Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

RECALIBRATION OF A WALL-CURRENT MONITOR
USING A FARADAY CUP FOR THE KEKB INJECTOR LINAC
T. Suwada#, S. Ohsawa, K. Furukawa, N. Akasaka, and K. Oide
Accelerator Laboratory, High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Abstract
An absolute beam-charge measurement of single-bunchelectron beams with a pulse width of 10 ps and shortpulsed electron beams with a pulse width of 1 ns was
performed by the beam test for the KEKB injector linac
using a Faraday cup. A wall-current monitor was directly
recalibrated by the beam test with an error of ±2%.

2 FARADAY CUP
A cross-sectional view of the FC is shown in Fig.1.
Secondary-ElectronSuppression Electrode Iron Frame
Vacuum Vessel
Air
Iron Block

Beryllia-Porcelain
Insulator
Carbon BlockLead Block

1 INTRODUCTION

0-7803-5573-3/99/$10.00@1999 IEEE.

e- Beam
660 mmThe KEK B-Factory (KEKB) [1] is an asymmetricelectron-positron collider comprising 3.5-GeV positronand 8-GeV electron rings. The KEKB injector linac [2]was upgraded in order to inject single-bunch positron andelectron beams directly into the KEKB rings. The beam
charges are required to be 0.64 nC/bunch and 1.3
nC/bunch for the positron and electron beams,
respectively. High-current primary electron beams (~10nC/bunch) are required in order to generate sufficient
positrons. About seventy wall-current monitors (WCMs)
have been newly installed in order to reinforce the beamcharge-monitoring system in the injector linac for the
KEKB. A bench calibration for the WCMs was performed
using fast test pulses with a width of nanoseconds, and thecalibration coefficients were derived from the pulse-height
response of the monitor, depending upon the pulse width[3]. It is, however, difficult to directly obtain thecalibration coefficient for a single-bunch beam with apulse width of about 10 ps, because the direct generationof such extremely fast test pulses is not very easy. Oneof the authors (T. Suwada) has tentatively derived
calibration coefficients for a 10-ps pulse width fromextrapolation based on the results of the bench calibration.
It is, however, not sufficiently accurate to estimate themby this extrapolation method, because the WCM has astrong frequency response and a beam-position dependence
for shorter pulses [4]. A recalibration of the WCM using a
Faraday cup (FC) has been performed using beam tests.
The purpose of this report is to give the results of precise
measurements of the amount of an absolute beam charge
for a single-bunch beam with a pulse width of 10 ps and ashort-pulsed beam with a pulse width of 1 ns generated
from the injector linac, and the recalibration for the WCM
by a beam test.
_______________________#
Email: tsuyoshi.suwada@kek.jp

VacuumWindow

Vacuum PortHV Signal
920 mmFigure 1: Cross-sectional view of the Faraday cup.
It was originally designed at the Laboratory of Nuclear
Science of Tohoku University about thirty years ago.
Since a detailed design report is available elsewhere [5],here, the geometry and a simple discussion about its
design are briefly presented. The FC comprises a vacuum
vessel made of iron, a lead block, an iron block and acarbon block with cylindrical symmetry. The FC
dimensions are about 1 m in total length to the beam axis
and about 660 mm in diameter; the total weight is about
1.8 t. The bottom thickness of the lead block is 190 mm,
which corresponds to 34 radiation lengths, in order toperfectly absorb incident electron beams with an energy ofless than 300 MeV. The FC can be separated by thevacuum vessel and a thin vacuum window with athickness of 0.01 mm made of stainless steel (SUS304)
from the beam line. The window thickness is required to
be as thin as possible in order to suppress as much as
possible any backscattering and multiplescattering ofincident beams. A carbon block with a thickness of 80
mm is used to suppress any rapid increase of anelectromagnetic shower cascade caused by incident beams
in the lead block. An opening hole with a diameter of
200 mm and a depth of 600 mm in the central region
needs to suppress the escape of secondary charged particles

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Proceedings of the 1999 Particle Accelerator Conference, New York, 1999
generated by an electromagnetic shower cascade in the
carbon block. A good vacuum condition needs to be
maintained through a vacuum port in order to suppress thegeneration of ions caused in the residual air gas. The innermaterials are sufficiently insulated to be larger than 1000
MΩ by several beryllia porcelains from the vacuum
vessel, which is connected to a ground line. A negativeelectric potential with several hundred volts is typicallyapplied to an electrode at the entrance of the monitor
through a vacuum feedthrough in order to suppress theescape of directly backscattered charged particles as muchas possible. The integrated beam charge is extracted from asignal port, which is connected to the lead block.

acquisition (DAQ) system was constructed for the beam
experiment. It comprises several front-end computers and ahost computer, which control the BPMs and the WCMs,
an electrometer (Keithley 617) used for the FC. All of the
BPMs and the WCMs are controlled by digital sampling
oscilloscopes (Tektronics TDS680B) and VME/OS-9based front-end computers. The host computer and thefront-end computers communicate with each other througha network system. Trigger-pluses synchronized with thelinac beam are provided to all of the devices at 0.33 Hz.
This rate is limited by the communication throughput
between the front-end computer and each device through a
GPIB line. A detailed report is available elsewhere [8].

