The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

UNUSUAL EDDY CURRENT STRAY FIELD OF PULSE SEPTUM OF
NEWSUBARU
Y. Shoji, LASTI, Himeji Institute of Technology, 678-1205, JapanK. Kumagai†, SPring-8, 679-5198, Japan

Abstract
NewSUBARU, 1.5GeV storage ring, has a magnetic
pulse to inject 1 GeV electron beam from the SPring-8
linac. Its maximum field is 0.36T with waveform of 1mswidth half-sine. Behaviour of stored electron beam
indicated an existence of a stray field which time structure
was cosine like. A beam pipe for an injected beam and abeam duct for a circulating beam made an electric loop
path of an eddy current induced by a magnetic field of theseptum. Off-beam measurement of magnetic field and theinduced current confirmed the existence and the path ofeddy current. An electric insulation to a metal support ofthe beam pipe cut the loop path and reduced the cosine
like stray field. There still existed a weak current, whichruns long path along a beam transport line.

1 INTRODUCTION
The synchrotron radiation facility NewSUBARU is a
VUV and Soft X-Ray light source at the SPring-8 site.
Laboratory of Advanced Science and Technology forIndustry (LASTI), at the Himeji Institute of Technology isin charge of its operation, collaborating with SPring-8.
The main parameters of NewSUBARU are listed in Table
1. The ring has two operation modes for users. In the 1.5
GeV mode, the beam is accelerated to 1.5 GeV and stored,
while in 1.0 GeV top-up mode, the beam current is kept at
250+0.15 mA by an occasional injection with the gaps ofundulators closed.
Two important points of top-up injection mode are thegood injection efficiency and the stabilization of beam
orbit during the injection. Lower efficiency means highradiation, which would damage the permanent magnets of
the undulators. The injection efficiency of NewSUBARU
is lower than 80% when the undulator gaps are closed,
although at the present this is acceptable because
NewSUBARU has no in-vacuum undulator.
In the early commissioning of the ring in 1998, weobserved an unexpected beam loss at about 0.4ms after
the injection. Later it became clear that the horizontal
closed orbit distortion (COD) was changing with theseptum field but its time structure was different from thatof the pulse septum. Main parameters of the septum arelisted in Table 2 and Fig.1 shows the flat view of theseptum. The time structure of the strength of COD wasalmost the same as a differential function of the main
septum field. We found that the beam pipe for the injectedbeam and the beam duct for the circulating beam made an________________________________________*Present address: RIKEN, Wako, Japan.

electric loop. The main field of the septum for the injectedbeam induced a current along the loop and produced theabnormal stray field. We reduced this current by simply
insulating the beam pipe to cut the loop. We will report
our experience on this eddy current induced stray field.
Table 1: Main Parameters of the storage ring
NewSUBARU.
0.5 ~ 1.5 GeV
EnergyCircumference
118.731 m
Injector
1.0GeV linac
RF frequency
499.956 MHz
Natural emittance
38 nm @ 1 GeV
Maximum current
single bunch 50 mA/bunch(at 1GeV)
multi bunch 500 mA /ring
Lifetime (at 1.5GeV)
14 hours at 100mA
6.30 / 2.23
Tune νx / νy
3.2 / 5.8
Chromaticity ξx/ ξyLinear coupling
1%
Table 2: Main Parameters of the pulse septum ofNewSUBARU.
Length1m
Maximum field
0.36 T
Pulse width (T)0.945 ms
Pulse Shapehalf sine
Maximum Voltage
Maximum current
Thickness
Inner duct size

382 V
4510 A3 mm
φ 11 mmconductor
yokebeam duct

vacuum beam duct
for circulationg beam

support arm

vacuum beam pipe
for injection beam

Figure 1: Flat view of the septum. The beam runs fromthe right to the left. The upper side, that was an outer sideof the ring, is a vacuum pipe for the injected beam and thelower side is the beam duct for the circulating beam.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

2 EDDY CURRENT PATH

3 MEASUREMENTS

Fig.2 is a schematic flat view of the pulse septum andtwo possible current loops. We assume an ideal septum,
which magnetic field, Bz, as a function of time, t, is
0<x<2r, 0<s<L, 0<t<T
Bz(t) = B0 sin(πt/T)
Bz(t) = 0other than the above condition .

(1)

Here B0, T, r and L are peak magnetic field, the width of
the half sine, radius of the inner pipe and the septum
length, respectively. The space coordinate we use is x, zand s as shown in the figure.
beam pipe for the injection beamxzs

θ

Loop1

3.1 COD change
We measured the change of the horizontal COD as afunction of time using the single-pass beam position
monitor. At that time the pulse septum was fired but thepulse bump system was not, neither the electron gun
trigger was. The analysis of COD at a time gave thelocation and strength of the error dipole field which
produced that COD. Fig. 3 is an example of the observed
COD, which showed that the error field existed at thelocation indicated by an arrow, where the septum was.
Fig.4 shows the calculated deflection by the error field
as a function of timing. The main time structurecomponent was cosine-like form.

Loop2

beam duct for the circulating beam

Figure 2: Schematic view of the pulse septum whichshows three possible current loops.
Along one loop the current goes outer side of the beam
pipe and goes back along the inner side of the same pipe(Loop 1 of the figure). The eddy current along this loop iswell known to reduce the magnetic field that deflects theinjected beam. The eddy current I as a function of time t
is roughly given byI(t) = (1/R) ³ dBz/dt da .

