Nuclear Instruments and Methods in Physics Research A 767 (2014) 26–33

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A
journal homepage: www.elsevier.com/locate/nima

Beam injection with pulsed multipole magnet at UVSOR-III
N. Yamamoto a,n, H. Zen b, M. Hosaka a, T. Konomi d, M. Adachi c, K. Hayashi d, J. Yamazaki d,
Y. Takashima a, M. Katoh daSynchrotron Radiation Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aich 464-8603, Japan
Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japanc
High Energy Accelerator Research Organization, KEK 1-1 Oho, Tsukuba, Ibaraki 305-0801, JapandUVSOR, Institute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japanb

art ic l e i nf o

a b s t r a c t

Article history:
Received 10 June 2014
Received in revised form
26 July 2014
Accepted 29 July 2014
Available online 14 August 2014

In this study, we designed and manufactured a pulsed multipole magnet for beam injection into the
UVSOR-III ring. A sextupole-like magnetic ﬁeld could be excited when using the multipole magnet. To
compensate for the residual ﬁeld at the center of the magnet caused by manufacturing imprecisions, thin
ferrite sheets were used. The injection experiments at UVSOR-III demonstrated multi-turn injections
with the pulsed multipole magnet. The injection efﬁciency was 23% and the electron beam was stored up
to the normal operation current of 300 mA. Moreover, we conﬁrmed that oscillations of stored beams
caused by beam injection were drastically suppressed compared with conventional pulsed dipole
injection.
& 2014 Elsevier B.V. All rights reserved.

Keywords:
Electron storage ring
Beam injection
Storage ring

1. Introduction
The pulsed multipole injection scheme was developed at KEK-PF
and KEK-AR [1,2]. In this scheme, the injection beam is captured into
the accelerator acceptance as a result of pulsed multipole kicks, andthe stored beam passes through the center of the multipole magnet,
where the ﬁeld strength is almost zero. The scheme thus avoids
exciting coherent oscillation in the stored beam and delivers a highquality photon beam for synchrotron radiation users.
The KEK group developed the qaudrupole and sextupole typemagnets and demonstrated to suppress the oscillation of thestored beam. At BESSY-II, the octupole type magnet was developed
and the injection efﬁciency around 80% was obtained [3]. Recently
the extensive beam dynamics have been studied for the MAX-IV
storage ring [4–6].
For relatively small rings, such as UVSOR-III [7], the pulsed multipole injection scheme provides additional advantages. In conventional
bumped injection, at least two pulsed dipole (bump) magnets are
required, and the perturbed (bumped) orbit is quite long. UVSOR-III
currently has three bump magnets at different straight sections, andthe distance between the ﬁrst and the third bump magnet is over onefourth of the circumference. In contrast, only one pulsed magnet isrequired in the pulsed multipole scheme. This scheme thus minimizesnCorresponding author.
E-mail address: naoto@nagoya-u.jp (N. Yamamoto).

http://dx.doi.org/10.1016/j.nima.2014.07.059
0168-9002/& 2014 Elsevier B.V. All rights reserved.

the injection system and realizes additional straight sections for
insertion devices or machine-improvement devices.
Another consideration is the “multiturn” effect during themagnet discharging, which needs to be considered when applying
the pulsed multipole scheme to relatively small rings. In this case,the injection beam travels through several large-amplitude turns
and is affected by nonlinear forces in the ring. In previous
experiments conducted at KEK-PF and KEK-AR, injection beams
were kicked less than third times [1,2]. At UVSOR-III, the numberof kicks could not be reduced further than seven because therevolution period is three times less than that of KEK-PF and thepulse period of the magnet could not be shortened for technicaland economic reasons. In this situation, we could not conﬁrm
whether the multipole injection method works; more complicatedconsiderations are imposed to apply this scheme to UVSOR-III.
Furthermore, with pulsed multipole injection, the residual ﬁeld
at the magnet center, where the stored beam transits, should be
minimized. The residual ﬁeld is so sensitive to the position of themagnet assemblies that extremely accurate manufacturing isrequired, achieving which involves high costs. To address thisproblem, we developed a ﬁeld-compensation technique in which a
set of soft ferrites is applied to the pre-manufactured magnets.
In this paper, we report the design of the multi-turn pulsed
multipole injection for the UVSOR-III storage ring and analyze its
operation. The pulsed multipole magnet was modiﬁed by using
the ﬁeld-compensation technique and then used for multi-turnpulsed multipole injection.

