JAERI-Conf 95-021

PA50
Single-Pass Measurements of Injection-Beam Positionat the Photon Factory Storage Ring
Tohru Honda
Photon Factory, KEK, National Laboratory for High Energy Physics
1-1 Oho, Tsukuba, lbaraki, 305 Japan

Abstract
A single-pass beam position monitor (BPM) systemthat monitor injection-beam positiOns is un<i!rdwelopment. A signal extracted through a button
electro<£ was retected using a high-speed digitizer.
Waveform of the button signal was recorded by real-timesampling. The waveform could be measured with enoughsensitivity even for positron beam as well as electron
beam. The beam position was retermined with aresolution of a few hundred microns.

I. INTRODUCTION

Photon Factory ring (PF ring) is a 2.5 GeVelectron
I positron storage ring dedicated to a synchrotron radiation
source. In 1997, reconstruction for a lower emittancelattice is scheduled. [I] The reduced emittance is achievedby replacement of quadrupoles and sextupoles in FODO
cells. The brilliance of synchrotron radiation will be
increased by a factor of I 0. In the normal-cell sections,vacuum ducts are replaced by new ones, and the number ofquadrupole magnets and BPMs are doubled. All electronic
components related to COD measurements will be alsoupdated. [2]In the low-emittance configuration, there is a weak
point of a small dynamic aperture. The small aperturewould remand a strict correction of the injection beampath. A COD correction in advance of beam storage will
be inevitable. So a BPM system to monitor the injection
beam is being prepared for the sake of efficientcommissioning after the large reconstruction.
Infollowing sections, we will report about a single-passmeasurements of button signals using a digitizing
oscilloscope and results of a preliminary position
measurement.

mm diameter. Thefourelectrodesnumberedin figure I (b)were used in the present measurement. An output voltageof these large buttons is estimated to be a factor of four
higher than that of the small buttons in the BT line.
A bunch signal extracted by a button electro<£ was
led to a digitizing oscilloscope through a double screened
coaxial cable of 20 m long. The cable had an attenuation
factor of 3 dB/I 0 m at the frequency I GHz. The digitizing
oscilloscope (Tektronix TDS684A) has four channels
with fourdigitizers. Each channel is a8-bit digitizerwith
a maximum sampling rate of 5X1 09 samples/s and aanalog bandwidth of I GHz. The signals from four
buttons weresimultaneously recordedin thefourchannels
by a real-time sampling. A time base for the measurement
is a trigger pulse of an injection kicker magnet. Amaximum record length was 15,000 for each channel. 10
turns of bunch signals that revolving at a period of 625 ns
could be recorded tum by tum with a sampling rate of
2.5x109 /s.

(a)

(b)

Figure I. Dimensions of button BPMs installed in the beamtransfer line (a) and the storage ring (b).

ll.MEASUREMENTPROCEDURE
We have installed three BPMs in a beam transfer(BT)line between the injector linac and the ring. A schematiccross section of the BPM is shown in figure I (a). It has· a circular cross section of 38 mm diameter. Four button
electrodes of 10 mm diameter are placed as in the figure. Inthe storage ring, a vacuum duct has a polygonal crosssection. EachBPM of the PF ring has six electrodesof30

A beam position is calculated from the peak-heightratios, U and V.

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U = ((V2+V3)-(V1+V4))/(Vl+V2+V3+V4),
V = ((Vl+V2)-(V3+V4))/(Vl+V2+V3+V4),

JAERI-Conf 95-021

where V# means a peak height of the #-th electrore's
signal. Because the injection beam position would changewith a large amplitude, nonlinear behavior of the ratios has
to be taken into account. For the BPM of the storage ring,
U and V were computed in a range, 0 mm <X, Y < 16mm. Figure 2 shows the U vs. V plot. We determine thebeam position using following quadratic equations.

good signal-to-noise ratio. Compared with simulated
waveforms of the button signals based on a Gaussian
approximation, bunch lengths (standard reviation) of theelectron and positron beams were estimated to be about 0.4
ns andO. 7 ns, respectively. For the both injection beams,
the present measurement system had a enough sensitivity
to deduce peak heights from the button signals.

X [mm] = (12.4 U 2 + 21.8- 17.7 V2) U.

60

y [mm] = (-79.0 U2 + 65.5 + 71.3 V2) v.

40

These approximations are valid up to 16 mm with an errorof about 3%. For the BPM in the BT line, following
equations are adopted. These are valid up to 10 mm withan error of I 0 %.

