 5] T.Mitsuhashi, these proceedings ( 1997 ) .
[ 6] T.Shintake, Frontier of Accelerator Technology(World Scientific, Singapole,1996) P.437.
[ 7] T.Obina et al., Nucl. Instru. Methods. in Phys.
Research A354 ( 1995) 204.
[ 8] K.Tamura, Dr. Thesis, Hiroshima Univ. ( 1997).
[ 9] P.Kuske, Synchrotron Radiation Sources ( WorldScientific, Singapole,1994) P.244.

[ 10] J.Fox et al., Proc. Int. Workshop on Multi-bunch
Instabilities in Future Electron Positron Accelerators ( tobe published ) .
[ 11] M.Tobiyama, Proc. Int. Workshop on Multi-bunch
Instabilities in Future Electron Positron Accelerators( Tsukuba, 1997) (to be published) .
[ 12] Y.Kobayashi et al., Proc. 1997 Particle Accelerator
Conference (Vancouver, 1997) (to be published) .
[ 13 ] K. Tamura et al., SPring 8 Annual Report ( 1994 ) ,p.128.
[ 14] K.Tamura et al., Proc. 1997 Particle Accelerator
Conference (Vancouver, 1997 ) (to be published) .

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