N. Yamamoto et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 26–33

27

X’SeptumWall
Beam monitor position
for stored beam oscillations

Phase
Advance

ΔX'
X(Existing)Pulsed dipole magnets

Stored beam
Injection
beam

ReducedinvariantInjection point

Injection
invariantFig. 1. Principle of pulsed multipole injection. Solid circles and open circle indicate
injection beam and stored beam in phase-space, respectively.

2. Principle of pulsed multipole injection
The principle of pulsed multipole injection can be explainedusing Fig. 1, which shows the beam trajectories and the Courant–
Snyder invariant in X–X 0 phase-space. The injection beam (solid
circles) is transported from the exit of the septum magnet to themultipole magnet with a certain phase advance, while the invariant is maintained constant. At the multipole magnet, the injection
beam receives the kick ΔX 0 and the invariant is reduced. When the
reduced invariant, indicated by the dashed inner circle, is sufﬁciently smaller than the accelerator acceptance, the injection beam
may be captured.
For convenience, we use the ratio of the reduced invariant tothe accelerator acceptance as a dimensionless ﬁgure of merit(FOM) for the injection efﬁciency. It is deﬁned as the ratio of theintegrated areas, where a smaller ratio (i.e., a small FOM) indicates
better injection efﬁciency. In contrast, the part of the injection
beam with FOM 41 cannot be captured.
For the stored beam, indicated by the small open circle at thecenter of Fig. 1, the ﬁeld strength of the multipole magnet isalmost zero and coherent oscillations cannot be excited in theideal case. In fact, the distribution of the stored beam should beconsiderable (see Fig. 2 in Ref. [2]).

3. UVSOR-III storage ring
UVSOR-III is an synchrotron light source at the Institute for
Molecular Science in Okazaki, Japan. The accelerator complex was
originally constructed as a second-generation VUV and soft x-ray
source. The upgrades of 2003 [8] and 2012 [7] led to improved
performance and qualiﬁed UVSOR-III as a third-generation source.
Top-up injection of the electron beam was started in 2010 tomaintain a near constant beam current 300 mA during user
experiments [9]. Because of the “close-bumped” orbit could not
be realized due to poor magnet performance after the optics
upgrades, the “open-bumped injection” scheme is now used forbeam injection at UVSOR-III. Injecting the electron beam into thestorage ring without using the closed bump orbit essentiallyrequires kicking the stored beam horizontally, which induces
betatron oscillations. These oscillations disturb user experiments
and cause undesired spikes in the data. To solve this problem, we
decided to use the pulsed multipole scheme instead of theconventional injection scheme.

Pulsed multipole magnet

Fig. 2. Schematic image of the UVSOR-III storage ring.

Table 1Main parameters of UVSOR-III storage ring for experiments.
Electron energy (MeV)
Circumference (m)Revolution time (ns)Natural emittance (nm rad)Natural energy spread
Horizontal tune
Vertical tune
Stored beam current

750
53.2
177
16.9
5.4  10  4
3.61
3.26
300 mA

Fig. 2 shows a schematic of the UVSOR-III storage ring, andTable 1 lists the main parameters. The electron energy is 750 MeV,
and the circumference is 53.2 m. The lattice consists of four
double-bend sections, four 4-m-long straight sections (long sections), and four 1.5-m-long straight sections (short sections). The
beam emittance of 16.9 nmrad was achieved by distributing thedispersion function to all straight sections and by using combinedfunction dipole magnets [7]. The sextupole ﬁelds that serve tocorrect the chromaticity are introduced by the combined quadruple and dipole magnets. During the injection experiments, thestorage ring was operated at the tune of (νx, νy)¼ (3.61, 3.26),
which is the designed operation point of the UVSOR-III ring.
The injection beam is provided by a 15 MeV-linac and a fullenergy booster synchrotron. During top-up, the injection rate is
1 Hz. An injection septum magnet, labeled as “injection point” inFig. 2, is located at one of the short sections. For conventional
dipole injection, three pulsed dipole magnets are used.
Numerical simulations indicate that only a few locations exist
where the pulsed multipole injection scheme can be applied withadequate kick angle. We select the position about 4.42 m downstream from the injection point because it required the smallest
kick angle for beam injection. The phase advance from theinjection point to the location of the ﬁrst kick was about 1461.
In addition, an existing pulsed dipole magnet and a ceramic chamber
were already located at this point. This conﬁguration allowed us to
examine the injection simply by replacing the magnet part; thevacuum system was not changed.

