RECALIBRATION OF A WALL-CURRENT MONITOR USINGA BEAM-INDUCED FIELD FOR THE KEKB INJECTOR LINAC
T. Suwada and S. Ohsawa, KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305, Japan
Abstract

new method and the results has been compared with theresults of the bench calibration.

About seventy wall-current monitors (WCMs) have been
newly installed along the KEKB injector linac. The
WCMs are used to measure three types of the beam
currents, that is, single-bunch electron (1.3nC/bunch)and positron beams (0.64nC/bunch), and primary highcurrent single-bunch electron beams (10nC/bunch) in orderto produce the positrons. Recalibration of the WCMs
using a beam-induced field has been performed in the beam
tests of the linac commissioning in December 1997, andthe beam charge /bunch for the primary high-current
electrons has been recalibrated.
The calibration
coefficients measured by the recalibration are larger than
23 % of that obtained by the bench calibration. This
report presents the calibration system of the WCMs by
the test bench, the recalibration method using the beaminduced fields, and the beam-test results in detail.
1INTRODUCTION

New wall-current monitors have been developed in order toreinforce the beam-monitoring system in the injector linac
for the KEKB [1,2]. The performance and characteristics ofa prototype WCM were reported in detail, mainly for thebeam-position dependence and the frequency response,
elsewhere [3]. It is furthermore important to obtain
precise information concerning the amount of beam
currents in order to keep the positron production and thebeam-injection rate to the KEKB rings higher. The
calibration of the WCM can be generally performed by thebench test. The bench calibration is done by using fast
test pulses with a width of nano seconds, and thecalibration coefficients are derived from the pulse-height
response of the monitor depending upon the pulse width.
On the other hand, it is difficult to directly measure thecalibration coefficient for the single-bunch beam with apulse width of about 10ps, because the direct generation ofsuch short test pulses is difficult. One of the authors(T.S.) has tentatively derived the calibration coefficientsfor the 10-ps pulse width from the extrapolation based on
the results of the bench calibration. It is, however, notsufficiently accurate to estimate them by thisextrapolation method, because the WCM has a strong
frequency response for the shorter pulses. A new method
for the precise beam-current calibration using a beaminduced field, that is, the fundamental longitudinal
wakefield, was proposed by the authors.
The new
calibration depends not upon the pulse characteristics ofthe beam, but only upon the amount of the beam charges.
The beam tests have been performed in order to check the2HARDWARE SYSTEM

2.1 Monitor Design
Figure 1 shows a schematic cross-sectional drawingof the monitor.
SpringAluminum Case

CeramicResistor

Toroidal Ferrite
Core

Wall Current
12mm

45mm

Beam

Insulating
Ceramics
Conductive Glue

Vacuum Pipe
RF Contactor
BNC-Type
Receptacle

Figure: 1 Cross-sectional view of the new wall-current
monitor. The induced wall current, which is spatiallydistributed around on the vacuum pipe, flows through theceramic resistor at the monitor. For the sake ofsimplicity, the wall current is drawn by a thick solid line.
The monitor comprises a disk-shaped ceramic solid
resistor, a Mn-Zn toroidal ferrite core inside an aluminum
case, and four pickup BNC-type receptacles with π/2
rotational symmetry. The aluminum case needs to cover
the monitor components in order not to affect the pickups
by the electromagnetic noise mainly generated by highpower klystron modulators. An insulated short gap(12mm wide) in a vacuum pipe (45mmφ) interrupts thereturn current on the inner surface of the pipe wall. Eachcomponent is separable into two pieces in order to beeasily mounted around the pipe.
The monitor iselectrically in contact with the outer surface of the pipe by
RF contactors. The Mn-Zn ferrite core is used to keep ahigh magnetic permeability (µ=4000 over a frequency
range less than 1MHz) and a comparatively high Curie
otemperature (>140 C). The resistor is made of a ceramic(Al2O3+SiO2) mixed with carbon powders sintered atotemperatures above 1000 C. The resistance was selected
to be 2.5Ω so as to have a good frequency response forshort-pulse beams.

