BEAM INTENSITY AND SPILL CONTROL BY A PERSONAL COMPUTERSYSTEM AT INS 1.3 GEV ELECTRON SYNCHROTRON
H.Masuda, M.Muto, K.Norimura, and K.Yoshida
Institute for Nuclear Study, University of Tokyo
Midori-cho 3-2-1, Tanashi, Tokyo 188, Japan

ABSTRACT
A personal computer system for controlling beamintensity and beam spill has been introduced to theelectron synchrotron at Institute for Nuclear Study,
University of Tokyo. When the beam intensity
deviates from the initial value beyond the upper orlower limits, a pulse motor system is operated torestore the beam intensity within the limits. The
beam spill is monitored by sampling the spill wave
form at two points in the spill time and when an
unbalance between the intensities at two monitoring
points, an analog signal to correct the unbalance issent to the rf system. With this system, we can keep
the beam intensity and beam spill stability for morethan 10 hours with no manual tuning of thesynchrotro n parameters .
INTORODUCTION
The 1.3GeV electron syncrotron (INS-ES) has been
extensivetoDue
1961.
since
operated
improveme nts, the operation stability as well as thebeam intensity and the duty cycle of the beam haveimproved. In current experiment s bybeen muchusing the synchrotro n beam, so-called photon
tagging method is often employed. For theseexperimen ts, it is needed to maintain the beam
intensity at a certain constant value during anexperimen tal run. Also the stability of the beam
spill from ... the machine is very important to reduce
accidental coincidenc es in the particle detection
system. We can attain the duty cycle of the beam as
high as 10-20% by using special devices introduced
in 1985 [ 1]. Since the repetition rate of thesynchrotro n is about 20Hz, above duty cycle
correspond s to a beam spill time of 5-lOmsec. The
beam spill should be uniform during this spill time,and it should be also stable throughou t theexperimen tal run.
The tuning of the machine for providing the beamwith above conditions is made once a week, usually
on Tuesday morning after one-day shutdown on
Monday for maintenanc e work. However, perfectstability can not be attained until the temperatur esof the room, cooling water and every machinecomponent s reach constant. Even after the initialinstability has damped, various disturbanc es canoccur such as fluctuation s of line voltage,
temperatur e and s.o on. We can, in principle,
suppress instability due to these disturbanc es byintroducing an appropriate feedback loop. The mostsimple way may be to introduce a single feedback
loop connecting the final output and the parameterthat gives the most strong influence on it.
Under these considerations, we have introduced asimple feedback system by use of a personal
computer to keep the beam intensity constant andto maintain the stable beam spill. The feedback loop
to control the beam intensity was originallydeveloped 9 years ago [2].
PRINCIPLE OF BEAM INTENSITY CONTROLThe major origin of the fluctuation of beam
intensity at INS-ES is the energy variation of thebeam from the injector linac. The beam from thelinac, whose standard energy is 15MeV, is deflectedby an electrostati c inflector to be guided on theinjection orbit of the synchrotro n. The injection
timing is determined in such a way that the guiding
field of the synchrotron at injection is just for thelinac beam energy. The successful injection occurs
when both of the inflector voltage and the injection
timing are correctly adjusted to the linac beamenergy. For maintainin g the beam injection stable,
we must keep the linac beam energy constant, or
adjust the inflector voltage and the injection timing
to follow the energy variation of the linac beam. For
the low-energ y electron linac, in which theaccelerated electrons are not perfectly relativistic , itis not possible to adjust the beam energy withoutchanging other beam parameters such as intensity,
emittance and so on. Thus we have chosen theinflector voltage and the InJection timing as
parameters for which feedback is to be applied.
Manual operation experience shows that we cankeep the final beam intensity constant by adjusting
above two parameters only.
Obviously there exists a correlation between thecorrect values of the inflector voltage and theinjection timing in accepting the linac beam. So wecan, in principle, adujst these two values by a single
parameter. But the inflector voltage and theinjection timing circuit themselves are not perfectly
stable. So we treat them as independen t parameters
to compensat e possible instabilitie s accompany ing
them.
PRINCIPLE OF BEAM SPILL CONTROLAt INS-ES, the final beam is spilled by controlling
the rf for acceleratio n. When the final beam energy
is high (>700 MeV), where the synchrotron radiation
loss is large, the beam spill can be well controlled
only by amplitude modulation in the rf voltage. For
the lower energy beam, we must apply rather
sophisticat ed method to get smooth spill during longspill time (5-10 msec), as described below [3].

-258-The frequency of rf, 138MHz, is frequencymodulated during the spill time desired ·by thebeam user. When this modulation frequency
concides' with the synchrotron oscillation frequencyof the electron bunches in the synchrotron, ormultiples of it, the electrons in rf backets undergo
the parametric resonance, and gradually spilled outof the backets. Since the synchrotron oscillation
frequency changes according to the acceleration,even in the spill time, we have to change themodulation frequency during the spill time to obtaina smooth beam spill. For this purpose, a voltagecontrolled oscillator (VCO) is used. By applying aprogrammed voltage pulse, whose width is to be thespill time, to VCO, we can generate desired
modulation frequency in any point of the spill time.
To avoid too much complication, we employ a
exponentially decreasing voltage pulse with anappropriate bias.
Thus the instantaneous spill intensity depends on
many parameters such as the rf amplitude, theamplitude of the frequency modulation, theelectoron beam intensity at that instant, and so on.
An example of the beam spill obtained by the above
mentioned method is shown in Fig.l.