4. Construction of pulsed multipole magnet
4.1. Design of pulsed multipole magnet
Numerical calculations and cost studies led us to select a
sextupole-like ﬁeld. In ideal, a higher order ﬁeld is more desirable

N. Yamamoto et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 26–33

60

Vertical Position [mm]from the perspective of the ﬁeld gradients around the center.
Furthermore, in case of an even order multipole ﬁeld, such as
qaudrupole and octupole, the injection does not work because thesign of the second kick becomes opposite to the required one for
UVSOR-III storage ring. In addition, the required load current forthe sextupole-like magnet was reasonable for power supply
manufacturing.
The magnet design should simultaneously obtain sufﬁcient ﬁeld
strength for injection beams and negligible strength for stored
beams. Small electromagnetic inductance is also important for the
temporal response of the magnet. A magnet made of a rectangularshaped yoke and a one-turn coil satisﬁes these requirements.
A small vertical gap ﬁtted to the vacuum chamber is preferredbecause it provides greater ﬁeld strength and a short single-turncoil is essential for reducing electromagnetic inductance.
In addition, we also aimed to minimize the residual ﬁeld at themagnet center, which was caused by manufacturing imprecision,
because the kick angle for stored beams is proportional to theresidual ﬁeld. To reduce the residual ﬁeld, extremely accurate
manufacturing is required. For the tolerance of the magnet
performance, some methods were proposed [4,6]. We derived
the tolerance to suppress the stored beam displacement to lessthan 10% of the beam size and a residual ﬁeld within 7 0.5 mm
from the magnet center to be less than 0.1 mT is required. This
indicates that the required precision in the placement of eachelectrode coil was less than a few micro-ms. This requirement is
difﬁcult to be achieved by using normal manufacturing processes,
whereby an electrode is ﬁxed to the yoke material by using a resin
insulator. In order to overcome these difﬁculties, we proposed amethod to compensate the residual magnetic ﬁeld by attaching
thin ferrite sheets after a ﬁeld measurement of pre-manufactured
magnet.

0

EMI sheet

EMI sheet−60
60

Vertical Position [mm]28

EMI sheet

0

−60
−100

−40

0

40

100

Horizontal Position [mm]Fig. 3. Schematic of the multipole magnet compensated residual ﬁeld. (a) Magnetic
ﬁeld shifts in the positive direction and (b) magnetic ﬁeld shifts into the negative
direction.

3.5
4.2. Compensation of residual magnetic ﬁeld

2.5

Magnetic Field [mT]

To compensate for the residual ﬁeld, we used an electromagnetic interference (EMI) absorption sheet (MAB, Kitagawa Industries Ltd.), which is a thin ﬂexible magnetic material with highresistivity. This EMI sheet is made of resin into which is incorporated a soft ferrite ﬁller, and is available in thickness from 0.4 to
4.0 mm. Before inserting the EMI sheet, we measured the saturation magnetic-ﬂux density as 0.338 T by using a vibrating sample
magnetometer.
Fig. 3 shows a schematic of the magnet structures for the
residual ﬁeld-compensation technique for a sextupole-like magnet. Fig. 3(a) shows a 1-mm-thick EMI sheet on the outer side of
the center coils, resulting in a uniformly increasing magnet ﬁeld
around the center. Fig. 3(b) shows the EMI sheet with the same
thickness on the inner side, resulting in a uniformly decreasing
magnetic ﬁeld.
The ﬁeld distribution calculated by the Poisson code [10] andthe magnetic ﬁeld measured by a Hall magnetic sensor are shownin Fig. 4. These data were measured for a pre-manufactured
magnet, which is made by IDX Co., Ltd. without any EMI sheets,with dc 30 A at a vertical position of 0 mm and scaled to a currentof 2.2 kA, where the ﬁeld linearity up to 3 kA is conﬁrmed by the
Poisson code. The maximum magnetic ﬁeld of 0.280 T at the ferrite
position, where the vertical position is about 720 mm, wassufﬁciently low compared with the saturation magnetic ﬂux
density of the EMI sheet because the required kick is expected tobe achieved for the current of 2.2 kA. In Fig. 4, the solid curve andopen squares correspond to the original conﬁguration (without
EMI sheets), the dashed curve and open triangles correspond to
the positive compensation conﬁguration shown in Fig. 3(a), andthe dot-dashed lines and open circles correspond to the negativecompensation conﬁguration shown in Fig. 3(b). Within the range

