Beam-Transport System of KEKB

M. Kikuchi, T. Honda, N. Iida, K. Kanazawa, T. Kubo,
T. Mimashi, H. Nakayama, Y. Sakamoto, K.Satoh,Y. Takeuchi, M. Yoshida and M. Yoshioka
KEK, National High Energy Accelerator Research Organization, 1-1 Oho,
Tsukuba-shi, Ibaraki-ken, 305-0801 Japan

Abstract
The transport lines of the KEKB for positrons and electrons convey the beams
separately from the injector linac to the KEKB rings. The length is about 500 m
for each line. In order to make the maximum use of the existing tunnels and also to
avoid interference with the AR, the beam lines took a serpentine course, resulting ina rather large curvature in the arcs. The consequences were a large number of bends
in the arcs with high ﬁelds and also a large dispersion function and, thus, a large
R56 component. The latter issue is crucial for the KEKB rings, since it results in alonger bunch length at injection. We adopted a special optics that reduces the R56
coeﬃcient suﬃciently. We have developed novel water-cooled ceramic chambers forkickers, eddy-current type septum magnets for injection, and a beam-abort system
for the rings. The present paper describes the design and current status of the beamlines, the injection system, and the beam-abort system.
Key words: beam, transport, KEKB, abort, kicker, septum, ceramic, bpm, wire

1

Introduction

KEKB is an asymmetric double-ring collider of 3.5-GeV positrons (LER) and
8-GeV electrons (HER), ﬁlled with current of 2.6 A for LER and 1.1 A for
HER in the design. The beam-transport (BT) lines transfer the positron andelectron beams separately from the injector linac to the rings, each having alength of about 500 m. The maximum repetition of the linac is 50 Hz. We
describe the beam transport and the injection system in sections 2 and 3.
The KEKB rings have a beam-abort system to protect the physics detector
and accelerator components. It ejects beams to the dump in a single turn. We
describe the beam-abort system in section 4.

Preprint submitted to Elsevier Science

16 October 2001

2

Beam Transport Lines

The design speciﬁcations of the beam parameters are given in Table 1. The
Table 1
Beam parameters
positrons

electrons

3.5 GeV

8.0 GeV

Emittance (Linac)

2.2×10−7 m

1.6×10−8 m

Acceptance (BT)

3.6×10−7 m

4.6×10−7 m

Energy spread (Linac)

1.25×10−3

1.25×10−3

Energy acceptance (BT)

2.5×10−3

2.5×10−3

Bunch length (Linac)

3.4 mm

1.7 mmBunch length (BT:end)

7.0 mm

7.1 mmEnergyenergy spread of the positron beam at the end of the linac is 2.5×10−3 and iscompressed to one half by the energy compression system (ECS) at the most
upstream part of the BT line.

Layout of the Beam Lines

A

c -1

AR

D

AR
AR Sout hExp. Hall

E

G

H

Inj .
t un
nel

K
J

A r c-

L

U

4

ji
Fu

Ri
ng
M
ai
n

e+

0

50 m

100 m

Arc

-4

nn
MR Inj. t u

el

V

e-

Linac
8 GeV
SY3

Ar

c-

3

I

A rc

-2

F

New t unnel

Ar
B
C

tu
nn
el

2.1

Fig. 1. Layout of the beam lines.

The layout of the beam lines, shown in Fig. 1, was decided under the guidelinethat maximum use should be made of the existing AR- and MR-injection tunnel and as large a separation as possible must be made from the Accumulator
2

Ring(AR), which had been planed to serve as a photon source. From theseexternal conditions, a new tunnel was built that connects the AR-injection
tunnel and the MR-injection tunnel, bypassing the South Hall of the AR. Another important design consideration was operational simplicity. If a single
beam line is used for both electrons and positrons having diﬀerent energies,
the need to re-adjust the magnet excitations or to switch their polarities tends
to create various operational complications. Thus, two beam lines have been
implemented separately for the electrons and positrons. The beam lines haveno common magnets nor common magnet power supplies, except for the ﬁrst
magnet that separates the two beams from the linac.
The positron and electron beams are separated at the exit of the linac andguided to each line. Fig. 2 shows a part of beam lines in an area called SY3,
which is immediately downstream of the the linac exit before reaching the
AR injection tunnel. Upstream of the SY3, an energy compression system was
tory

ton

Fac

T

Acc. structures

e+
LINAC

ho
oP

eLinac beam dump
AR

injec

tion

0

10

20

30

40

50

tunn

el

60 m

Fig. 2. Layout of the Beam Switch Yard (SY3)

