HIGH-POWER TEST OF THE S-BAND ACCELERATOR GUIDE FOR
THE KEKB INJECTOR LINAC
S. Yamaguchi, Y. Igarashi*, K. Kakihara and A. Enomoto, KEK, 1-1 Oho, Tsukuba, Ibaraki, 3050801 Japan, * Mitsubishi Heavy Industries, Ltd., Nagoya Aerospace Systems, Oye 10, Minato,Nagoya, Aichi, 455-0024, Japan
2A high-power test of an accelerator guide (S-band, 2mlong, quasi-constant gradient, made by electroforming) foran energy upgrade of the KEKB injector linac [1] wasperformed. An average accelerating electric field of 36
MV/m was achieved for 50-Hz pulses from a SLED. The
dark current caused by field-emitted electrons wasmeasured as an index of progress of rf conditioning of theaccelerator guide. The obtained field-enhancement factor
from modified Fowler-Nordheim plots of the dark current
converged to about 70 after rf conditioning for 250 hours.
In order to obtain information about rf breakdown,radiation (neutron as well as gamma-rays) emitted fromthe accelerator guide was measured by a NE213 liquidscintillation counter.
HIGH-POWER TESTIn the usual operation of the linac, the klystron outputpower (41 MW, 4µs and 50 Hz ) is pulse-compressed by
the SLED and fed into four 2-m accelerator sections.
However, all of the power was fed into one accelerator
section in this high-power test. A schematic layout of theexperimental setup is shown in Figure 1. The maximum
klystron output power and the electric-field strength in theSLEDTV

Klystron

FC2

VM

VM
Dummy loadTVS

10-1

CCG

200

Figure 2: Modified Fowler-Nordheim plot. Numericalvalues show the number of shots from the start of rf
conditioning. Es(maximum surface field at disk edge) =Eacc (accelerating field) × 2.1.
plot [2]. The field-enhancement factor (β), which can beobtained from the gradient of the F. N. plot, is shown inFigure 3 together with the amount of dark current (I), for
an electric field of 21 MV/m. We can see from this figurethat while the β value converged to 70 at 4.5x10 7 shots(250 hours at 50 Hz), I continued to decrease after thistime and converged at 7×107 shots (400 hours).

FC11858∆Ω/4π=7.7E-4z [mm]

AM:Analyzer magnet
FC:Faraday cup
S:Slit
CCG:Cold cathod gauge
VM:Vacuum manifold
CT:Current transformer

collimator
scintillator
stepping
motor

acutuator

Figure 1: Schematic layout of the experimental setup.
accelerator guide averaged over the pulse were 30 MW
and 36 MV/m, respectively, which shows a large margin
for the KEKB operation (10.5 MW, 21 MV/m,respectively).

Field enhancement factor, β1800

964

2.4 E+7
3.5 E+74.1 E+74.6 E+75.2 E+75.6 E+76.8 E+710-3
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
1/E s [m/GV]

AM7010-2CTA-432m-long accelerating structure

CCG

The total amount of the dark current (I ) was measured bya Faraday cup 1 (FC1 in Figure 1). The SLED wasdetuned during the measurement. The measured result isshown in Figure 2 as a modified Fowler-Nordheim (F. N.)

400

0.07

350

0.06

300

0.05

250

0.04

200
150

β100

I

0.03
0.02Dark current, I

1MEASUREMENT OF DARK CURRENT

I/E s2.5 [A / (V/m) 2.5 ]Abstract

0.01

50
00.00
01234567
Number of shots (x10 )78Figure 3: History of the field-enhancement factor (β )and
the amount of dark current (I) for Eacc=21 MV/m.
Figure 4 shows the momentum spectra of the darkcurrent measured by the analyzer magnet (AM) and4µs

delay [µs]Dark Current [nA]10-101234567810-2

4E acc=33.5 [MV/m]
E acc=35.7 [MV/m]
E acc=36.4 [MV/m]

10-3
0103
width [µs]

2030

4050

12Figure 6: Relation between the rf pulse and gate signal.
The gate is open for the black area.

