The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

WIRE SCANNER FOR MULTIBUNCH BEAM
H. Hayano, KEK, Tsukuba, Ibaraki, Japan
M. Higuchi, S. Hongo, Y. Inoue, Tohoku-gakuin Univ., Tagajyo, Miyagi, Japan

Abstract
The tungsten wire scanners are used for a measurementof the very small beam size and the emittance inAccelerator Test Facility (ATF). They are installed in theextraction beam line of ATF damping ring. The extracted
beam emittance are Ex=l.3xl0- 9 m.rad, Ey=l.lxl0- 11 m.radwith single bunch 2x 109 electrons/bunch intensity at
1.3GeV energy [1]. The detail of the system and theperformance are described in elsewhere [2]. In the end ofyear 2000, multibunch operation has started. The fast
gamma detector is commissioned for the measurement ofeach bunch size in the multibunch by the same wire
scanners. An avalanche photo-diode (APD) is employed
for the fast Cerenkov gamma detector.

1 INTRODUCTION
ATF is a test accelerator to realize a small emittancebeam which will be used in an electron positron linear
collider. The beam emittance measurement in the ATF
extraction line is required for a single bunch and 20 multibunched beam which has 2x 10 10 electrons in each bunchwith 2.8ns spacing [3]. The required resolution of thebeam size monitors is less than lJtm for the beam of 6 7Jtm vertical size. On the other hand, the horiwntal beam
size is around 50 -150Jtm, bigger than vertical one. The
wire scanner beam size monitor is the most appropriatemonitor for the required performance. The five wire
scanners together with a gamma detector at cbwnstream
are installed at the region of no dispersion in theextraction line between quadrupole magnets. The beam
sizes from 4 or 5 scanners are used to fit an emittanceassuming the optics between the scanners [4].
Precise measurement of such a small vertical beam sizewith big horiwntal size is discussed in elsewhere [2]. In
case of multibunch beam, gamma rays from a beam-wireinteraction have also the same time structure as themultibunch beam. A fast rise and fall response within
2.8ns is required for the gamma detector of themultibunch. APD was chosen instead a photo-multipliertube for the Cerenkov light detector. The performance ofthe APD and measurement of multibunch beam size aredescribed here together with multibunch emittancemeasurement.

2 WIRE SCANNERS
The wire mount shown in Fig.l in the wire scanner
vacuum chamber is supported by the two arms fromoutside of the chamber. A 50Jlm diameter gold plated
tungsten wire is stretched simultaneously to X, Y and U

directions which are 45 degree tilted from the X direction.
In the other side of the mount, a lOJlm diameter gold
plated tungsten wire is also stretched to X, Y, U and 10
degree tilted from X direction. Both end of wire are fixed
to copper pillars with holding by solder. The moving
direction of the wire stage is 45 degree between X and Y
axis has an advantage of that the only one moving
direction is necessary for 3 axis scans. The error from thedesign angle is less than l degree. The installation into
the beam line was done to have 10]1 m horiwntal wire to
sit in the precise horiwntal direction. Using an alignment
telescope, tilt angle of the whole wire scanner chamber
was adjusted within 0.2 degree.
The two arm supports of the wire mount inside thevacuum chamber are fixed to the two stages with twobellows at both ends of support arm. This two-arm
support reduces the vacuum pressure loading for the stage
movement. Furthermore a vibration of the wire mount isalso reduced by using this cbuble support stage comparedwith a single end support. The one end of the wire moverstage is powered by a 5-phase stepping motor stage
assembly (Physik Instrumente M-510.10). This stage isdriven by a ball bearing spindle of lmm pitch for one
revolution. The stepping motor performs one rotation by
2000 steps. Combining the ball bearing spindle and theprecise stepping motor, the resolution of step is 0.5Jlm
and repeatability of the positioning is less than 0 .I Jim.

moving direction
45deg

50pm
Tungsten wire
0.5Jlm-stcp stepping moter stage
O.Spm resolution digital scale