implemented. Its purpose is to reduce the energy spread of the positrons fromthe linac by a factor of 1/2, and to relax the the energy acceptance of thebeam-transport line as well as that of LER. The ECS consists of a chicane,
composed of six 1.5 T bends, and two 2 m-long S-band accelerating structures.
The electrons are simply separated from the positrons by the ﬁrst bend of the
ECS. Since the AR injection tunnel does not have suﬃcient space, 3 m wideand 2.5 m high, the lines were stacked vertically, with the positron line on topof the electron line.
In the middle of the new tunnel, immediately after the ﬁrst arc (Arc-1), thetwo lines are separated horizontally. The elevation of the positron line is reduced in the subsequent straight section to meet the MR injection tunnel
at an appropriate point in the middle of the MR injection line: the verticalseparation of two beams is 0.9 m in the Arc-2 and Arc-3. After passing thedownward slopes only minor modiﬁcations to the injection lines are required,since the injection points to the KEKB rings are at nearly the same positions
as those of the TRISTAN MR. Fig. 3 shows a typical view at the Arc-1 of the
new BT tunnel.
3Fig. 3. Typical view from upstream at Arc-1.

2.2

OpticsTight constraints on the geometric conditions led to a large curvature of thebeam lines in the new tunnel. A simple repeat of identical FODO cell would
produce a large dispersion function in the arcs, which would result, combinedwith the small bend radius, a large R56 component. Here, R56 refers to the(5,6) component of the transfer matrix, i.e., the path-length dependence on
the momentum. A larger R56 generates a longer bunch length at the end of
BT due to drift in the longitudinal phase space. A beam with a large bunch
length, correlated with momentum, induces a large quadrupole synchrotron
oscillation. We adopted special optics that reduces R56 : the optics around thethree arcs in the new tunnel was arranged such that the dispersion function was
reversed in the middle of the arc or upstream of one arc generated negative
dispersion downstream of the arc so that in total R56 was decreased. Thisapproach demands, however, that almost all quads are powered independently.
In the present optics, the bunch length is still dominated by the longitudinal
drift eﬀects, as shown in the Table 1. In this optics the largest energy amplitudeof the induced bunch oscillation immediately after injection is much smaller
than the longitudinal acceptance of the rings in the worst case (0.5 %). Fig. 4
shows the optics function of the beam lines. The dispersion function is kept
relatively small and closed within each region. The parameter R56 is -4.9 m
for positrons and -5.5 m for electrons at the end of the beam lines.
4

Fig. 4. Optics functions: In order to maintain R56 to be as small as possible, thedispersion function is reversed in the middle of the arcs.

2.3

Beam Halo Collimation

Beam halo collimation is vitally important to protect the fragile detectorsof the Belle from lost particles. Although the KEKB rings have their own
masks, it is preferable to cut the tail part of the beams before injection. The
collimation system is also necessary to reduce the beam loss in the undesirable
part of the transport lines. We installed 12 pairs of collimators; among them,
two are for collimating the energy-tail of positrons and the others are forcollimating the emittance-tail for both beams. Two sections, SY3 and the Arc1, are equipped with energy collimators. The dispersion function was made to
be as large as 4 m at both locations while keeping the beta function as small
as possible to obtain a higher energy resolution. In the current operation, theenergy collimation in the SY3 has been eﬀective to suppress the radiationlevel, while both collimators have had no distinct eﬀect in reducing the beam
backgrounds of Belle.

2.4

Operation Status

In the commissioning we ﬁrst made a relative calibration of the quad strength
to the bend strength. This procedure was necessary because we had made ﬁeld
measurements using diﬀerent coils for quads and bends and had not made acalibration between coils. An unknown factor was thus common to all quads.
We measured the leakage of the dispersion function downstream of one arc,
where the dispersion is zero in design, as a function of the common factor for
the quads included in that arc. After the calibration, the dispersion function
in all of the arcs of the beam lines was found to be locally closed. Secondly, we
performed optics matching between the linac and the BT using wire scanners.
This was very eﬀective, especially in recovery of the linac after a long shut5

down. Since we could not make an intensity calibration between the BPMs
the transmission eﬃciency has an ambiguous factor. A precise calibration wasdiﬃcult because the cable length of the BPMs diﬀers signiﬁcantly. The injection eﬃciency is 100% at maximum and is strongly dependent on the aperture
condition of rings, deﬁned with masks etc.

2.5

2.5.1

Instrumentation

Beam Position Monitor

Both the positron and electron lines are equipped with beam-position monitors(BPM) of 61 and 56, respectively. The BPMs are attached to every
quadrupole magnet. A BPM comprises four strip-line type pickup electrodes,
shown in Fig. 5. The length of the electrode is 175 mm. Each electrode spans
45◦ to the beam center and forms a part of a cylinder whose diameter is 52 mm,slightly wider than the standard aperture, 49 mm, of the beam chamber inthe transport line. The characteristic impedance of the electrode is matched
50 Ω. In order to avoid any adverse eﬀect of synchrotron light, electrodes areplaced such that the synchrotron light escapes through the space between twoelectrodes. The mechanical precision for the assembly of the electrodes wascontrolled within ±0.1 mm.
175