Momentum [MeV/c]

Figure 4: Momentum spectra of the dark current for anaverage electric field strength of 33.5, 35.7, 36.4 MV/m.

2.5t w=1µst w=2µst w=3µs
t w=4µs

1.50.5

PMT:HAMAMATSU R1828-01
Divider:HAMAMATSU H1949-50

PMT
scintillator
NE213
51mmφx51mmLtrigger

ORTEC 113

Divider

Pre Amp
ORTEC 416A-1800V

GDG

HVPS

ORTEC 460

1.510123456Gate delay time,t d [µ s]780123456Gate delay time, t d [µ s]78163t w=1µs
t w=2µst w=3µst w=4µs2(c)t w=1µst w=2µst w=3µst w=4µs

1412Count rate [/s]2.5Count rate [/s](b)0.50MEASUREMENT OF RADIATION

In order to obtain information about rf breakdown,radiation (neutrons as well as γ-rays) emitted from theaccelerator guide was measured with a NE213 liquidscintillation counter ( 51 mm in diam. by 51 mm inheight) using a pulse-shape discrimination technique. A
schematic diagram of the electronic circuit is shown in
Figure 5.

2103t w=1µst w=2µst w=3µst w=4µs(a)Count rate [/s]2Count rate [/s]Faraday cup 2 (FC2). It is observed that the peak energy
is much lower than the accelerating energy (about 70MeV), calculated from accelerating gradient and structure
length.

2.5

1.5
110(d)

86
40.5

2000

123456Gate delay time, t d [µ s]780123456Gate delay time, t d [µ s]78Figure 7: Count rate of radiation for various vs. gate
delay for various gate widths and locations. (a) z=964, γrays, (b) z=964, neutrons, (c) z=1858, γ-rays, (d) z=1858,
neutrons.

AMP

OKEN 723-1CDelay AMP

RHCgategate

pulse height

105FAST ComTec MPA/IInpulse heightγnrise time

E=19.7 MV/m
E=20.4 MV/m
E=21.1 MV/m
E=21.8 MV/m
E=22.5 MV/m
E=23.1 MV/m

4
382

65
4
3
1
0

00

We first, examined the dependency of the count rateof radiation on the gate width (tw) and gate delay (td) time.
Figure 6 shows the relation between the rf pulse and gate
signal width and delay. It is shown in Fig. 7 that radiation(both γ-rays and neutrons) is uniformly emitted during apulse at the location z = 964 mm; however, at z = 1858mm (near output coupler) and tw = 1µs, neutrons7(b)21Figure 5: Schematic diagram of the electronic circuit.

E=19.7 MV/m
E=20.4 MV/m
E=21.1 MV/m
E=21.8 MV/m
E=22.5 MV/m
E=23.1 MV/m9(a)Count rate [/s]countscountspulse heightmulch parameter MCA
γnγ
rise timeADC2

CANBERRA
8701

counts

ADC1Count rate [s-1]

ORTEC 427A
PMT : Photomultiplier Tube
HVPS : High Voltage Power Supply
GDG : Gate and Delay Generator
AMP : Amplifier
RHC : Rise Time to Height Converter
MCA : Multichannel Analyzer

at the pulse's edge are five-times more than that at themiddle region of the pulse. This fact suggests that rf
breakdown can easily occur at a pulse's edge. In themeasurement mentioned below, tw and td were fixed as4µs.

unibi

500

1000
z [mm]

1500

2000

0500

1000z [mm]

1500

2000

Figure 8: Dependence of the count rate on the detector
position. (a) γ-rays, (b) neutrons.
Figure 8 shows the dependence of the count rate ofradiation on the detector position. A glance at the figurez=90z=368z=666z=964z=1262z=1560
z=1858Count rate [/s]

3.0
2.5
2.0

9.0(a)

7.0

1.5
1.0

6.0
5.0
4.0(b)

1.0
0.0
21

22

23

24

Neutron flux

1E+0
1E+3
1E+2
1E+1

1E-1
1E-2
1E-3
UP
LOW

1E-4
1E+0

1E-5
1E-6
12
34567
Gamma-ray energy [MeV]801234 56 7
Neutron energy [MeV]

8Figure 11: Energy spectra for z=1858 mm. (a) γ-rays, (b)neutrons.