w. - compensation
w. + compensation
w/o.compensation

3

2
1.5
1
0.5
0
-0.5
-1

-40

-30

-20

-10

0

10

20

30

40

Horizontal Position [mm]Fig. 4. Magnetic ﬁeld distributions for three models of multipole magnets with thescaled current of 2.2 kA. The calculated results are shown by the three curves andthe data are plotted with error bars. The measurements were performed at thevertical position of 0 mm in Fig. 3. The solid line and open squares indicate themultipole magnet without EMI ﬁeld-compensation sheets. The dashed line andopen triangles correspond to the positive compensation conﬁguration shown in
Fig. 3(a). The dot-dashed line and open circles correspond to the negativecompensation conﬁguration of Fig. 3(b).

of measurement error, the data reproduce the results of thecalculation. We found that the magnetic ﬁeld can be shifted
70.6 mT by using a 1-mm-thick EMI sheet. It should be noticed
that the shifted value of the ﬁeld depends strongly on the ﬁeld
strength and distribution at the location of EMI sheet. Then thesituation described above is only one example by using 1-mm-thick
EMI sheets for a sextupole-like magnet.

N. Yamamoto et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 26–33

29

0.018

60

0.014

Integrated Field [T.m]Vertical Position [mm]0.016

EMI sheet
0

−60
−100

−40

0

40

100

0.012
0.010
0.008
0.006
0.004
0.002

Horizontal Position [mm]Table 2
Parameters for the multipole magnet.
Sextupole magnet
Iron yoke length
Vertical gap
Horizontal gap
Magnet inductanceYork material
Power supply
Max. peak current
Max. charging voltage
Pulse period
Max. repetition rate

240 mm
45 mm
136 mm
2:15 μH
Laminated silicon steel(0.2 mm thick)
2.4 kA
22 kV
1:33 μs
3 Hz

0
-40

-30

-20

-10
0
10
20
Horizontal Position [mm]3040Fig. 6. Integrated magnetic ﬁeld of the manufactured multipole magnet with thescaled current of 2.2 kA. The measurement was made with dc 30 A. The solid line isevaluated by multiplying the calculated magnetic ﬁelds by the effective core length.
The measured values are plotted as open circles with error bars.

1
0.8

Output Current [a.u.]

Fig. 5. Cross-section of manufactured multipole magnet with calculated ﬁeld
distribution. Open circles indicate the electrode coils and the two open squares
indicate the EMI sheet that was introduced to compensate the error in themagnetic ﬁeld.