ø52+/- 0.1
A

C

SMA

45

D

B

250

Fig. 5. Strip-line electrodes for the BPM

Single-pass measurements of BPM signals utilizing a waveform digitizer or adigital oscilloscope have been used in the electron accelerators of KEK[2,3].
Fig. 6 shows a schematic diagram of signal combination in the present system.
In order to save the amount of cable, the signals of four BPMs are combinedin a single cable in the tunnel, and these four combined signals are, moreover,combined in a single cable in the local control room; eventually, 16 signals are
combined in a single cable. The peak-to-peak amplitude of the BPM signal
is detected with a VXI-based waveform analyzer. A coaxial switch selects thesignal from the two beam lines; thus, a single channel of the waveform analyzer
covers 32 BPMs. We used a 8D-class coaxial cable having a corrugated copper
tube as an outer conductor. The transmission length ranges from 100 m to
300 m. The total cable length used for the BPM system amounts to 50 km.
64-BPM signals (100~300 m)
Sub-Control RoomA
BPM #1 D

BCA
BPM #2 D

B
C

4x1
B

4x1
A

A
BPM #3 D

B
C

4x1
C

4x1
B

A
BPM #4 D

B
C

4x1
D

4x1
C

4x1
A

4x1
D

BT tunnel

Coaxial Switch (VXI)
eWaveform Digitizer
(VXI)
e+ A
eCH 1
e+ B
CH 2
ee+ C
ee+ D

CH 3
CH 4

16-BPM signals

4-Way Power Combiner

Fig. 6. Schematic diagram of the BPM signal combination.

The maximum sampling speed of the digitizer is 5 GS/s and the voltage resolution is 8 bits. Since the fundamental frequency of the signal is 1 GHz, whichis comparable to an input bandwidth of 1 GHz, we have limited it to 250 MHz
with a low-pass ﬁlter. The peak-to-peak value is thus more precisely detectedwith more immunity to the timing jitter. The power combiner has a bandwidthof 1 GHz. The total insertion loss is about 7 dB.
In the present system, the four signals from one BPM are recorded with diﬀerent channels of the waveform digitizer. Diﬀerences of a few meters in the cable
lengths have no eﬀect on the precision. Diﬀerences in the transmission loss
among the four electrodes, however, cause an oﬀset of the BPM center. The
transmission loss, including cables and the power combiners, were measured
at two frequencies of 200 MHz and 500 MHz at a resolution of 0.1 dB. The
measured amplitudes of the beam signals were corrected based on the dataof the transmission loss. An error of 0.1 dB in the signal amplitude approximately corresponds to an error of 0.1 mm in the beam-position oﬀset. During
operation, the bunch charge of electrons (positrons) was 2 nC (0.6 nC). The
observed amplitude of the beam signal was from 0.2 V to 2 V, depending on
the cable length, when the bunch charge was 0.6 nC. A position resolution of
100 µm has been achieved, which is dominated by the digitizing resolution of
the waveform analyzer.
The beam orbit of the beam transport line is continuously monitored during
injection. Fig. 7 shows an example of the positron orbit along with the wholebeam transport line. A typical cycle of data acquisition is 1 to 2 seconds. The
beam intensity is plotted in the third row of the ﬁgure. The beam intensity
was estimated from summing the four electrode signals.

2.5.2

Wire Scanner etc.

In order to make matching the optics between the linac and BT, four wire
scanners have been installed at the long straight section in each beam line
in between SY3 and Arc-1, where the dispersion function is zero in design.
Another wire scanner was also installed at the entrance of the Arc-4 section ineach line for measuring the residual mismatch and for its correction prior to
7

Fig. 7. Example of the beam orbit in the positron line.

injection. For monitoring the energy spread of the positron beam, we installeda wire scanner in the short arc before the long straight section, where thedispersion amounts to 3 m. Details are given elsewhere[1].
Three tungsten wires of 100 µm in diameter are strung on a frame for measuring the horizontal, vertical, and diagonal beam size. The frame moves along aline rotated by 45◦ to the horizontal plane, perpendicular to the beam direction. The stroke is 120 mm and the minimum step size is 4 µm. The vibrationof the wire accompanied by the start and stop motion, which dominates theamplitude, was measured and found to be suﬃciently small (±10µm) compared to the beam size[4]. In the actual measurement we moved the wire ina “continuous mode”; the wire does not stop at the scheduled positions, but
continuously moves, recording the signal coincident with the beam. In thismode we do not measure the wire position, but instead infer the position fromthe repetition rate of the beam and pulse counts of the stepping motor. Thismode of operation greatly reduced the time necessary for a scan: 30 seconds
for a scan.
We have mounted photomultiplier tubes (PMT) far downstream of the scanner
to detect any gamma-ray coming from the wire due to the bremsstrahlung
process. One PMT serves as gamma-ray monitors for two upstream scanners.
Fig. 8 shows typical beam proﬁles measured with four wire scanners. The threepeaks in each inset correspond to, from left to right, vertical, diagonal, andhorizontal scan. Since we have no beam monitor that measures the positions
or intensity of the beam in the pulse-to-pulse mode, no correction was madefor the position jitter nor the intensity jitter of the beam.
Using the beam-size data at four wire positions, one can estimate the beta
functions and the emittance at the most upstream wire based on an assumption that the beam has no correlation terms between the position and energy.
The procedure of beta matching usually converges with a single iteration, witha criterion on convergence that the beta mismatch between the measured one
8Fig. 8. Typical beam proﬁles in the positron line measured with four wire scanners.

