BEAM TESTS OF A WIRE SCANNER FOR THE KEKB INJECTOR LINAC
AND BEAM TRANSPORT LINE
Yoshihiro FUNAKOSHI, Naoko IIDA, Tsuyoshi SUWADA, Takashi KAWAMOTOand Mitsuo KIKUCHI, KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305-0801, Japan
Abstract
Beam tests of a prototype wire scanner which will be usedin the KEKB injector linac and in the beam transport line
have been successfully conducted. The main purpose ofthe tests was to investigate effects of beam background onsystem performance by using electron beams of the injectorlinac and to prove that the wire scanner does work in an
actual beam situation. This report describes results of thebeam tests in detail.
1INTRODUCTION

Since the KEKB is a factory machine, a well-controlled
operation of its injector is required for minimizing tuning time and a stable operation. To this end, beam diagnostic and monitoring tools are essentially important. Of
those tools, a wire scanner (WS) system is used to measure transverse beam distributions non-destructively and todetermine beam emittances and Twiss parameters which
are used in optics matching. To determine these parameters without changing magnet strength, at least 3 WS's areneeded in a localized area. For redundancy, we plan to install WS's in sets of four. At several critical points of theinjector linac and the beam transport (BT) lines, such a setof monitors will be installed.
Wire scanner monitors themselves are well-establishedand common techniques used in various machines. Empirically, however, performance of WS's is seriously affectedby beam background and seriousness of beam backgroundto a detector is rather different from machine to machine.
The main motivation of the present beam study was to investigate background situation in our machine. For thispurpose, we carried out beam tests three times by using
prototypes of WS's.
The first beam test was conducted in July 1997 using abeam of the KEKB injector linac. We used a 2.5GeV electron beam with a beam current of around 0.2nC/bunch. Results were reported elsewhere[1]. Here, we summarize theresults very briefly. We found out that seriousness of beam
background depends strongly on a beam tuning condition,a detector arrangement and a detector shield condition. Asignal to noise (S/N) ratio was easily affected by a verysmall beam loss in upstream of the monitor and we neededa very careful beam tuning to maximize the S/N ratio. TheS/N ratio of the order of 102 was finally obtained after sufficient beam tuning. As for a detector arrangement, we
compared two cases of (1) a small angle detection (mainlydetects Bremsstrahlung photons) and (2) a large angle detection (mainly detects electrons from multiple scattering).
Since the latter brought much better S/N ratio than the for-mer, we adopted the large angle method with the detection
angle of 65o .
In the second beam test conducted in December 1997,
we investigated beam background in different beam conditions. We used a beam of 1.5GeV with a charge of1∼10nC/bunch. We also checked out to what extent we canreduce detector shied for saving space. We measured thebeam emittance and Twiss parameters using the WS. In thispaper, we mainly describe results of this second beam test.
The third beam test is now in progress. In this report,
we show very preliminary results very briefly. In the beam
test, we employed another prototype of WS, which wasredesigned to fit the narrow room for the monitor in thelinac[2].

,,,,,,,,,,,,

Magnescale,,

Pulse Motor
Linear Guide

V wire,,,U wire,,,,,,H wiree- Beame- or γ - ray
Scintillator

IN

PM

Figure 1: Schematic view of prototype wire scanner.
2EXPERIMENTAL SETUP

A cross-sectional drawing of the prototype WS used in thesecond (and first) beam tests is shown in Fig. 1. The WS
is composed of a tungsten wire of 300-mm diameter and adriving unit to drive the wire. The driving unit consists ofa pulse motor and precise linear guides. Each of 3 wires
meets the others at an angle of 45 degree. The three wires
are actually different parts of a single wire. The wires,
which are called H-,V-, and U-wire, go across the beamhorizontally, vertically and in the 45-degree direction bydriving a wire holder in the direction perpendicular to Uwire. The motion of the wire holder is constrained by twolinear guides installed on both sides of the holder. A smallport with a SUS foil of about 100µm thickness is attachedto the vacuum chamber to guide scattered electrons at anangle of 65o with the beam line. Fig. 2 shows a block
diagram of the system. The pulse motor is driven with astep resolution of 1µm by the motor driver controlled bya pulse-train generator(PTG) at the pulse rate of 512Hz.
A tuner controller was used to drive the pulse motor in abench test. The drive position of the wire is monitored by
Magnescale with a resolution of 1µm. The driving unit iscontrolled by a VME/CAMAC system and a host computer
through a network. We used a photomultiplier (PM) tube towhich a 40 × 50 × 10mm3 plastic scintillator was attached.
PM signals were directly fed to a CAMAC-based charge
ADC(LeCroy 2249W) through a 15m-long coaxial cable.
A beam timing signal was used to gate the ADC. A control
software has been developed based on the EPICS system.
A beam test was performed at the end of the B sector of
the linac by using electron beams at a pulse repetition rateof 5Hz. An outline of the linac is described in ref.[3]. Thebeam energy and beam intensity were 1.5 GeV and 0.7∼10nC/pulse at the WS. An experimental setup is shown inFig. 3. The detector was placed in the downstream of theWS and at the same level as the beam line. The detector
was sufficiently shielded by using about one-hundred lead
blocks against beam background. The detector acceptanceswere about 1.7 × 10−3 sr.
The third beam test is being done at the Arc section ofthe linac using another type of WS[2].
UNIX Computer

VMESDIOCCAMAC ADC
SD:Serial Driver
TC: Tuner Controller
CC: Crate ControllerPM: Pulse Motor
NIM
LG: Linear Guide
MS: Magnescale
CM: Control Module
WS: Wire Scanner
HVWCM:Wall Current Monitor
PhM: Photomultiplier
Scinti: Plastic Scintillator
PhM
Scintie - ,γ