Fig .I: wire mount in the wire scanner chamber
The stepping motor stage assembly has a radiation
resistant. Also, the stage position sensor must have aradiation resistant. The Magnescale position sensor isacbpted, because it cbes not have processing electronics
near the sensor. It includes a magnetized rod with very-375-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

fine pitch and a pickup coil. The processing electronics isplaced outside of the accelerator tunnel. The resolution ofthe Magnescale is 0.5Jtm for IOOmm travel, enoughsmall for the beam size measurement of lOJtm.

a GPffi interface in a CAMAC crate. Since this reading
cycle is not fast enough compared to BPM reader task, theMWOXOONOVI7_0242.DATii

200ii

150

---

i-- . _,
Ii

100- .. ·+-i_,

I-100-200

0100

200300wil-e position [mu]

50

f.so

I

'-"-'--'--'--'--~~~'-'--"--'--"--'--'~.LL..C--'-"--'-~_w_._~o

1 ur• 2 ur• 3 ur• 4 ur•

s ur• 6 ur• 1 ur• s ur•time [sec]Fig.2: APD waveform of multibunch gamma rays

3 GAMMA DETECTOR USING APD
A breamsstrahlung gamma rays is used for the signal ofbeam and wire interaction. The gamma detector is placedat the last bending magnet in the end of beam diagnostic
section. There are two types of Cerenkov detector which
consists of 2mm thick lead plate converter, air light guideand a photo-detector. As for the single bunch detector, aphoto-multiplier tube IS used with its HV voltage
operated from 600V to 950V depending on the beam size
and the wire diameter. On the other hand, APD assemblyunit (Hamamatsu C5658) with around 200V DC bias
voltage, temperature stabilization circuit and pulse
amplifier included is used for multibunch signal detection.
The multibunch detector is installed behind the single
bunch detector. Since the Cerenkov light is emitted like a
sharp cone, two mirrors are used for the lighttransmission and a lens for focusing into APD cell. The
multibunch wire signal out from the APD which is
amplified following the fast built-in pulse amplifier ofgain 100 is a pulse train signal with around Ins width and2.8ns spacing. Typical output pulse height is around
150mV. The output signal from the APD assembly issent out from the damping ring tunnel by 20m Suco-flex(Suhner) cable, and is digitized by TDS694C (Tektronics)
oscilloscope of IOGs/s sampling and 3GHz bandwidth.
Fig.2 shows a typical waveform captured by the scopeduring the multibunch beam hitting on the wire. The
processing software finds a peak amplitude of each bunch
signal inside of +/-1.4ns window at around given
sampling point which is determined in advance. Theinterference of tail ripple at 2.8ns behind into the next
signal peak is less than 9%.

~~~

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020~

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wil-e position [mu]-!Ill-100

050

•

100

wil-e position [mu]MW4X Yprofiles

4 BEAM SIZE MEASUREMENTS

i-150

The acquisition of the wire signal is cbne by theoscilloscope reading task which read the APD signal from-100

sowil-e position [mu]-!Ill0100

1!111

Fig.3: Y profiles of multibunch in 5 wire scanners-376-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

wire signal, the beam intensities and the beam positions
are not fully synchronized each other for the same beampass. However, in case of0.78Hz beam repetition, almost
all data are synchronized. On the other hands, single
bunch wire scan cbes synchronization in any repetition. A
fluctuation of the wire signal caused by the beam
intensity is corrected by normalizing the wire signal by
the extraction beam intensity.
The scanning speed is around 30sec for one profile. The
slow scan is coming from both the 0.78Hz beam
repetition, the slow stage movement and slow acquisition
from the oscilloscope. The scan is made as follows; bysetting the wire stage to the initial position at first, thenwait the beam passing, get the wire signal of each bunchtogether with the beam intensity, go to the next stageposition, then repeat again until the end position coming.