3 EXPERIMENTAL SET-UP

4 BEAM TEST

Figure 2 shows a schematic layout of the experimental
set-up and the beam line at sectors A and B of the injector
linac.
VacuumPump

VacuumWindow

BPM
Electron Beam

WCM

FC
WCM-R0-DT
Screen Monitor
WCM-R0-01
BPM-R0-02
Screen Monitor

Pre-Injector
Optical-TransitionRadiation Monitor

to RF-Power
Measurement System
B-8

B-7

C-1

B-6

C-2

B-5 B-4
Sector B
C-3

B-3

C-4 C-5
Sector C

B-2

C-6

B-1

C-7

A-4

C-8

A-3
A-2
Sector A
1-1

1-2

Electron Gun

A-1

1-3 1-4
Sector 1

1-5

1-6

1-7

Bending Magnet
180-Degree
LINAC ARC

Figure 2: Schematic layout of the experimental set-up andthe beam line for the sectors A and B at the injector linac.
Single-bunch electron beams can be generated by a new
pre-injector [6], which is composed of two sub-harmonicbunchers, a prebuncher and a buncher. The electron gun
can generate a beam charge of about 20 nC/pulse. The
nominal beam energies are about 500 MeV and 1.5 GeV
at the end of the sectors A and B, respectively. A
longitudinal beam profile can be monitored using an
optical transition-radiation monitor (OTR) with a streakcamera system [7] installed after the exit of the module A1. The beam experiments were carried out using electron
beams ejected from a straight line after the sector B. The
FC was installed 2.3 m behind the first bending magnet,
which guides the electron beams to the 180-degree arc
section. The beam energies and the energy spreads were
measured after the bending magnet using a screen monitor
installed behind. The vacuum of the FC was separated
from the beam line with a 10 µm-thick vacuum window
30 mm φ in diameter made of stainless steel, and thevacuum pressure inside the FC was maintained at 1.1×10-4
Torr using a turbo-molecular vacuum pump. Twenty-two
stripline-type beam-position monitors (BPMs) and twenty
WCMs measure the beam positions and the beam charge
in order to control them stable without any beam loss,
respectively. Another WCM and a screen monitor were
mounted just before the FC in order to observe the beamlosses and the spatial beam sizes, respectively. A data-

4.1 Beam experiment
Two kinds of electron beams, single-bunch and shortpulsed beams with the energies of 1.5 GeV and 325 MeV,
were tuned for the beam test. Single-bunch beams weregenerated using the two sub-harmonic bunchers; on theother hand, short-pulsed beams were accelerated withoutthem. A single-bunch beam with the energy of 1.5 GeV is
a nominal-injection beam for the KEKB ring. The otherbeams with the energy of 325 MeV were generated inorder to reduce as much as possible any systematic error
for the beam-charge measurement. The energy spread of
the 325-MeV beam was 5.5%, which was greater than that(2%) of the 1.5-GeV beam due to the longitudinal beam
spread caused by a deacceleration at the sector B. The
bunch length of the single-bunch beam and the envelope
width of the short-pulsed beam were measured by the OTR
to be 8.64 ps and 1 ns in FWHM, respectively. The beam
orbits were carefully tuned for all of the beam modes byusing the BPMs without any observational beam loss.
The spatial beam size was measured to be about 6 mm indiameter in front of the FC by a screen monitor. The
negative bias voltage applied to the secondary-electronsuppression electrode was chosen to be –1 kV bymeasuring the beam-charge reduction using single-bunch
beams with the energy of 1.5 GeV by changing theapplied voltage. Two background measurements werecarried out before the beam-charge measurement. The firstwas a dark current generated from accelerator structures andthe second was a dark current from the electron gun. The
first background was estimated by the beam-charge
reduction by disturbing the dark currents using a screen
monitor in front of the FC with a “beam off” condition,
which meant no high voltage applied to the electron gun.
The second background was obtained by measuring thebeam-charge increment with a “beam wait” condition,
which meant no triggers were fed to the gun with a highvoltage applied. The background contribution from thefirst one was negligibly small compared with the second
one (0.3 nC/scan) which was corrected in the analysis.
The beam-charge measurement was carried out by2239

Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

5

Charge Ratio (QFC/QWCM-R0-01)

