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

FIRST BEAM TEST RESULT OF A PROTOTYPE WIRE SCANNER
FOR THE KEKB INJECTOR LINAC AND BT LINES
Tsuyoshi SUW ADA, Naoko IIDA, Yoshihiro FUNAKOSHI, Takashi KAWAMOTO and Mitsuo KIKUCHI
KEK, High Energy Accelerator Research Organization
1-1 Oho, Tsukuba, lbaraki, 305 JapanAbstract
The first beam test of a prototype wire scanner for theKEKB injector has been successfully conducted during themachine time of July 7-11 this. year. The purpose of the test
was to investigate effects of beam background on system
performance by using electron beams of the injector linac.
This report describes results of the beam test in detail.

1. Introduction
Since the KEKB is a factory machine, a well-controlled
operation of its injector is required for minimizing tuning
time and a stable operation. For this purpose, beamdiagnostic and monitoring tools are essentially important. Of
those tools, a wire scanner (WS) system is used to measuretransverse beam distributions non-destructively and todetermine beam emittances and Twiss parameters which areused in optics matching. To determine these parameterswithout changing magnet strength, at least 3 WS's areneeded in a localized area. Forredundancy, we plan to install
WS's in sets of four. At 6 critical points of the injectorlinac
and the beam transport (BT) lines, such a set of monitors will
be installed. The basic design for the injector linac and BTlines was reported in detail elsewhere[l,2]. The resolution
for the beam size measurements is required to be the order ofI%. Empirically performance ofthe WS is seriously affectedby beam background. The main purpose of the beam test
was to investigate how serious background is and how we
can suppress its effects. We studied effects of beam tuning,detector geometry, detection methods and shield by lead
blocks on beam background.

2. Prototype wire scanner and the detection systema block diagram of the system. The pulse motor is driven
with a step resolution of ljlm by a motordrivercontrolle d bya pulse-train generator (PTG, 512Hz). A tuner controller canalso drive the pulse motor in a manual mode. The drive
position of the wire is monitored by Magnescale with aresolution of ljlm. The driving unit is controlfed by a
VME/CAMAC system and a host computer through anetwork. Two types of detection methods were tested in thisexperiment: one detects y.rays generated through thebremsstrahlung and another detects secondary electrons
scattered at large angles through the M<j>ller scattering. For
both detection methods, we used photomultipliers (PMs) towhich a 30x10x10 mm 3 or40x50xl 0 mm 3 plastic scintillator
was attached. The PM signals were directly fed to aCAMAC-based charge ADC(LeCroy 2249W) through 15m-long coaxial cables. A beam timing signal was used togate the ADC. A control software has been developed basedon the EPICS system.[3]3. Experimental setup
A beam test was performed at the end of the fourth
sector of the linac by using electron beams with a pulse widthof Ins in FWHM. The beam energy and beam intensity were
2.7 GeV and 0.2 nC/pulse (5Hz) at theWS. An experimental
setup is shown in Fig. 3. Three detectors were placed in thedownstream of the WS and at the same level as the beam line.
One is an electron detector (e-) and the other two are photon
detectors (}'f, rz), whose detection angles are 65°, 10.2° and13" with the beam line, respectively. Each detector wassufficiently shielded by using about one-hundred lead blocks
against beam background. The detector acceptances were
1.7xl o-\ 1.1 Xl0"4 and 4.1 XI o- 5 sr for thee·, }'J and Y2 detectors,respectively. Beam intensity of each pulse was measured
using a wall-current monitor placed 5m upstream from theA cross-sectional drawing of the prototype WS isshown in Fig. I. TheWS comprises a tungsten wire of 300jlm diameter in a vacuum chamber and a drivingunittodrive
the wire. The driving unit consists of a pulse motor andprecise linear guides. Each of 3 wires meets the others at anangle of 45 degree. Actually the three wires are different partsof a single wire. The wires, which are called H-,V-, and Dwire, go across the beam horizontally, vertically and in the45-degreedirection by driving a wire holder in the direction
perpendicular to U-wire. The motion of the wire holder isconstrained by two linear guides installed in both sides of theholder along the driving direction. A small port with apolyimide (Kapton) foil of about 76-jlm thickness is attachedto the vacuum chamber to guide secondary scatteredelectrons at an angle of 65" with the beam line. Fig. 2 shows

WS to make corrections for variations of beam intensity.