2.0

0.0
20(b)

1E+1

3.0

0.5
19

1E+2UPLOW

1E+40

z=90z=368
z=666
z=964
z=1262z=1560z=18588.0Count rate [/s]3.5

1E+3(a)

1E+5

1E-1

10.0

4.0

1E+6

Gamma-ray flux

reveals that the count rate increases exponentially, except
for at E = 23.1 MV/m.
The graph in Figure 9 illustrates the dependence ofthe count rate of radiation on the electric-field strength. Itis shown that the count rate of γ -rays increases
exponentially; however, neutrons suddenly increase at theoutput coupler, except for at z = 1858 mm.

192021222324

Electric field [MV/m]

Electric field [MV/m]

Figure 9: Dependence of the count rate on the electricfield strength. (a) γ-rays, (b) neutrons.
Energy spectra of the γ-rays and neutrons wereobtained by unfolding the pulse-height spectra. Response
matrices of γ-rays and neutrons were calculated by EGS4
[3] and SCINFUL [4] code, respectively (see, Figure 10).
The energy(a)the low-energy component is dominant in the direction of90 degree, where the detector has been set. It is reasonable
to think that the neutrons are generated by a giant
resonance reaction of copper nuclei based on the facts thatthe peak energy of neutrons is a few MeV (Fig. 11) andthe peak energy of field-emitted electrons is around 10
MeV (Fig.4). Also, the reaction cross section of the giant
resonance has a maximum at 10 to 20 MeV.
5

An accelerating gradient of 36 MV/m (4 µ s, 50 Hz,
averaged over SLED pulse) was obtained for an accelerator
guide for the KEKB injector linac. The field-enhancementfactor (β) obtained from a modified Fowler-Nordheim plot
converged to 70 after 250 hours of conditioning. From ameasurement of the optical neutrons, it has been revealed
that the neutron are emitted in large quantities at the rf
pulse's edge, which suggests that rf breakdown can easily
occur at that time.
6(b)ACKNOWLEDGMENTS

The authors wish to acknowledge valuable discussions
concerning a high-power test of the accelerator guide withT. Higo. They wish to thank Y. Namito for his valuablecomment on the angular distribution of the opticalneutrons. We also acknowledge F. Maekawa for
supplying the FORIST code. The SCINFUL code isprovided by NEA data bank.
7

Figure 10: Response matrices for the NE213 scintillator.
(a) γ-rays, (b) neutrons.
resolution of the detector for γ-rays and neutrons wasdetermined by comparing the measured pulse-heightspectra for standard γ-ray sources and the calculated onesby EGS4.
Figure 11 shows the energy spectra unfolded using
the FORIST code [5] for z=1858 mm (near outputcoupler). The γ-ray energy is fairly low (the flux has amaximum at energy of 1.5 MeV). It is likely that this isdue to the fact that the γ-rays have strong anisotropy andCONCLUSIONSREFERENCES[1] A. Enomoto: ‘Construction Status of the KEKB 8-GeV
Injector Linac’, these proceedings.
[2] G. A. Loew and J. W. Wang: ‘RF Breakdown Studies in
Room Temperature Electron Linac Structures’, SLACPUB-4647 (1988).
[3] W. R. Nelson, H. Hirayama and D. W. O. Rogers: ‘The
EGS4 Code System’, SLAC-265 (1985).
[4] J. K. Dickens: ‘A Monte Carlo Based Computer Programto Determine a Scintillator Full Energy Response toNeutron Detector for En Between 0.1 and 80 MeV:User's
Manual and FORTRAN Program Listing’, ORNL-6462(1988).
[5] R. H. Johnson: ‘FORIST:Neutron Spectrum Unfolding
Code - Iterative Smoothing Technique’, PSR-92,
ORNL/RSIC (1975).