PTG

TC

CC

Optical Fiber (RS232C)

Gate

Klystron Gallery

PM
LG

Linac Tunnel

CMAmp

WCM
WSMS

Figure 2: Block diagram of the wire scanner system.
3PROCEDURE OF BEAM TEST

To obtain an optimum value for high voltage (HV) fed tothe PM, we compared signal strength in the case that thebeam was on the wire with that off the wire with scanning
the HV values. We chose the value of -700V for HV which
maximized this ratio. Several parameters were tuned byusing the beams before the data taking. The wire holder

Plastic Scintillator +PhotomultiplierPb shield
1.5GeV electrons

65

Wire scanner

Linac Beam Lineat the end of B sector

Figure 3: Schematic view of the setup for the second beamtest.
was driven step by step in units of 0.5mm/step. A whole
drive length was set to be 80mm. The number of data points
for one whole scan was 160 points/scan (one data point wasconstructed by a 7-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 startof the data taking to avoid any wire vibration. Prior to thebeam study, we made a beam orbit correction. An orbit
distortion was reduced from 0.5 mm in rms to as much as
0.2 mm in rms for both the horizontal and vertical orbits.
This correction was effective to get a better S/N ratio. On
the other hand, we did not make any beam tuning on optics
for better S/N ratio of the WS. This was because we can notalways perform such a tuning for the WS in an actual beam
operation. To investigate a beam current dependence of thebeam background, we tested three different beam currents;
1nC, 7nC and 10nC/bunch with a beam energy of 1.5GeV.
To investigate effectiveness of Pb shield, we intentionally
removed some Pb blocks and observed changes of beam
background.
In the third beam test, we used the new wire scanner withtwo different bunch currents of 1nC/bunch and 7nC/bunchso far.
4RESULTS

In the current dependence measurement in the second beam
test, we measured beam profiles with bunch currents of
1nC/bunch, 7nC/bunch and 10nC/bunch. We could measure beam profiles clearly with these current and did not
see any significant difference among S/N ratios for thesethree bunch currents. A typical value of the S/N ratio wasaround 3/1, although it depended on a beam tuning condition. Fig. 4A shows a typical result for 7nC/bunch. In thefigure,the left, middle and right peaks correspond to V-, Uand H-wires, respectively.
Fig. 4 shows a result of a study on effectiveness of Pb
shields performed in the second beam test. The bunch current was 7nC/bunch in this measurement. In the study, weintentionally removed some Pb shield. Before removal, adetector (a plastic scintillator and a photomultiplier) wasshielded closely with Pb blocks of 20cm in effective thickness on every side except for a path through which signal
particles can reach the detector from the wire. We removed

Pb blocks on one side which faced the upstream. As isseen in Fig. 4, the S/N ratio got worth when we reduced
the shield thickness from 20cm(a) to 10cm(b). The shieldthicknesses of 10cm(b) and 5cm(c) gave almost the sameresult. Here, we noticed that a large crevice in front of thedetector was made unintentionally when we removed thePb blocks. When we narrowed this crevice in the case ofa thickness of 5cm, the S/N ratio was improved(d). Although the S/N ratio was not recovered to the level of the20cm-shield, this may be due to a residual crevice. In a
restricted machine time, we could not fill the crevice sufficiently. From these observation, we concluded that the5cm-thickness may be enough but we should pay close attention not to make unnecessary crevice. The WS system
was used to measure beam emittance. Results were wellconsistent with those by using a screen monitor near to theWS.
In the third beam test, we prepared a Pb shield structurewith the thickness of 5cm based on the result of the second
beam test. With the bunch current of 1nC, the S/N ratio wasat a practical level. However, in the case of 7nC/bunch,
we could not measure the beam profile at all, owing to abad S/N ratio. This was because we could not cure a heavy
beam loss which amounted to 20% of the whole beam in theArc section where the WS was installed. A further study
with additional shield blocks showed that we need a shieldwith a thickness of 10cm in that case.
5(a) 20cm-lead shield(b) 10cm-lead shield

SUMMARY

A prototype system of the wire scanner worked well witha detector shield of 5cm-lead in effective thickness in therange of bunch current of 1nC∼10nC/bunch, unless thebeam loss is heavy. When the beam loss is such that it
amounts to several tens percent of the whole beam aroundthe monitor system, we need thicker lead, say, of 10cmthickness for suppressing the background.
6

ACKNOWLEDGMENTS(c) 5cm-lead shield

The authors gratefully acknowledge S. Yoshida in Kanto
Information Service for his help on the software development using EPICS. Thanks are also devoted to the KEKB
commissioning group who helped the preparation of thisexperiment.
7

REFERENCES[1] T.Suwada et al.: `FIRST BEAM TEST RESULT OF A
PROTOTYPE WIRE SCANNER FOR THE KEKB INJECTOR LINAC AND BT LINES ' , Proceedings of the 11thSympo. on Accelerator Science and Technology, 1997, Hyougo, Japan, KEK Preprint 97-184,1997[2] N.Iida et al.: `A METHOD FOR MEASURING VIBRATIONS IN WIRE SCANNER BEAM PROFILE MONITORS' , in these proceedings.
[3] `KEKB B-Factory Design Report' , KEK Report 95-7, 1995.

(d) 5cm-lead shield (narrowed crevice)
Figure 4: Results of study on effectiveness of Pb shieldwith a bunch current of 7nC/bunch. The abscissa and ordinate denote wire positions in units of mm and ADC counts,respectively.