BEAM MONITOR OF INJECTOR SYNCHROTRON FOR UVSOR
Hiromichi Yamamoto
Department of Electronics, Fukui Univ.
Toshio Kasuga
Institute for Molecular ScienceIntroduction
Cylindrical beam monitors were designed and made to detect both theposition and the bunch shape of the electron beam in the injector synchrotron
for UVSOR. The synchrotron consists of six bending sections and six straightsections in which the quadrupole magnets, the perturbators and the otherdevices are installed. Position monitors will be set up one by one in eachof the straight sections, and a fast intensity monitor will be installed
separately.
Electrons injected from the 15MeV linac are accelerated by the RF
power of 90.2MHz. In the beginning of the acceleration cycle, the bunch
length is about 1.6m, and about 0.19m at the end of the cycle. The electrostatic type is chosen as the fast intensity monitor which detects the changeof the bunch length because of its simple structure and its good frequency
response. And it is also used as the position monitor. As the space in thestraight section is limited, we adopt the monitor with four sectors becausethat is capable of detecting both the horizontal and vertical displacementwith single monitor head.
1.

Cylindrical Beam Monitor
The angular distribution of the electric field arising from a pointcharge at rest is uniform, but in the case of the moving charge, the more
the velocity of the point charge increases, the less the electric fieldcomponent parallel to the velocity becomes by the Lorentz contraction.
As the electric field of the point charge moving at the velocity of light
has the same distribution as the line charge, it is easy to calculate theamount of charges induced on the monitor electrodes.
We calculated the sensitivity and the output voltage from the amountof charges. Two electrode configurations of four sectors shown in Fig. 1
may be adopted. The formulae of the relative sensitivity are also given inFig. 1. The sensitivities, Sx(X,Y), of Type A and B were calculated. The
calculated values of Sx(O,O) are 13.8(1/m) for Type A and 19.5(1/m) for Type
B. The maximum deviations of Sx(X,O) from Sx(O,O) are 5% for Type A and
9% for Type B when the horizontal excurtion of the beam is within 30(mm).
When the vertical coordinate of the beam is 20(mm), they increase up to 10%
and 13% respectively. The linearity of Type A is better than that of Type B.
And the output signal processing of Type A is easier than that of Type B.
We chose the configuration of Type A from these calculations. The dimensionof our monitor is shown in Fig. 2.

2.

Measurement and Accuracy
We measured the linearity of Type A monitor on a bench assembly. The
measurements were performed as follows. From a signal generator of 50MHz,
RF signal was supplied to an aerial which simulated an electron beam going
through the monitor. The signals appearing on the electrodes were fed toa matching transformer with an impedance ratio 4:1 in order to transformthe impedance of the monitor head into the impedance of the observing system
of 50n because the time constant of the monitor, T=RC, must be longer than
the time length of the bunch, where C is the capacitance of the monitor, R is
the load resistance. The differences between the signals from two facing
electrodes were obtained by means of a hybrid transformer, and were observedwith an oscilloscope. The results of the measurements are given in Fig. 3.
The vertical and horizontal axes show the amplitude of the difference signal
3.

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and the position of the aerial respectively.
The good linearity is obtained
within X=±l8(mm). The measured sensitivity agrees well with the calculatedone.
We will now discuss how the errors of the capacitances of the electrodes
cause an offset on the measured beam position. The offset, ~Xe, due to thecapacitance error is represented by~X=~·~e8/2C
where r is the radius of the monitor, ~C and C are the capacitance differenceof two facing electrodes and the average capacitance of the four electrodes
respectively.
In our case, r is 65(mm), Cis 18.4pF and ~Cis 0.7pF. The
offset of the position measurement will be 0.6(mm).
4.

Conclusion
The cylindrical beam monitor of Type A is better than the Type B monitor
with respect to the linearity and the easiness of the signal processing.
Though the offset due to the capacitance error is not satisfactory, it isimproved provided each of the capacitances of electrodes is corrected. The
frequency range of the whole monitor system including the matchingtransformer is from 40MHz to lGHz and it is sufficient for our purpose.

Type A2_.......

3(-)14

Sx·.dX:Ot-01

o,.ova 3 .a,

TypeS
o: Y= O(mm)
X:Y:lO(mm)

2[\1>
E·

3\_A.

>
0(J)

Sx·.dXz 0,-0!-01.0<

a 1 •C:·~·a,(2)/Fig. 1 Two configurations offour sectors and relative sensitivity.

III)?
/30X(mm)shieldelectrodeFig. 2 Dimension ofmonitor.

Fig. 3 Measurement of the linearityof Type A at 50MHz.

Reference
T.Katsura and S.Shibata
BEAM POSITION MONITOR FOR THE PHOTON FACTORY STORAGE RINGKEK Report 79-27, November 1979-124-