and that of the design is less than 2, for example. Occasionally, however, it
does not converge. In such a case, the incoming beam from the linac, itself,
may have a dispersive correlation. Fig. 9 shows the variation in the optics
parameters of the positron line for a period of twenty months operation. The
horizontal parameters are displayed in the left ﬁgures while the vertical onesin the right. The bottom row shows the beta mismatch. In usual operation
the optical parameters are measured once a day and optics matching is madeif the beta-mismatch is greater than the criterion.

Fig. 9. Variation of the optics parameters in the positron line. The parameters in
the horizontal plane are drawn in the left column while those in vertical planes
are in the right. From top to bottom, the parameters of α, β, emittance, and beta
mismatch are shown.

Besides wire scanners, 40 proﬁle monitors and 23 beam-loss monitors utilizing
9air-ﬁlled coaxial cables have been installed for the lines. The proﬁle monitors
have played a great role in checking the validity of the optics matching.

3

Injection system

Since the beam scattering in the window, even for Be, produces an enormous
emittance increase, we adopted a scheme that does not utilize the injection
window: the vacuum is common for the transport line and the ring. The dimensions of the beam chamber in the injection area of the ring are 65 mm inwidth, 48 mm in height for HER while 76 mm wide and 48 mm high for LER,
which are maintained throughout the whole injection area in order to maintain smoothness. Four septum magnets for HER and two for LER are used for
injection. They are all housed in vacuum chambers. Two septum chambers for
HER and one septum chamber for LER, which are nearest to the ring, accommodate the beam chambers of the ring inside of them. The beam chamber ofthe ring is smoothly connected in the septum chambers, while keeping its inner dimension. The incoming beam is bent through the septum magnets and,after the quad, injected through a thin slit which is cut in the beam chamberof the ring. The gap height of the slit is 12 mm. A schematic injection orbit
is shown in Fig. 10 for the HER case.
Since the total bend angle of septa is relatively large, 150 mrad for HER and
100 mrad for LER, an angle jitter of 0.1% gives rise to a sizable increase of
the eﬀective beam size. This means that the tolerance of each septum magnet
must be less than .02%, which should be relatively tight for a pulsed magnet.

3.1

Injection septum

We adopted the “eddy current” type instead of the “current sheet” type,because it simpliﬁes the electrical insulation of the coil, and also helps tomake the septum thinner. In the “eddy current” type, on the other hand, theleak ﬁeld outside of the septum is relatively large. In addition, it has a slow
component that lasts even after the drive current has ceased. The leak ﬁeld
has a position dependence; it is smaller at longer distance from the septum.
Since the position dependence of the slow component is much smaller thanthat of the fast component, the slow component exists even at the position of
the circulating beam. We adopted an approach in which the slow component
is partially canceled out by using a full-sine pulse, the positive swing being
canceled with the succeeding negative swing. The leak ﬁeld due to the eddy
current is proportional to the pulse width. For a shorter pulse width, however,a higher voltage is needed, and it is more diﬃcult to select a good power
10

am

d be

jecte

f In
pe o

elo

Env

SeptumRing chamber

Slit

Kicker orbit

Envelopeof captured

beam

QI4E

Fig. 10. Injection orbit of the electron beam. The vertical axis is the horizontal
position from the beam center of HER, while the horizontal axis is the longitudinal
position along the HER beam. The inner radius of the ring chamber is 32.5 mm.
The beam is injected through a thin slit of the ring chamber, which is shown by
a horizontal broken line. Also shown are the envelope of an injected beam of 2.5σ,
the kicker orbit, and the envelope of a captured beam.

switch. As a compromise we adopted a pulse width of 280 µsec.
Table 2
Design parameters of the septum magnets.