4. Beam test procedure
We found that the signal-to-noise (S/N) ratios werevery sensitive to beam background. Prior to measurements,
sufficient beam tuning was needed to minimize beam loss inthe upstream. With a good beam condition, the two detection
methods with several detector setup configurations werecompared with each other by measuring the SIN ratios. The
setup configurations examined are shown in Fig. 4. High
voltages (HVs) fed to the PMs were adjusted so as tomaximize the SIN ratios within the linear response region ofthe PMs. The HV s were set to be -800, -800, and -950 V for-423-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997e-, }'I, and }2 detectors, respectively (see Fig.5). Severalparameters were tuned by using the beams before the datataking. A drive step length was chosen to be 0. 7 mm/step. A
whole drive length was set to be 85mm. The number of data
points for a scan was 135 points/scan (one data point wasconstructed by a ten-pulses average). As a result, the total
measuring time was about ten minutes for one whole scan
including waiting time at each drive step before the start ofthe data taking to avoid any wire vibration. Fig. 6 shows
typical signals using the setup configuration [ 1-a]. In Fig.4
the measured SIN ratios are shown, which are average valuesof the data from the three wires. We found that the edetection is the best method and that the setup configurationof [ 1-a] or [2-b] gives the best results. This seems reasonable,
since the e- detector has an advantage of backgroundsuppression over they detectors. A part of the beam pipe seen
directly by the e- detector is rather smaller than those for the rdetectors. A larger incident angle of background particles tothe e- detector is also contributed to background suppression.
On the other hand, the e- detector with much largeracceptance has comparable signal strength with that of the ydetectors. We changed strength of beam background andinvestigated its effect for each detection method. Beambackground was controlled by changing field strength of aquadrupole magnet located about 39.2m upstream from theWS. The quadrupole strength was changed in a range of±15% for the nominal strength. The Twiss parameters andemittances were also measured in this method. To investigate
the shielding effect of the lead blocks, we intentionally
removed some of them for the setup configuration [3-a'] andobserved changes of the SIN ratios. This showed that theseshield blocks are necessary to suppress beam background.

5. Data analysis and discussions

from a finite wire thickness and another is an error in the wiredrive. As for the former, the measured beam size am isexpressed as am 2= Oj 12+ aw2 , where Oj1 is a true beam size andaw is the effective wire thickness dependent on the shape ofthe cross section. In the case of the round cross section, aw is
D/4. Here, D is a wire diameter and is 300!lm for the presentwire. Its effect can be corrected in the first order, although
the amount of the correction is small in this case. As for thelatter, we have to consider wire-position ing errors arisingfrom a winding motion of the wire holder along the driving
direction. In this experiment, however, the wire
misplacemen t is small and is estimated to be less than IO!lm,since the both sides of the holder are constrained by the linear
guides. The emittances were measured using theWS. Results
were Y£x= II 01t mm•mrad and "r'Cy=827t mm•mrad which areconsistent with those measured using a screen monitor 5m
downstream of theWS within the errors.

6. Conclusions
A prototype wire scanner was tested by using electron
beams in the injector linac. We found that the e- detection
method at an angle of 65" was better than the r detection
under large beam background. The SIN ratio of the order of
102 was obtained after sufficient beam tuning. We obtained
the resolution in the beam size measurement of about 3%.

Acknowledg ment
The authors gratefully acknowledge S. Yoshida in
Kanto Information Service for his help on the softwaredevelopment using EPICS. They also thank to K. Oide for adevelopment of the data analysis tools and H. Koiso for ahelp in the emittance measurement .. Thanks are also devoted
to the KEKB commissioni ng group who helped thepreparation of this experiment.