2.2 Data-Acquisition SystemA new data-acquisition (DAQ) system has been
constructed in order to get the amount of the beamcurrents in real time.
The software and hardware
developments are described in detail elsewhere [4], andhere only the hardware system is briefly summarized.
Figure 2 shows a schematic drawing of the DAQ system.
Digital OscilloscopeI x y 5GHzGPIB
VME/OS9estimated using an extrapolation of the straight line
calculated by the least-squared fitting procedure. It is not
self-evident whether or not this extrapolation is correct
because the frequency response of the monitor is not clear
in such a short pulse. We can derive a correct singlebunch calibration coefficient from a model calculation of
the monitor; however, in practice, it is difficult to get
accurately some parameters, for example, capacitance of
the insulated gap and precise frequency response of theferrite core, etc.. This is the reason why the recalibrationis necessary.
to DAQ
Signal Combiner(Mini-Circuits ZFSC-4-1)

Combiner Box
4chCombiner

Combiner forBPM

Coaxial Broadband
Transformer

Calibration Tube35-m-longcoaxial cable

Klystron GalleryLinac TunnelCombiner

WCM
50- Terminating
Resistor

WCM

The four pickup signals of the WCM are combined by asignal combiner, and then the combined signal is sent up
to a signal-combiner box, which combines the signal withthe other WCM signals, in order to be fed to one channelof a digital oscilloscope with a sampling rate of 5 GHz.
The same oscilloscope is used for the beam-position
monitor system. The digital data are sent to the front-end
computer (VME/OS9) through GPIB. The beam currents
are calculated on this computer in real time using thecalibration coefficients described later. A UNIX-based
host computer is interconnected with the eighteen frontend computer through ethernet.
3Pulse Generator(HP8131A)

Figure: 3 Bench calibration system using a 50-Ωmatched-coaxial calibration tube.
10WCM-B5-2
8Calibration Coefficient (nC/V)

Figure: 2 New data-acquisition system of the wall-currentmonitors.

642Extrapolated

0BENCH CALIBRATION

Bench calibration was performed in order to get calibration
coefficients which were defined by the ratio of an unit
charge to the output voltage of the WCM signal. Figure
3 shows a schematic drawing of the bench-calibration
system. The monitor is installed on a 50-Ω matchedcoaxial tube (calibration tube). The test pulses with thewidth of nano seconds are fed into the calibration tube andthe output voltages of the monitor are measured by thedigital oscilloscope (Tektronics TDS680B) after
combining signals from the four pickups. Figure 4
shows the results of the bench calibration. It is clear thatthe calibration coefficients have the strong frequency
dependence in the nano-second region of the pulse width.
The calibration coefficient for the single-bunch beam, ofwhich pulse width corresponds to the order of 10 ps, wasMeasured

02468Pulse Width (ns)1012Figure: 4 Pulse-width dependence of the calibration
coefficients measured by the bench calibration.
4RECALIBRATION USING A BEAMINDUCED FIELD

The basic conception of the recalibration using afundamental beam-induced field is that a square of theamount of the beam charge is proportional to a power
amount generated by the beam-induced field in an
accelerator tube [5]. The beam-induced power P isrepresented as follows:
P = (eNb )2∞∞−∞z˜∞∫ λ (z˜ )dz˜ ∫ λ (z)W (z − z˜ )dz, ∫ λ (z)dz = 1,||−∞

LPF(Fc=4GHz)

Attenuator(10dB)RF Amp(G=30dB)

accelerator tube, respectively. The power of the beaminduced field was stably measured by the peak-power meterwithin the error of 1 %. Figure 6 shows the result of thebeam tests. A solid line shows the average beam charge
measured by the nano-second-pulse beams. The differenceof the beam charge based on the recalibration was 23 %compared with that estimated by the bench calibration.
Table 1 Recalibration result for the 10nC beam.