1st kick

0.6
0.4
0.2

6th kick0-0.2

4.3. Manufacturing the magnet-0.4

Fig. 5 shows a cross-section of the multipole magnet on which
the calculated ﬁeld distribution is superimposed. The speciﬁcationsof the magnet and the pulsed power supply are summarized inTable 2. The magnet yoke was made of 0.2-mm-thick laminatedsilicon steel sheets to reduce the effects of eddy currents. The open
circles in Fig. 5 indicate the electrode coils and the coil is formed by
a 5-mm-diameter single-turn copper bar. The iron yoke length andcross-sectional geometries were dictated by the shape of theceramic chamber and the requirement of a 1-mm clearance. Two
open squares represent the EMI sheets described in Section 4.2. The
position and thickness (4.4 mm) of the EMI sheets were determinedbased on ﬁeld measurements of the pre-manufactured magnet.
The residual ﬁeld of the pre-manufactured magnet was 4 mT.
The measured inductance of the magnet was 2:15 μH, which is
35% greater than the calculated inductance. We considered that theconnection parts of electrode coils contributed to this additional
inductance.
Fig. 6 shows the horizontal distribution of the integrated ﬁeld.
The solid line indicates the integrated magnetic ﬁeld evaluated byscaling the calculated magnetic ﬁeld by the effective core length of
280 mm. The magnetic ﬁeld was estimated by using the Poissoncode and the effective core length was determined by measuringwith a Hall magnetic sensor. The open circles indicate measurement data, which were measured at dc 30 A and scaled to theoperating current of 2.2 kA. These results are less than thecalculated results because the magnetic ﬁeld around both edges
is canceled by the magnetic ﬁeld generated from the parts thatconnect the single-turn coil.

0

500

1000

1500

2000

2500

Time [ns]Fig. 7. Power supply output current monitored by a current monitor. Solid circles
represent the kick timing when a beam is injected at the peak timing ofdischarging pulse.

The power supply was produced by IDX Co., Ltd. and installednear the multipole magnet. The connection cables with the pulser
and magnet were 30 cm long. Fig. 7 shows the output current of
the power supply as measured by a current monitor.
The power supply has a maximum charging voltage of 22 kV
and produces a 2.4 kA output current in a half-sine pulse shapewith a full width of 1:33 μs. A reﬂection current appears with anamplitude that is 30% the main peak. The estimated total inductance is about 4:0 μH, which is sum of the contributions from themagnet and the pulser circuit. The required peak current of 2.2 kA
is achieved for a charging voltage of 19.7 kV. The maximum
repetition rate is 3 Hz. No electric discharge between the EMI
sheets and the electrode coils was observed during magnet
operation.
For simple and efﬁcient beam injection, the single-kick scheme
is preferred. However, a period of revolution for the UVSOR-III
storage ring is about 177 ns; thus, to create a pulse that is as shortas two revolution period is difﬁcult both technically and economically. For this reason, we used a pulse period of 1:33 μs. When theinjection timing is tuned at the peak, the beam experiences morethan seven kicks. The timing of the kicks for the injection beam isplotted as solid circles in Fig. 7.

30

N. Yamamoto et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 26–33

5. Injection calculation

50

Horizontal divergence [mrad]

8

Multipole Location

2nd
Turn

6

1st Turn

4

4th Turn

2
0

5th
Turn

6th
Turn

-2

Injection Location

1st Turn

Horizontal Position [mm]40

2nd Turn

30
20
10
0
-10
-20
-30
-40

w. pulsed multipole
w/o. pulsed multipole

-50
0

10

20

30

40

50

60

70

80

90

100

Longitudinal Position [m]Fig. 9. Calculated Horizontal trajectory as a function of the longitudinal position forthe pulsed multipole injection. The solid and dashed lines represent with andwithout pulsed multipole magnet, respectively. The arrows indicate the location of
the pulsed multipole magnet at each turn.

2
1
0

Kick Angle [mrad]