Deﬂection angle(mrad)

Number of magnets

e−

e+

37.5

50.0

4

2

Peak magnetic ﬁeld

(T)

1.04

0.79

Septum thickness

(mm)

2.5

2.5

Magnet core length

(m)

1.0

0.8

Magnet free aperture(mm)

11H ×44W

11H ×44W

Laminate steel thickness

(mm)

0.1

0.1

Magnet inductance

(µH)

7.1

5.7

Peak current

(kA)

9.1

6.9

Peak voltage

(kV)

1.9

1.4

Voltage stability

(%)

0.02

0.02

(µsec)

280

260

Pulse width11

3.1.1magnet structure

The main parameters of the septum magnets are given in Table 2. The septum
thickness in the table includes that of the shield plates. The magnet cores are
made of 0.1 mm thick laminations stamped from silicon steel. The end plates
are made of 15 mm thick stainless steel. The cores are surrounded by copper
plates in order to reduce the leakage ﬁeld. The coil is wound at the back-yokeby one turn and extracted outside of the chamber without any connection in
the chamber. It makes use of a copper hollow-conductor, which has a coatingof 0.3 mm thick ceramics to protect against voltage breakdown and radiation
damage. In order to reduce any leakage ﬁeld due to an eddy current in thecopper septum, we attached two thin silicon steel plates of 0.5 mm thickoutside the copper septum.

3.1.2

Power supplies and ﬁeld measurements

The main capacitors are charged up precisely, with the voltage being controlled
in precision by a deQing circuit at a ﬁxed voltage level after the trigger. The
capacitor is discharged by turning on six switch modules in parallel, each ofwhich utilizes three thyristers in series; in a negative swing of the full-sine wave
it is re-charged through a diode which is connected to the switch modules inparallel. The pulse height of the negative swing is 85% of the positive one, due
to resistive loss in the coil and in the circuit. This reduction brings anotherimperfection in the cancellation of the leak ﬁeld. Fig. 11 shows the current andvoltage form. We have measured the magnetic ﬁeld in various conditions. The

Fig. 11. Left: Voltage wave form, vertical scale 500V/div, horizontal scale 1ms/div,
Right: Current wave form, vertical scale 4kA/div, horizontal scale 50µs/div

peak leak-ﬁeld at a distance of 30 mm from the septum was 9 Gauss without
a beam chamber. In this case the slow component of the leak ﬁeld was 3% of
the peak leak-ﬁeld. In order to simulate the eﬀect of the beam chamber wemeasured the leak ﬁeld at the same place within an aluminum pipe of 2 mm
thick and 20 mm diameter. The result was 2.5 Gauss. Since the real beamchamber has a much larger thickness, a much smaller leak ﬁeld is expected in
12the real conﬁguration. We have achieved a voltage jitter of less than 0.02%,
at least in short period of the observation time.

3.1.3

Operation status

The septum magnets have been successfully operated since December, 1998.
The vacuum level in the septum chambers is, however, poor, though it has
been improved slowly. This may become an issue in the future when the beam
current will be increased. The high pressure in septum chambers is mainly
due to out-gas from the silicon steel surface, which is coated with CrO3 that
includes an organic material as glue. When septum magnets are powered thetemperature of the core is increased due to iron loss. Fig. 12 shows the temperature at various places of the septum magnets as well as the vacuum level inthe septum chamber as a function of time during beam ﬁlling in HER, where
the current was increased from 540 mA to 770 mA in 3.3 minutes. Fortunately,
Temp at septum(Cu)Temp at end-plate(SUS)
Temp at power feed-through

-7

70

7 10

-7

60

6 10

-7

50

5 10

-7

40

4 10

-7

3 10

-7

Temperature (deg C)

Pressure in septum-chamber

30
-5

0

5

10

Pressure (Torr)

8 10

80

15

Time (minute)Fig. 12. Temperature of the magnet and the vacuum pressure. The septum waspowered from 0 min to 3.3 min.

thanks to bad conductance between the septum chamber and the ring chamber, the poor vacuum has a relatively small eﬀect on the vacuum in the ring.
The abrupt increase in the vacuum pressure at the last minute of beam ﬁlling,
seen in the Fig. 12, is also observed in the vacuum pressure in the ring. Thisphenomenon depends on the bunch current of the beam in the ring, and has athreshold. We suspect a HOM resonance near the septum. The vacuum pressure of the ring around the septa is roughly 5 to 10-times higher than those of
the normal arc, while the pressure in the septum chamber is as high as 3 ordersof magnitude. The outgassing rate of silicon steel that we used is comparablewith that of rubber[5]. It might be necessary in higher current operation to13

replace it with new steel that does not include any organic material in thecoating.

3.2

Injection Kicker

The kicker bump orbit is generated by two groups of kickers whose horizontal
phase diﬀerence is 180◦ . Each group consists of three kickers, i.e., six kickers
are used in total for each ring. Single-turn injection needs 1.4 mrad and 2.8
mrad for a group of kickers for HER and LER, respectively. In usual multipleturn injection we need 0.9 mrad and 1.8 mrad at maximum for HER andLER. Since the repetition rate is 50 Hz, which is comparable to a transverse
damping time of 45 msec of the rings, a smaller pulse width compared to therevolution time of 10 µsec is necessary to make 50 Hz injection possible. We
adopted a pulse width of 2 µsec.