It is important to carefully analyze the beam loss between
the end of the module B-8 and the FC, any generation of
secondary charged particles (shower) at the vacuum
window, and the effect of a beam halo, because suchphenomena may produce systematic errors. The first andsecond phenomena can be estimated by relating the ratiosof the charge measured by the FC to that of the WCMR0-01, and to that of the WCM-R0-DT depnding upon the
beam positions obtained by the BPM-R0-02 (see Fig.2).
The systematic error analysis is given elsewhere in detail[9]. Figure 3 show the results after only a dark-current
correction for a 1.5-GeV single-bunch beam. We can findbeam losses slightly before the FC in the region of thehorizontal beam-position displacement of less than zero.
An analysis was performed after data reduction due to thebeam loss and after rejecting data larger than ±2σ obtained
from a Gaussian-function fitting procedure for the chargeratio distributions in order to exclude any noisy events
caused by the klystron modulators. Figure 4 shows alinear relation of the beam charge of the WCM-R0-01 tothat of the FC by using a least-squares fitting procedure
after data reduction. The slope (a) corresponds to arecalibration coefficient of the beam charge for the WCMR0-01. The errors in the figure give only statistical errors.
For the other beam modes, the analysis was also
performed in the same way after the data-reduction
procedure while carefully taking into account the beamloss and the shower events. Table 1 summarizes theanalysis results for all of the beam modes and the
combined recalibration coefficients for the single-bunch (sb) beam, and for the short-pulsed (s-p) beam, respectively.

REFERENCES[1] KEKB B-Factory Design Report:KEK Report 95-7
(1995).
[2] Design Report on PF Injector Linac Upgrade for
KEKB: KEK Report 95-18, 1996.
[3] T. Suwada et al.,Nucl. Instrum. & Methods A 3 9 6(1997) p.1.
[4] T. Suwada et al., APAC’98, KEK, Tsukuba, Japan,
1998, p.528.
[5] T. Ohama et al., Mitsubishi Electric Advance 43 (5)p.739 (1969).
[6] S. Ohsawa et al., LINAC’96, CERN, Geneva,
Switzerland, 1996, p.815.
[7] Y. Ogawa et al., APAC’98, KEK, Tsukuba, Japan,
1998, p.534.
[8] T. Suwada et al., APAC’98, KEK, Tsukuba, Japan,
1998, p.531.
[9] T. Suwada, to be in preparation.

Counts

4.2 Experimental results

7

1.5-GeV Single-Bunch Beam
1.1

1.1

1.08

1.08

1.06

1.06

1.04

1.04

1.02

1.02

1.

1.
-0.2

0

0.2

0.4

0.6

0

10

Counts
20
10
0
-0.2

0

0.2

0.4

0.6

Horizontal Beam Displacement (mm)Figure 3: Scatter plots for the charge ratio (QFC/QWCM-R0-01)
to the horizontal beam displacement measured for the1.5GeV single-bunch electron beam. The allows and lines
indicate the data-reduction conditions.
1.5-GeV Single-Bunch Beam3

QFC (nC)changing the bias voltage applied to the gun four times,
which were 172.8, 191.2, 209.6 and 228.4 V; themeasured beam charge was about 1 to 3 nC for all of thebeam modes.

2

Fitting Function y=ax+b,
Reduced Chi-Square= 5.89×10-4,
a=1.086+/-0.001,
b=0.145+/-0.002

1

CONCLUSIONSAn absolute beam-charge measurement for single-bunchand short-pulsed electron beams was performed by using a
Faraday cup at the KEKB injector linac. A wall-current
monitor was recalibrated according to the analysis results;
the recalibration coefficients were obtained to be 1.11±
7×10-4 (stat.)±2×10-2 (syst.) for the single-bunch beam,and 1.04±2×10-3 (stat.)±2×10-2 (syst.) for the short-pulsed
beam, respectively.

6 ACKNOWLEDGMENTS
We are grateful to Prof. T. Terasawa (Tohoku University,
Japan) for allowing us the use of his group’s Faraday cup.

0
0

1

2

3

QWM-R0-01 (nC)

Figure 4: Linear relation of the beam charge measured by
the WCM-R0-01 to the FC for the 1.5-GeV single-bunchelectron beam.
Table 1: Results for the beam experiment. “a” and “δa”
show the recalibration coefficient and its error,
respectively.
Beam Modeaδa (stat.,syst.)
1.5-GeV s-b beam
1.086
±1×10-3±2×10-2
1.5-GeV s-p beam
1.031
±2×10-3±2×10-2
325-MeV s-b beam
1.126
±1×10-3±2×10-2
325-MeV s-p beam
1.061
±3×10-3±2×10-2
Combined s-b beam
1.11
±7×10-4±2×10-2
Combined s-p beam
1.04
±2×10-3±2×10-2

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