The SAD code [11] was used for beam-tracking simulations andthe lattice model was modiﬁed by using measured parameterssuch as betatron tunes, chromaticities and a horizontal dispersion.
In the tracking simulations, the ﬁeld distribution calculated by the
Poisson code was used as the ﬁeld from the pulsed multipole
magnet. Tune shifts determined by ﬁnite beam amplitudes andsextupoles for correcting the chromaticity were taken into theconsideration.
The results of a six-turn tracking calculation and the acceptanceellipse at the location of the pulsed multipole magnet are shownin Fig. 8. An injection beam is generated from an injector with theemittance of 680 nmrad and an energy spread of 2:3  10  3 .
It passes 40 mm horizontally from the central orbit of the ring at
the injection point. The beam distribution at this point is plotted as
green dots on the left inset of Fig. 8, in which the twiss parametersbeta and alpha are assumed to be 1.5 m and 0 at the injection
point, respectively. The twiss parameters beta and alpha at thepulsed multipole magnet location are 1.8 m and 0, respectively.
In the calculation, the injection beam is represented by a thousandmacro-particles with three-sigma Gaussian distribution in sixdimensional phase-space, and we assume that the injection timing
for the magnet pulse is set on the peak of the discharge current(i.e., 660 ns in Fig. 7) and the output voltage of power supply is
22 kV, which corresponds the output current of 2.4 kA.
Calculated horizontal trajectory as a function of the longitudinal position and an estimated kick for the injection beam ateach turn based on the pulsed multipole magnet are shown in
Figs. 9 and 10, respectively. The arrows in Fig. 9 indicate thelocation of the pulsed multipole magnet at each turn. At the ﬁrst
turn, the injection beam has a horizontal amplitude of 35 mm anda divergence of 2 mrad. The kick angle for the ﬁrst turn iscalculated to be  6 mrad, and the invariant quantity increases
because the kick is too strong. At the second turn, the beam with ahorizontal amplitude of 19 mm and a divergence of 5.8 mradexperiences a kick of 2.6 mrad, and the reduced invariant
becomes slightly smaller than the acceptance of the accelerator.
Furthermore, the invariant quantity is also slightly reduced at thesixth turn, which corresponds to the reﬂection peak of the power
supply (see Fig. 7). These results indicate that the ﬁrst kick servesto adjust the phase advance for the second and sixth kicks.

-1
-2
-3
-4
-5
-6
-7

1

2

3

4

5
6
7
Turn Number

8

9

10

11

Fig. 10. Estimated kick angle provided by the pulsed multipole magnet for injection
beam at each turn. The beam is assumed to be injected at the peak of themagnet pulse.

In addition, the effect for the sixth kick was expected by the beam
tracking in the design process of the power supply and therequirement for the pulse shape was eased, which led the cost
reduction of the power supply.
After the discharging pulse is dumped, the ratio of the reduced
invariant to the accelerator acceptance (i.e., the FOM) becomes
0.46 for a center particle of the beam and the injection efﬁciency isestimated to be 65%. In comparison the resulting injection efﬁciency for the conventional dipole injection scheme is around 45%.

-4

3rd Turn

-6
-8
-50

6. Results and discussions

-40

-30

-20

-10

0

10

20

30

40

50

6.1. Beam injection test

Horizontal Position [mm]Fig. 8. Calculation of six-turn beam distribution in phase-space based on pulsed
multipole injection at the output voltage of 22 kV. The initial distribution of thebeam at the injection location is drawn in green. The beams before and after eachturn's kick, which occurs at the multipole magnet, are drawn in red and blue,
respectively. The beam trajectories of ﬁrst and second kicks are emphasized by theusing solid ellipses. The dashed ellipse represents the acceptance at the location of
the pulsed multipole magnet. (For interpretation of the references to color in thisﬁgure caption, the reader is referred to the web version of this paper.)The injection efﬁciencies as functions of the injection timing
and output voltage are plotted as solid circles in Figs. 11 and 12,
respectively. A maximum injection efﬁciency of 23% was achieved
with an injection timing of 600 ns and an output voltage of 22 kV,
which correspond to a timing near the discharge peak of the pulse(see Fig. 7) and to the maximum output voltage. In this situation,
we also injected the electron beam up to the normal operating

N. Yamamoto et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 26–33

25

1

8

0.6

10

0.4

5

0.2

0
300

400

500

600

700

800

900

Horizontal divergence [mrad]

15

Figure of Merit (FOM)

Injection Efficiency [%]0.8

Multipole Location

2nd
Turn

6
20

31

1st Turn

42

4th Turn
5th
Turn

0
Injection Location-4

3rd Turn

-6
-8
-50

0
1000

6th
Turn

-2

-40

-30

-20

-10

0

10

20

30

40

50

Horizontal Position [mm]Injection timing [ns]Fig. 11. Measured injection efﬁciencies as a function of the injection timing. The
measurements were performed with the output voltage of 22 kV.