(2)

Here R and a are resistance along the loop and area,taking integration over the area enclosed by the loop.
Along the second loop (Loop 2) the current goes the beam
pipe and goes back along the beam duct, passing themetal support of the pipe. The current is also calculated
by Eq. (2). We integrate the segmented current, dI(t),around the circular beam pipe as shown in Fig.2. The
dI(t) is, using an angle of the segment θ,dI(t) = σ (w/LB)rdθ (1-cosθ )rL (dBz/dt).

(3)

The σ, LB, r, w are conductivity of stainless steel (1/σ=7.4X10-7Ωm), length of the beam pipe (LB =1.2m), mean
radius of the beam pipe (r=5.75mm) and thickness of thebeam pipe (w=0.5mm). We ignored the resistances of thebeam pipe support and the beam duct because they are
negligibly small. The integration over θ gives the induced
current of Loop2.
I(t) = σ (w/LB)r2L (dBz/dt) ³02π(1-cosθ )dθ0<t<T. (4)= 2π2σ w r2 (L/LB)(B0/T) cos(πt/T)
For B0= 0.36 T we obtain
I(t) = 140 cos(t/πT) A 0<t<T.

(5)32
10-1- 2 septum-303

691218 BPM #15Figure 3: Example of horizontal closed orbit distortion
around the ring produced by an error kick at the location
indicated by the arrow.
Horizontal deflection (mrad)septumdX (mm)x0.31ms

0.2
0.1
0-0.1
-0.2-0.3

withinsulation

without insulation

-1000
01000
Delay (revolutions)2000

Figure 4: The deflection by the stray field of the septum
as a function of time, counted by a number of revolutions.
The solid line and the broken line are deflections without
and with the electric insulator.
Later we insulated the injection beam pipe from thesupport arm. The result of the same measurement is overwritten in Fig.4. The cosine-like component wasremarkably reduced but was still existed. The injection
beam pipe and the ring was still weakly connected bywater cooling pipes, beam duct support and so on.

3.2 Measurement of Eddy Current
The AC current along various electrically conductivelines were measured using crampon probe. The resultswith and without the insulation of the injection beam pipe
is shown in Fig.5 The measured current had cosine-likeform and the current without the insulation was 260 A p-p,which almost agreed with the calculation of section 2.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

DCsep6The AC current along various electrically conductivelines were measured using clamp-on probe. The resultswith and without the insulation of the injection is shownin Fig.5. The measured current had cosine-like form andthe current without the insulation was 260 Ap-p, whichalmost agreed with the calculation of Section 2.
The insulation reduced the eddy current. Fig.6 shows acomplicated paths of induced current with the insulation.

DC
sep

67

260

260
pulse
sep

pulse
sep

4260

441

40

25

Figure 5: The septum induced AC current without(left) and with (right) the insulator between thebeam pipe and the supporting arm. The peak to peakcurrent (Ie) and its initial direction are shown.

67
GV
M158
QQ

1616

Q

117

BendstvDC
sep2(5)53
11

support

2SIP

Beam Transport from Linac(9)Storage Ring
16Q Sx11Q Sx2water pipe

stv

11pulse
sep

12

bump

67
40

16

water pipe

Storage Ring

bump

29

Sx Q
stv

Sx Q

1514
B

B

20
support2water2

23
BPM

support

10

1.2
SIP

9

1525(2)

support0.7

2

supportSIP

10

30.4

BPM

waterSx

QSx

Q

21water

5

7Figure 6: The septum induced AC current with the insulator of the beam pipe.

3.3 Magnetic Field Measurement50

The magnetic stray field in the ring beam duct with thepulse septum was measured using a pick up coil. During
the measurement the septum was replaced from the ring,which meant that the septum was isolated from the weakcurrent path shown in Fig.6. The pulse shapes of theinduced voltage at the pick up coil with and without theinsulator are shown in Fig.7. The magnetic field also
confirmed the existence and the disappearance of theeddy current field.

40

X=0mm
X=9mm

B (Gauss)

X=13.5mm
X=0mm

30X=9mm
X=13.5mm

20Halfsine
Halfcosine

100- 100- 50

050100s (mm)Figure 8: Distribution of the stray field. We defined x=0at the designed centre of the circulating beam, where thewall of the vacuum duct is at x=21mm. The septum exists
at s<0 and ends at s=0. The thick lines are cosine-likecomponents and the thin lines are sine-like components ofthe stray field.
Figure 7: The pulse shapes of the induced voltage at thesearch coil without (left) and with (right) the insulator. Inthe left main magnetic field and the integrated shape ofthe search coil voltage (=current) is shown. In the rightthe cosine-like shape was the induced voltage and thesine-like shape was the integration of the voltage.
The distribution of the stray field without the insulator
near the edge of the septum was shown in Fig. 8. The
shapes shown in Fig.7 (left) were separated into cosinelike component and sine like component. The sine-likestray field existed only at the edge of the septum. On theother hand the cosine-like (eddy current induced)
component existed along the septum.

4 SUMMARY
We show some proofs of the existence of the eddy
current induced stray field in the pulse septum ofNewSUBARU.
We succeeded to reduce the stray field just by theinsulation of the beam pipe from the support arm. The
new septum was already constructed and is waiting forthe installation. It is capable of insulation from the long
beam transport line and shall not have eddy current
induced stray field.