3.2.1

Kicker magnets and power supplies

The kicker magnets are of the conventional window-frame type using a ferrite
core. The ferrite core is 225 mm long and 90 mm high in the free gap. The
kicker is mounted on a ceramic vacuum chamber. The maximum voltage andcurrent of the power supply are 35 kV and 2000 A and the magnetic ﬁeld is
550 Gauss. The voltage is low enough for stable operation of the thyratrons in
the air. The shape of the current is a half sine (right bottom inset in Fig. 13).
The same type of kicker magnet and the power supply are used for both rings.
The stability of the peak current is less than 0.1%.
Klystron Gallery

40m

Sub-tunnel

10m

Beam Line
6m

250V
20kHz
CX1154C

Impedance
Matching Kicker
MagnetUnit
Thyratron Unit
Kicker magnet pulse shape

High Voltage Unit (Oil Tank)

12 Circuits
Connected in Series
0

1

2

3 4 5
( sec)

6

KEKB Injection Kicker system power supply

Fig. 13. Schematic diagram of the kicker-magnet system.

A deuterium-ﬁlled ceramic thyratron (CX1154C, Marconi Applied Technologies) was adopted as the power switch of the main circuit. Since many bunches
exists in the time span of a kicker pulse it is important that the kicker orbit
14

bump is precisely closed; otherwise, the stored bunches would receive residual
kicks, which would give rise to an adverse eﬀect on the fast feedback system,
for example. It is, thus, crucial to make the pulse shapes of kickers identicalas well as to keep the phase advance of the two group of kickers 180◦ . Fig. 13
shows a simple circuit diagram of the power supply. By adjusting the resistor
in the impedance match unit and also the stray inductance, we were able tocontrol the pulse width and the undershoot depth.

3.2.2

Ceramic Chambers

We have developed novel water-cooled ceramic chambers[6] for HER and LER,
as shown in Fig. 14. The length of the ceramic is 420 mm. Kovar was used
as an intervening material between the ceramic and the SUS ﬂange, since its
thermal expansion matches that of the ceramic and eases the braze. From the
Cooling Water
ChannelCooling Water
Channel

HER

LER

Fig. 14. Structure of the water-cooled ceramic chambers

view point of high-order mode loss, high-dimensional accuracy was required
for the inner wall of the ceramic. It was ground so that the ﬂatness of the innersurface would be less than 0.1 mm in 10 mm square area. At the junction of
the ceramic and kovar, the step size was less than ±0.2 mm.
The ceramic chamber for LER consists of two concentric chambers; in between
them cooling water ﬂows. It has fairly good cooling ability. Ti was adopted
for braze metallization with an activated metallization method. The ceramicchamber for HER is, on the other hand, a single chamber which has 16 holes forcooling water drilled through the ceramic body. The water enters the chamberfrom upper and lower inlets, and after going back and forth twice it leaves fromleft and right water pipes. This type of ceramic chamber needs only one pieceof ceramic, and is relatively easy to assemble. Since Mo-Mn is weak to water,
though it is typically used for braze metallization of a ceramic, we adopted
W-Mn.
A thin metallic coating is required on the inner wall to carry the beam image
current and to conﬁne the beam ﬁeld in the beam chamber. The thickness ofthe coating should be, on the other hand, small enough so that the kicker ﬁeld
is not reduced too much due to an eddy current in the coating. The power from15

the image current had been measured in the AR under a stored current of 500
mA. Extrapolating the data to the case of KEKB, it had been estimated to beapproximately 1 kW for a 6µm Ti coating to the design current of 2.6 A. With
this thickness, the power from eddy current was negligibly small, 20 to 50W,
under a 50 Hz repetition rate. The inner surface was coated with Ti using
Ti-Mn wire at the center of the chamber. The uniformity has been checkedwith a microscope for cut samples. Since the chamber shape is racetrack, thethickness was asymmetric; it was relatively thicker at the upper and lower
sides and 60% thinner in the side arc.

3.2.3

Operation status

It was checked whether the kicker orbit bump is closed, making use of a singlepass beam position monitor of the fast-feedback system, which measures thebeam position of each bunch on a turn-by-turn basis. Fig. 15 shows snapshotsof the horizontal positions of each bunch around the injected bucket. Every

Fig. 15. Snapshots of the horizontal position of bunches around the injected-bucket.
Left: pattern from kick angle error, 2 µs/div, Right: pattern from timing error,
0.5 µs/div

4th rf bucket was ﬁlled in this measurement. Since the location of the BPM isoutside of the kicker orbit bump, no signal would be observed if the bump wasclosed. In the left ﬁgure of Fig. 15, the kick angle was diﬀerent between thetwo groups of kickers. In this case, the shape of kicker pulse is imprinted on thepattern of bunch oscillation. By adjusting the kick angle, one can reduce theamplitude to a certain level, where, however, another type of pattern emerges,
as shown in the right ﬁgure. Note that the vertical scale of the right ﬁgure ismuch smaller than that of the left. This pattern could be attributed to the time
diﬀerence of the two kicker groups, and was able to be minimized by adjusting
the trigger timing of the kicker magnets to some level that corresponds to 1.5%of the pulse shape distortion, or 10-30 µrad kick. The temperature rise at theceramic surface was less than 1◦ C for a LER current of 950 mA and a HER
current of 800 mA.
16