Output current [kA]
1.8

25

2.0

2.2

2.4

Fig. 13. Calculation of six–turn beam distribution in phase–space based on pulsed
multipole injection at the output voltage of 26 kV. The initial distribution of thebeam at the injection location is drawn in green. The beams before and after eachturn's kick, which occurs at the multipole magnet, are drawn in red and blue,
respectively. The beam trajectories of ﬁrst kick are emphasized by the using solid
ellipses. The dashed ellipse represents the acceptance at the location of the pulsed
multipole magnet. (For interpretation of the references to color in this ﬁgure
caption, the reader is referred to the web version of this paper.)1

1st Turn

15

0.6

10

0.4

5

0.2

2nd Turn

20
10
0
-10
-20
-30
Measurements
Calculation

-40
0

0
14

16

18

20

22

3rd Turn

30

0.8

Horizontal Position [mm]20

Figure of Merit (FOM)

Injection Efficiency [%]40

24

Output voltage [kV]Fig. 12. Measured injection efﬁciencies as a function of the output voltage. The
measurements were performed with the injection timing of 600 ns.

currents of 300 mA. However, the maximum injection efﬁciency
was only one-third of what we estimated.
Figs. 11 and 12 show the measured injection efﬁciencies andthe FOMs calculated by the beam-tracking simulation. The results
indicate that the maximum injection efﬁciencies occur where thereduced invariant is minimum.
This indicates that the experimental results can be qualitatively
understood by beam-tracking simulations. In the experiments, thepower supply limited the output voltage to 22 kV; however, we do
not expect better injection efﬁciencies for higher output voltages.
If the output voltages are greater than 22 kV, the kick angle at ﬁrst
turn would increase but at the second turn would decrease
because of the larger phase advance shift of the beam, and because
the horizontal position of the beam approaches the magnet center.
Then, the resulting reduced invariance becomes larger, and moreparticles would be lost.
The results of a six-turn tracking calculation for the output
voltage of 26 kV are shown in Fig. 13. The kick angle at ﬁrst turnincreases and is calculated to be  8 mrad. At second turn, thebeam with horizontal amplitude of 15 mm, which is slightly
smaller compared with the same turn of 22 kV case shown in

0

20

40

60

80

100

120

140

160

Longitudinal Position [m]Fig. 14. Horizontal trajectory as a function of the longitudinal position for thepulsed multipole injection. The solid line and solid circles represent the calculated
result and the data acquired using turn-by-turn beam-position monitors, respectively. The arrows indicate the location of the pulsed multipole magnet at
each turn.

Fig. 8, experiences a weaker kick. Furthermore, at third turn, thebeam has larger amplitude and more than half of the beam are
kicked out of the acceptance and lost as a result of the kick. Thenthe injection efﬁciency become worse than the case of output
voltage of the 22 kV. We show the calculation result for the output
voltage of 26 kV for the explanation, the same consequences are
obtained in the case for the greater output voltage than 22 kV. Thismeans that the difference of the phase advance for the injection
beam affects the injection efﬁciencies and it limits the ﬂexibility of
the ring optics. Especially for the multi-turn injection scheme, theamplitude-dependent tune shift is being changed with turn byturn according to the each kick angle. Then the ﬂexibility of theoperation tune is signiﬁcantly limited.
The horizontal trajectory of the injection beam for the pulsed
multipole injection was measured using turn-by-turn beam-position monitors [12] and is plotted, as shown in Fig. 14, along withthe calculated trajectory. This plot shows that the injection
conditions for the horizontal direction are reproduced by thecalculation.

32

N. Yamamoto et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 26–33

10
Pulsed multipole injection
Pulsed dipole injection

Horizontal Position [mm]Horizontal Position [mm]

8
6
4
2
0

w. field compensation tech.
w/o. field compensation tech.

2

1

0

-1

-2
-2
0

2

4

6

8

0

10

4

6

8

10

Time [μs]Time [μs]Fig. 15. Measured stored beam movements with pulsed dipole magnet (opencircles) and pulsed multipole magnet (close circles).

2

Fig. 16. Measured stored beam movements with (close circles) and without (opencircles) ﬁeld compensation technique for pulsed multipole magnet.