BeamIntensity

A block diagram of the beam intensity and thebeam spill control system is shown in Fig. 2. The
2222
CAMACFig.2 Block diagram of beam intensity and beam
spill control system.
circulating current in the synchrotron is monitored
by a current transformer installed in a straightsection of the ring. Using an amplifier, the output of
1V corresponds to the circulating current intensityof 100 rnA. Whereas the beam spill is monitored bya scintillation counter located at the beam channel,
which measures the time structure of the beam sent
to the experimental hall. Examples of these signals
are schematically shown Fig. 3. The final beam
Beam lntensty

Beam Spill

0P.G outBARFVoltage
Beam Spill

Fig.l An expample of signals of beam intensity,
beam spill, voltage pulse for frequency
modulation on rf, and rf voltage. Small beam
spill behind the main spill is not relevant forthis work.
Since the beam spill is affected by many
parameters, it often suffers from the instability. The
most typical case is such that the spill is enhanced atthe begining of the spill time and is diminished at
the end, or vice versa. We have found that this"asymmetry" of the spill can be corrected byadjusting the voltage pulse for VCO, simply varying
the bias of it. Thus the feedback to correct the beam
spill asymmetry is applied to the voltage pulse
generator. By this simple way, we can expect tohave a stable beam spill for fairly long time.
APPARATUS

DC-2

0+2

msec

Fig.3 Schematic feature of the beam intensity andbeam spill signal. Voltages at arrows aresamled.
intensity is measured as the difference of thevoltages sampled at the points A and B in the figure.
Difference of the voltages at A and B is sent to ADC.
To know the asymmetry of the beam spill, theintensities at points C and D, which are at, forexample, ±2msec from the end point of theacceleration, are measured by use of sample-holdcircuits.
These analog signals are digitized by ADC circuits
in a CAMAC system, which is controlled by apersonal computer.

-259-Stepping motors to adjust the inflector voltage
and the injection timing are driven by the CAMACsystem. On the other hand, a DAC module in theCAMAC system generates a de voltage to control thebaias of the voltage pulse to the VCO circuit foradjusting the spill asymmetry.

ContorolBeam Intensity

Range

139/ll/07 22: 35: 00
start 17:32:23

S6:0.173 - 0.186

Spi II As'lfl\.

: -3.6 '+3.8.4.3

Not Controlled

Controlled.2~~~~~~$~= =~*~'~~~~~==~~~~~.I

SOFIWARES
After manual tuning of the machine parameters
to provide the beam as required by the beam user,
the computer control program is started. The beamintensity and the spill intensities (voltages at twopoints C and D, Vc and Vd, as described in thepreceding section) are read 100 times every 30
second. As for the beam spill, the following
calculation is made for each read out :Vc- Vd
S=-Vc+Vd
For every 100 samples, mean values and standard
deviations of the beam intensity and the asymmetry
parameter S are caluculated. They are plotted in thegraphic display. Initial values of them arememorized as references for the following control.
When the mean value of the beam intensity
and/or the spill asymmetry parameter deviate fromthe reference value by f times standard deviation,
where f is assigned beforehand, control commands
are generated. All the above process is finishedwithin 30 sec. If the beam intensity has decreased,
the inflector voltage is optimized, followed byoptimization of the injection timing. For the case ofintensity increasing beyond the limit, the inflector
voltage is shifted from optimum value until thebeam intensity come back within the limit. It is
usually needed several times of monitor and control
cycle, each cycle is repeated every 30 seconds,'before the correction is completed.
The logic to control the beam spill asymmetry issimpler than the one to cntrol the beam intensity.
According as the spill asymmetry parameter
becomes positive or negative beyond the limit, thebias of the voltage pulser for VCO ·is increased ordecreased.
RESULT AND DISCUSSION
Fig.4 is a hard copy of the computer display
showing the variation of the beam intensity and thebeam spill asymmetry during 5 hours. From thisresult, we can remark the following characteristics.
(1) For the computer controlled period, the beam
intensity is quite stable. (2) The beam spill
asymmetry is also well controled to be kept around
S=O. (3) The beam intensity is also stable even for
the non-controlled period. It is known that the beam
intensity becomes stable one or two days after
starting the operasion, espescially at midnight. Thisdata was taken just under such condition. (4) The+4~ f======== ====;==== ======== =l• 20

Beam Spill Asymmetry·<~C.<.c':•;•.:,•.:,·.>:,~;:,_,.

-:::;"·:·"~·<,

...

~.,

-40 l.J......J_J..._jJ~--'-'----'-1~~'--'-J-'---L.-'-'-L......iJ-'-'--'--'-l.....JI~
Time (IW!DivlFig.4 Beam intensity (upper graph) and beam spill
asymmetry (lower graph) monitored for 5
hours. Mean values and their standard
deviations are plotted. Right hand side is theresult with the control, whereas the left handside is the one without control.
beam spill asymmetry is very unstable when it is
not controlled by feedback system.
It is seen in Fig.4 that the standard deviation of
the spill asymmetry is large in the controlled region
while the mean values are well controlled to bearound zero. This is because present control system
works with a time constant longer than 30 seconds,
and has no ability to control short term deviations.
Thus this system aims at keeping long term stabilityof the synchrotron beam.
ACKNOWLEDGEMENTS
The authors wish to thank Prof. H.Okuno for his
carefull reading of the manuscript, and K.Watanabe
for his. advice on electronics circuits. They are also
thankfull to the memburs of INS electron
synchrotron group for their cooperation on thiswork.
REFERENCES[1] K.Yoshida et al., IEEE Trans. Nucl. Sci., Nill(1985)
2688[2] S.Asaoka et al., Proceedings of the 3rd
Symposium on Accelerator Sience andTechnology. P.133.
[3] T .Fukushima et al., INS-TH 72(Institute for
Nuclear Study, University of Tokyo, 1971)-260-