20g>

0> -20X [mm] = ( 13.5 U2 +12.4- 15.0 V2 ) U-40y [mm] = (- 15.0 U2+ 12.4 + 13.5 V2) v-60
00.5468108

10time (ns)

0.3

••••

0.20.1(a)•• •• •• • ••• • •0.4>

2302010g>0 ~ww~~~ww~~~~~wu~~
00.1

0.2

0.3

0.4 0.5 0.6

0> -100. 7-20

u-30
0Figure 2. A (U,V) plot for the BPM of the storage ring. Solidsquares are plotted by a 4 mm step for a (X,Y) range from 0mm to 16 mm.

246time (ns)(b)
Figure 3. Measured waveforms for electron (a) and positron(b) injection beam pulses.

III. MEASUREMENT RESULTS
An injector of the PF ring is a 2.5-GeV linac.
Positron or electron beam is available for usual operation.
Measured waveforms for electron and positron pulses areshown in figure 3 (a) and (b), respectively. Those wererecords of the first-tum's signals observed with one buttonin the storage ring, while RF acceleration system was not
powered Solid squares in the graphs are the sampling
points at a rate of 5Xl09 samples/s, and the solid lines are
curves drawn by a 3rd orrer spline interpolation. Charge
per bunch for the electron beam was about 2Xl o-w C, thatwas estimated from an output of a current transformer inthe BT line. Charge of positron beam was an orrer ofmagnitude less than that of electron beam. The waveformof the positron bunch was measured with an amplifier of
20 dB gain and I GHz bandwidth. The amplifier waslocated in the neighborhood of the BPM in order to keep aFigure 4 is records of revolving injection beams(positron) observed with one BPM in the storage ring.
Four lines in a figure were signals of the 4-button
electrores, and they consist of 15000 points at an rate of
2.5x I 09 samples/s. Nine spike-like peaks at an interval of
about 600 ns correspond to the first- to ninth-tum's
signals after the injection. Figure 4 (a) was a record
measured when all operation conditions of the storage ringwere the same as the usual ones except for the RFacceleration system. Beam would be possible to be stored
if the RF system is powenrl. On the other hand, figure 4(b) was a record measured when an abnormally highoctupole fields were applied At this condition, no morestorage was possible because of the very small dynamic- 169-

JAERI-Conf 95-021

aperture. Rapid attenuation of the peak heights wereobserved in the several turns.

0.3 mm and 0.8 mm, respectively. The poor value for thevertical was mainly due to a rectangular arrangement of thefour electrooos. It is not the case for the BPM in the BTline that has the same position sensitivity for bothdirections.

642

85 0>--4time (1 us/div)

-6(a)

l

Ilf'I

-2-6-4-2

0

2

46X(mm)I!

I!

Figure 5. The first 9-turn beam positions deduced from thepeaks in the figure 4 (a).

>~EIV. SUMMARY

0~>

il

1:

I iI

I:

Ii'I!

time (1 us/div)(b)
Figure 4 The first 9-tum signals from four electrodes of one
BPM in the ring measured with usual condition (a) and withabnormally high octupole fields (b).

In the present measurement, it would be confirmedthat the button signal of the injection beams could bemeasured with good sensitivity to ootermine the beam
position. Now, we are going to develop single-pass BPM
systems for the BT line and the storage ring on the basis of
the present method. When any number of button signals
from different BPMs are combined to one channel, thepeaks for every buttons can be recorded at the same time.
[3] So using a small number of high speed digitizers, theBPM system that extended over the whole ring would beconstructed.
V. ACKNOWLEDGMENTS

Beam positions of the first nine turns dxhlced fromthe records in thefigure4 (a) were plotted in figure 5. The
BPM used in the present measurement was located just
upstream the injection point. Numbers besioo the solid
circles means the data of the N-th tum. The peak heights
were determined by interpolated curves for the sampled
data, and the approximation equations described above
were adopted to the calculation of positions. A horizontal
injection bump generated by the kicker magnets had aduration of about 5 us. A timing of the injection pulse
was adjusted to the center of the bump duration. The bumpwould disappear just before the fourth~tum pulse go backto the injection point. It seems that a large horizontal
Aoscillation occurred after the bump disappeared
resolution of the position measurement was estimatedfrom variations of data in measurements for large numberof injection pulses. For the BPM of the ring, theresolution for horizontal and vertical measurements wereThe author would like to thank M. Tejima, T.
Kasuga, and K. Shinoe for their support and useful
discussion. I also acknowledge Y. Hori andY. Takiyama
for their support in installation of BPM heads in the BTline.
VI. REFERENCES[1] M. Katoh et al., in these proceedings.
M. Katoh andY. Hori, "Report of the design study ona high brilliance configuration of the PF storagering", in Japanese, KEK Report 92-20, 1993.
[2] N. Nakamura et al., "Beam position feedback systems
for the PF storage ring", KEK Preprint 95-29, 1995.
[3] M. Tejima, private communications.

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