Fig. 7 shows a comparison of the SIN ratios for the edetector to those for the y (}'I and }2) detectors. The SIN ratio
Referencesof thee- detection is clearly larger than that of they detection
with the ±15% variation of the quadrupole strength. The data
[I] S. Kurokawa, et al., KEK Report 90-24 (1991 ).
taken with the WS and shown typically in Fig. 6 was fitted[2]
I. Sato, et al., KEK Report 95-18 ( 1996).
with a function which has three peaks and a background.
[3]
L.
R. Dalesio, et al., ICALEPCS 91, KEK Proceedings
Each peak was assumed to have an asymmetric Gaussian
92-15( 1992) p.278.
shape and we used an exponential function for thebackground. We estimated the resolution of the beam size/)measuremen t with fitting residuals. Fig. 8 shows a{/-Pulse Motorcomparison of the beam sizes measured with thee- detectorl ,incar Guideand the r detectors. The result shows that the beam sizes
obtained with the r detectors seem to be somewhat larger •than those with the e- detector. The reason is now under\H-\"t'ir~\ e- or Y- rayinvestigation . Fig. 9 shows a dependence of the resolution(&r/a(% )) of the beam sizes on the SIN ratios. With a good/Scintillator
INSIN ratio we obtained a resolution of about 3%. This/PM
resolution is limited by the drive step length and the numberof beam pulses at each step. A better resolution is probably
attainable by improving them. Here, we briefly discuss
Fig. !.Cross-secti onal drawing of the prototype wire scanner.
about two systematic error sources: one is the error derived/-424-The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

600
CC: Crate Controller
TC: Tuner Controller
PhM: Photomultiplier
WCM: Wall-Current Monitor
PuM: Pulse Motor
LG: Linear Guide
WS: Wire Scanner
MS: Magnescale-E 600=0uu 400Q<2000500
Unac Tunnel

100Drive Length (mm)Fig. 6. Typical transverse beam distributions. The left,middle and right peaks correspond to V-, U- and H-wires,respectively.

Fig. 2. Block diagram of the system.
. ··r#';:""

e· ,ri6.~.

"' 100=-...
-..
0~~=,-! .

z.._f!M4400

111~1-:-_,-=00,PMY2 detector

Fig. 3. Experimental setup.

,_ I ..
,_ I t-Set-up#I >electors

121

Ill

ac

1~1ha'SIN- l."ic

SIN= 40

SIN= ."i2hS/N = ."i.'i

SIN= 2.~ylc~c

SIN= X.6~°10 0.4-~S/N -'1.0

I .ight guide

Plaslk scinLillaLor size:a ... ~Ox~Ox 10
h.c ... 40x."i0x 10(unit: nlm)

2Beam Size (mm) [e·2.5detection]

3Fig. 8. Variations of the beam sizes( 1cr) measured using thee· detector to those ofthe r detectors. The same symbolsasin
Fig. 7 are plotted. A solid line shows a relation of cr,_ = crr.

Pulse Height-SIN

15........
• ------/
0.5

Fig. 7. Correlation plots ofthe SIN ratios for thee· detectorto
those of the y detectors. A solid line shows a relation ofSIN[ e·]=SIN[ }1.

20~.

=-1000

SIN- ~.X

10 4=="' 10 1:;

10100
SIN [e· detection]

Plastic scintillator

Fig. 4. Detector setup configurations.

·;:;"" 10 2

S/N[e"]=S/N[y] .

...

·.
c

SIN- 6.~

~l,

a0.1
0.011

~··----------;:;7c;-3 -;:-50;;-n-m-'~'1l.':'d':.et~e,._c~to,._t:_-_·----=-~~--·

5 10 3.:=10"CC

Linac Beam Line- _...,, ..

:;;-0~w~'- ·-- -~ ~t10.2 ) 11 0Y2.. "I•0.6

0.7 0.80.9 110"'z5~~oO

0

HV [kV]

Fig. 5. SIN ratios and signal pulse heights depending on thehigh voltage fed to the photomultiplier. This figure shows aresult for thee· detector using the setup [1-a].

SIN

Fig. 9. Resolutions of beam-size measurements dependent
on the SIN ratios. A solid line is an eye's guide.

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