4

Beam abort system

The beam-abort system is an important element of KEKB for the protectionof sensitive components of the Belle detectors and the accelerator components
from damage due to intense beams. It is also indispensable for radiation safety.
A beam-abort trigger is issued from many accelerator components as well as
Belle when their parameters of, for example, temperature or radiation doseetc., exceeds their threshold levels.
The beam-abort system is composed of two(one) horizontal kicker magnets,
a vertical kicker, a Lambertson DC septum magnet and a beam dump for
HER(LER). If one of the kickers fails to kick, an improperly deﬂected beammight hit some unexpected parts of the Belle or accelerator components andmight give rise to irrecoverably serious damage. In order to avoid such risks,
we adopted one common power supply for the kickers, where only one powerswitch is used.

4.1

Outline of the abort system

Ti Window
Orbit
icker
ntal Kbit
Horizoker Oral KicVertic

80mm

40mm

0mm

HER Beam
0

2

4

Horizontal Direction

Inner
Chamber Wall

QX7E

Vertical
Abort Kicker

Horizontal
Abort Kickers

QX6E

Beam Dump

QX5E

Lambertson
Septum Magnet

A schematic layout of the beam abort system of HER is shown in Fig. 16.
Circulating beams are kicked horizontally with two horizontal kickers. The
deﬂected beam, exiting the the ring through a thin Ti window of 1.4 mm thick,is bent downward by 100 mrad with the Lambertson DC septum magnet, andenters the beam dump. The beam traverses in air from downstream of thewindow. Power dissipation at the window due to the extracted beam is a

6m

Fig. 16. Schematic layout of the abort system of HER

serious issue especially for LER. Since the beam size is very small, 500 µm
in horizontal and 100 µm in vertical, for which we assume 3% coupling, thewindow would not bear the temperature rise. In order to relax the power
17

density at the window, we have used two tricks. Firstly, the vertical kicker,
with a rise time greater than the revolution time, sweeps the beam in thevertical direction. The very small cross section of the beam in the verticaldirection makes the vertical sweep eﬀective in diﬀusing any heat deposit on
the window. Secondly, we introduced an intentional ripple of ±10% at the ﬂat
top of the horizontal kicker pulse. Combining two dimensional deﬂections, thebeam trajectory draws a serpentine pattern at the window, and the maximum
power density is reduced by a factor of 5×10−3 . The maximum temperatureof the window is estimated to be 660◦ C for a current of 2.6 A. We have notobserved any hint of degradation in the window under the maximum currentof 1 A.
The beam-abort systems have been working well since December, 1998. Now,
both rings have an abort gap of 1µsec, or 10% of the circumference. This is required for the rise time of the horizontal kicker magnets. For higher luminosity,
an improvement of the rise time of the horizontal kicker would be necessary.
4.2

Kicker magnets and power supply

The basic parameters of the kicker magnet are listed in Table 3. The horizontal
Table 3
Parameters of kicker magnets
HER-H

HER-V

LER-H

LER-V

Deﬂection angle(mrad)

1.875

1.2

1.93

1.4

Integrated ﬁeld Bl

(T m)

.0525

.0336

.0238

.0173

Peak magnetic ﬁled

(T)

.05

.0961

.0227

.0494

Peak current

(kA)

1.79

2.68

1.62

1.23

2

3

1

4

Number of turns
Inductance in design

(µH)

11.4

8.64

4.4

11.6

Gap height, width(mm)

90, 160

105, 17690, 160

125, 170

Ferrite Length

(mm)

900×2

300

900

300

kicker has three separated cores with a common coil, each having a separate
ceramic chamber, because a single long ceramic chamber would be expensive
and separate chambers would be beneﬁcial when using the ceramic chamber
developed for the injection kicker as a common part. Details of the ceramicchamber are found in section 3.2.
Fig. 17 shows a schematic diagram of the main part of the power supply for
the HER, which supplies the pulse current simultaneously to the three kickers.
18

The power supply for LER has the same circuit with diﬀerent parameters. The
horizontal kickers are connected in parallel to make the rise time shorter. The
Power Crowbar Circuit;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
;
;
;
;;
;
;
;;
;
;
;;
;
;
;;
;
;
;
;;
;
;
;
;
;
;
PC
;
;;
;
;
;
;
;
;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

1.6kV

C

Z 0 =10 Ω
50m

T MAIN

50 µ F

35kV

C MAIN
1.2 µ F

THY

Horizontal Kickers

Z 0 =25 Ω;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
25m ;;;;;