The discrepancies of the injection efﬁciency between thecalculated and measured results are attributed to the injection
beam qualities or unexpected nonlinear forces. In our calculation,
the injection efﬁciency depends strongly on beam blurring in thephase-space. We considered only the sextupole ﬁeld of thedesigned lattice and evaluated the FOM, including the off-center
beam, to be 0.68 70.4 for all particles. Note that particles with aninvariant greater than 1 cannot be captured. If the quality of theinjection beam decreases slightly or if additional nonlinear forcesarise somewhere in the ring, the efﬁciency would drasticallyreduce.

respectively; the data corresponding to ﬁeld compensation are thesame as those in Fig. 15. Betatron oscillations beyond the beam
position monitor resolution occurred only without the ﬁeld
compensation, and the amplitude of these oscillations is consistent with the calculated result based on a residual ﬁeld of 4 mT
(which was measured for pre-manufactured magnet). Therefore,
these experiments demonstrate that the proposed ﬁeld-compensation technique effectively cancels the residual ﬁeld.

6.2. Coherent oscillations of stored beams

To introduce pulsed multipole injection into the UVSOR-III
storage ring, we designed a rectangular-shaped pulsed multipole
magnet and tested it for electron beam injection. The goal of thedesign was to compensate for residual magnetic ﬁelds by using
thin ferrite sheets applied to the pre-manufactured magnet.
The injection experiments demonstrated that multiturn injection, which uses the pulsed multipole magnet, is successful
technique for UVSOR-III. This is the successful result of the pulsed
multipole injection in case of as many as 7–9 kicks being applied
to the same injected bunch on consecutive turns.
After optimizing the injection conditions, we obtained a maximum injection efﬁciency of 23% and an electron beam injection
up to 300 mA. The injection efﬁciency obtained was less than
expected from beam simulations, but the efﬁciency was sufﬁcient
to maintain the top-up operation at 300 mA. We suggest that thequality of the injection beam may have caused the lower injection
efﬁciency because an injection efﬁciency lower than expected also
occurred when using conventional dipole magnets. Thus, to obtaina higher injection efﬁciency, we shall focus our future investigations on the injector system.
Finally, this work demonstrates that the ﬁeld-compensation
technique based on using thin ferrite sheets to cancel the residual
magnet ﬁeld is an economical and powerful way to improve
performance. Coherent oscillations were drastically suppressedby applying this technique to pre-manufactured magnet that had aresidual ﬁeld of 4 mT. This compensation technique may also be
applied to other types of pulsed magnets.

Fig. 15 shows the horizontal displacements of the stored beam
as a function of time for both the conventional pulsed dipole andthe completed pulsed multipole. The measurement accuracy forthe position was about 80 μm. The solid and open circles indicate
the data of pulsed multipole and the conventional pulsed dipole,
respectively. The beam position was measured about 31 m downstream from the injection point, as shown in Fig. 2, and bothpulsed magnets were discharged at 0 μs. The horizontal betatronfunction at the monitor position is 7.4 m and the maximum
betatron function of the ring is 7.6 m.
For the pulsed dipole case, the orbit was distorted and thebeam underwent betatron oscillations with amplitudes of about
2 mm.
However, for the pulsed multipole case, although some spikes
occurred for the same magnet discharge timing, no signiﬁcantoscillations were observed after the discharge pulse. The spiky
signals are attributed not to the stored beam but to the electric
noise generated by the pulsed magnet. No observable coherent
oscillation of stored beams occurred. Note that the estimatedamplitude based on the measured ﬁeld shown in Fig. 6 frombeam-tracking simulations is 8 μm.
These results indicate that coherent oscillations of the stored
beam are drastically suppressed on using pulsed multipole injection. It is considered that the betatron amplitude of 2 mm for thepulsed dipole reduced to at least 80 μm, which is the measurement accuracy, for the pulsed multipole. Then the reduction isestimated to be 96%.
Furthermore, for the pulsed multipole magnet, the horizontal
displacement of the stored beam with and without the ﬁeldcompensation techniques is shown in Fig. 16. The solid and open
circles indicate the data with and without ﬁeld compensations,

7. Conclusion

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