RF sensor& Peak power meter

Coaxial cable

Bethe-Hole Coupler

Wave Guide
Dummy LoadWCM
Beam

10

Measured by WCM-B4-2
Measured by WCM-B5-2
Average Charge
02

468

1012

Pulse Width (ns)Beam-Induced Fields
WCM

long2
10
13.7
1.90
14.2
14.4

155
Klystron

long1
555.8
0.942
13.3
13.2

20

Subcontrol room
Linac tunnel

short
2.1
47.3
1.0213.0
13.0

single
16ps
49.5
10.9
10.4

pulse width (ns)power (mW)power ratio
WM-B4-2 (nC)
WM-B5-2 (nC)

Beam Charge (nC/bunch)where e is the charge of a particle, Nb the total number ofparticles in the bunch, λ(z) the normalized linear
distribution of a beam to the beam axis, and W || thelongitudinal wake function. If the fundamental (2856
MHz in our accelerator tube) beam-induced fields generated
by both the single-bunch and nano-second-pulse beams are
precisely measured, assuming no variation of the bunch
distribution, the recalibration for the single-bunch beam
can be performed by using the proportional relation of thesquare-root power ratio, that is, Qs=√(Ps/Pp)×Qp, where Psand Pp are the measured powers for the single-bunch andpulsed beam, respectively, and Qs and Qp are the beam
charges of the single-bunch and of the pulsed beam,respectively. Figure 5 shows a schematic drawing of themeasurement system of the beam-induced field. The
beam-induced field can be measured by a bethe-hole
coupler [6] attached to a waveguide coupled to an output
port of a 2-m-long accelerator tube. The pickup power issend to a subcontrol room through a 37-m-long coaxial
cable, and is detected by a peak-power measurement
system.

Figure: 6 Recalibration result by using the beam-inducedfields.

Accelerator Tube

Figure: 5 Recalibration system of the WCM using thebeam-induced field.
5BEAM-TEST RESULTS

The single-bunch electron beams can be generated by the
new pre-injector [7] which is composed of two subharmonic bunchers, a prebuncher and a buncher. Theelectron gun can generate the amount of the beam chargeof about 18 nC/pulse, and the single-bunch electron
beams greater than 10nC/bunch were stably accelerated
from the outlet of the buncher until to the end of thesector B without any observable beam loss. The beamenergy was about 1.5 GeV at this end. The beam tests forthe single-bunch electron beams and several pulsed beams
with the width of 2 (short), 5 (long1) and 10 (long2) ns
have been performed in order to check the new method.
The obtained beam charges/bunch were summarized intable 1. The beam-induced fields were measured byfeeding the signals picked up from a bethe-hole coupler
mounted in the acceleration tube B5-2 which is located ata distance of about 93 m after the electron gun. The two
WCM signals (B4-2 and B5-2) were simultaneously
measured, which were installed before and behind this

6CONCLUSIONSThe new wall-current monitors has been recalibrated byusing the beam-induced fields for the primary high-current
single-bunch electron beams. The calibration coefficientmeasured by the recalibration was larger than 23% of thatgiven by the bench test. It is very useful method for thebeam-current recalibration because the stability of thepower measurement is very well within the error of 1 %.
7

ACKNOWLEDGMENTS

The authors gratefully acknowledge Drs. F. Hanaki andM. Suetake for their help in supporting the beam test.
They also thank the KEKB linac commissioning groupfor their help in the bench-calibration work.
8[1][2][3][4][5]

REFERENCES

I. Sato, et al., KEK Report 95-18 (1996).
S. Kurokawa, et al., KEK Report 90-24 (1991).
T. Suwada, et al., Nucl. Instr. and Meth. A396 (1997) 1.
T. Suwada, et al, to be presented in this conference.
H.Wiedemann, Particle Accelerator Physics II (SpringerVerlag, New York, 1995), p.355.
[6] H. Hanaki, PF Activity report #13 (1995) P-20.
[7] S. Ohsawa, et al, Procs. the XVIII International LINAC
Conference, CERN, Geneva, Switzerland, 1996, p.815.