6 µ H ;;;;;

T H ;;;;

CV

3µF

CH
48nF

25m T V;Z 0 =100 ;;;;;;;;;;;;;;;;;;;;;;;;
Ω
;;
;
;
;
;;
;;
;;
;;
;
;
V
;;
;;
;;
;;
;;
;;
;
;
;;
;;
;;;
;;;
V
;;
;;
;
;
;;;;;;;;;;;;;;;;;;;;;;;;;

L H ;;;;

;
;
;
;
;;
;;
;;
;;
;
;
;;
;;
H
;;
;;
;
;
;;
;;
;;
;;
;;;;;;;;;;;;;;;;;

I

I

L

Vertical Kicker

9µH

under groundFig. 17. Main circuit of power supply for the HER abort kickers. The charging
circuits are not shown.

voltage (VMAIN ) of the main capacitance (CMAIN ) is 35 kV (19 kV) and thevoltage of the capacitance (CPC ) is 1.6 kV (0.6 kV) for HER (LER). Charging circuits are not shown in the ﬁgure. We have used thyratrons as powerswitches, CX1175 and CX1154 (Marconi Applied Technologies), for HER and
LER, respectively. Their maximum voltages are 70 kV and 40 kV, which givea large margin in voltage.
As the thyratron turns on, negative voltage pulse starts to travel through acable (TMAIN ) 50 m long; when it reaches the other end, the voltage of the main
capacitor (CMAIN ) jumps up to 70 kV, twice the charged voltage. This voltage
continues for up to 500 nsec. The discharging current of CMAIN starts to ﬂow
through mainly the coils of the horizontal kickers. Its rise time is determinedby VMAIN and the inductance of the horizontal kicker (LH ). The droop of thecurrent (IH ) is compensated for by a discharge current of CPC . On the other
hand, the charge of CV is resonantly discharged through the coil of the verticalkicker (LV ). From its resonant frequency, the rise time of the current (IV ) is
determined to be ∼10µsec. The ripples at the ﬂat top of the horizontal kicker
pulse are generated while making use of a resonant circuit composed of CH
and LH . The period is about 3.3µsec. The amplitude of ripples is about ±10%4.3

Measurement of the magnetic ﬁelds

The integrated ﬁelds of kicker magnets along the beam direction in the ceramicchambers have been measured. Fig. 18 shows the results. The rise time of thehorizontal ﬁeld is about 1µsec and the ripples have suﬃcient amplitude during
the time for a ring circumference of 10µsec. The ﬁeld strength of the horizontal
19

BL [Tm]

Time [µsec]

Fig. 18. Field strength of HER kicker magnet measured in the ceramic chamber

kicker satisﬁes the requirement in Table 3. The ﬁeld strength of the verticalkicker at 10µsec is, however, 25% smaller than the value given in Table 3.
To remedy this, we made the rise time faster by adjusting CV , and also byadding a resistance to CV in parallel. With these changes the ﬁeld strength
has satisﬁed the speciﬁcations.

5

Conclusion

We have constructed two beam lines from the injector linac to HER and LER of
the KEKB, and also the injection system comprising the kickers and the eddycurrent type septa. We have also implemented beam-abort systems for bothrings. Speciﬁcally, we have developed novel water-cooled ceramic chambers forthe kickers. These systems have been successfully operated since December,
1998.
Acknowledgments The authors appreciate the personnel of the KEKB groupfor their contributions to the construction of the present system, especially
J. Flanagan for the beam-loss monitor, T. Suwada for proﬁle monitors, N. Yamamoto for the control system, and S. Kurokawa for his help in magnet construction.

References[1] N. Iida et al., “Recent Progress of Wire Scanner Systems for the KEKB Injector
Linac and Beam Transport Lines”, Proc. of EPAC 2000, Vienna, Austria,
pp1738-1740.

20[2] T. Honda, M. Katoh, T. Mitsuhashi, A. Ueda, M. Tadano and Y. Kobayashi,
J. Synchrotron Radiation 5 (1998) 618.
[3] T. Suwada, N. Kamikubota, H. Fukuma, N. Akasaka, and H. Kobayashi,
Nucl. Instrum. Methods A440 (2000) 307.
[4] N. Iida et al.,“A Method for Measuring Vibrations in Wire Scanner Beam ProﬁleMonitors”, Proceedings of the APAC98, Tsukuba, Japan.
[5] Y. Satoh et al., “Outgassing Measurement of the Laminated Cores for use ofPulse-operating Magnets”, J. Vac. Soc. Jpn., vol. 43, No. 10, 2000, pp.24-28 (inJapanese)[6] T. Mimashi et al., “Water Cooling Ceramic Chamber for KEKB Kicker
Magnet”,Proceedings of the EPAC 2000, Vienna, Austria, June 2000, pp24442446.

21