So far, the multibunch injection into DR is not stablepulse-to-pulse, accumulating the gamma signal andaveraging them are used for a clear beam profiles of eachbunch. Measured typical beam sizes of y-direction arefrom 9f!m to 26f!m, x-direction from 50f!m to 200f!m.
Fig.3 shows y profiles of each bunch in the 5 wire
scanners. The beam size of each bunch is not change,
however, profile centroids are shifted from head bunch totail. It seems to be an x-y coupling between two
extraction kickers, however it will be studied in the next
machine time.

5 EMITTANCE MEASUREMENTS
In damping ring multibunch operation, verticalemittance blow-up in the tail was observed at the initialstage. After making horizontal bump orbit tuning
especially at the end of south straight section, the verticalemittance blow-up was disappeared. Even after careful
tuning of damping ring, vertical emittance is still twotimes higher than the one in single bunch at the same
beam intensity. As for the wire scanner measurement, acareful tuning of the extraction diagnostic region is madeby usual way using single bunch condition, such as orbitcorrection and dispersion correction. Then multibunch
beam emittance is measured by using APD detector based
wire scanners after switching to multibunch condition. In
case of the PMT based detector for use of single bunch,
the projected average of the beam profile can be measured.
When the beam of damping ring is well tuned for themultibunch, their detectors give the similar value of theemittance. Fig.4 shows the vertical emittance of eachbunch at around 3xl0- 11 with bunch intensity around
3x 109 electrons/bunch, which is twice much as the one in
the single bunch. The reasons of this bigger emittancecompared with single bunch emittance 1.4xl0- 11 of thesame intensity might be come from residual gasscattering, some resonant mode acting evenly to everybunch and so on, because the vertical emittance of eachbunch are kept constant from head bunch to tail bunch,and the vacuum pressure is twice much higher inmultibunch operation. On the other hand, it can not be
minimized by the tune, RF voltage, and bump orbit.
Further study is needed to reduce the vertical emittance inmultibunch.

Y emi (each bunch) 6/15/2001
5 10-ll ,...---,-,.........---,-,...........--,-,.....,--,-.....--.---,-,...-,--,..............

..= 4 10-ll§~..=:;Gl 3 lo-ll ~.!

....c:oGl:eJGl~•....•.:,.. • •~.~ 04.

.....

• ....

•y4••

2 lo-ll f-··································;·····································;·····································+···············"'··· ..
110-ll0 o~~~5~~~1~0~~~1~5~~~20

BlDlch number
Fig.4: measured Y emittance of each bunch

6SUMMARIESThe five wire scanners in the ATF extraction line areused to measure the multibunch beam size of 50- 200}lm
in horizontal and 9 - 26Jlm in vertical using APD based
gamma detector. The measured emittance is near the ATF
target even in case of multibunch operation. The twice
much emittance growth in the vertical direction comparedwith single bunch is still not understandable.

7 ACKNOWLEDGMENTS
The author would like to acknowledge Prof. Sugawara,
Prof. Kimura, Prof. Kihara, Prof. Iwata, Prof. Kamiya,
Prof. Takata and Prof. Yamazaki for their support of ATF
and the wire scanner developments. The author also thanktoM. Ross, D. McCormick of SLAC and the member of
ATF for their strong cooperation and useful discussion.

REFERENCES[l] J. Urakawa, "Present Status of KEK ATF" 13'hSymp. Of Ace. Science and Technology,Osaka(October 200 1);
K. Kubo et. al.,"Beam Tuning for Low Emittance in
ATF Damping Ring" EPAC, Vienna (June 2000)[2] H. Hayano, "Wire Scanners for small emittance beammeasurement in ATF" Proc. LINAC2000 p146,
Monterey (August 2000)[3] F. Hinode et. al., "ATF Design and Study Report"
KEK Intemal95-4, (June 1995).
[4] T. Okugi et. al.,"Evaluation of extremely small
horizontal emittance" Physical Review Special
Topics-Accelerator and Beams, vol.2 022801 (1999)-377-