DESIGN STUDY OF EDDY CURRENT SEPTA FOR JPARC MR FASTEXTRACTION
K.Fan#, K. Ishii, T. Sugimoto, S. Fukuoka, H. Matsumoto
KEK, 1-1 OHO tsukuba Ibaraki, Japan.
The present low-field septa of the JPARC main ring
fast extraction system are conventional design multi-turn
septum magnets. The major problems of the septa are
weak mechanical structure, poor gap field uniformity andlarge leakage field. These defects may have significant
effects on high power operation and cause large beamlosses. Eddy current septa have been studied to replace
the present multi-turn septa. In order to generate sufficient
flattop width for the beam extraction, the eddy current
septum will be excited by a fundamental sinusoidal pulse
with a superimposed third harmonic. The paper will
discuss the methods to realize the superposition of third
harmonic and to suppress the end fringe field.

However, several problems involved in the septa of theFX system have been encountered. One of the big
problems of the septa is the large leakage. The septa workin pulsed mode having the same excitation pattern as themain dipole. The resulting error fields due to the eddy
current impair the circulating beam particularly at thestart of acceleration because of the low energy. Fig. 3
shows the beam closed orbit distortion in the bothhorizontal and vertical direction changes with theexcitation of FX septa. At the start of acceleration, thebeam orbit changes quickly, which indicates the eddy
current effects are predominant.
0.2

COD, mm

Abstract

INTRODUCTION

Figure 1: FX septa system
The beam orbit in the septa region is shown in Fig. 2.
Due to the limited physical aperture of both the QFR andthe QDT, the separation between the extracted beam andthe circulating beam orbit is very narrow. Thus theleakage field from the septa will have significant effects
on the circulating beam.
Amplitude, m

SM33

0.4

SM11/12 SM21/22

SM30

FX septa field

Fast extractiont

0.2

170

340

510

680

850

1020

1190

1360

1530

1700

Time, ms (Run_42, BPM157)

Figure 3: FX septa leakage effects on COD
Some of the FX septa problems have been corrected but
some are still remained. To realize high power operation,it is extremely important to keep beam loss within thecollimator capacity that is determined by the radioactivation control scenario. However, the remainingproblems may lead to severe beam loss becoming roadblocks toward high power operation. In addition, several
potential problems may arise because of the highrepetition rate operation and high beam intensity.
Therefore, the entire FX system needs to be upgraded forhigh power operation. Since the upgrade of FX system
involves many elements in the system extensive andfurther studies are needed. This paper focuses on the lowfield septa upgrade only.

90Figure 2: FX orbit in septa region
The system has been operated successfully since 2008.
#kj.fan@kek.jp

0.4

The four low-field septa are installed inside twovacuum chambers as shown in Fig. 4. The basic
parameters of the septa are summarized in table 1.

-0.1____________________________________________

0.6

LOW-FIELD SEPTA PROBLEMS

0.080
85
Insertion_C coordinate, mAcceleration start
FX septa synchronize with Acc.

0.80SM31

QDT0.175

COD_Y0.0SM32

QFR

-0.4

-0.8
Inj. 160 ms
1.00.30.2-0.2-0.6

The fast extraction (FX) system of the J-PARC mainring is composed of five kicker magnets, four low-field
septa (SM11/12, SM21/22) and four high-field septa(SM30/31/32/33). The layout of the septa system isshown in Fig. 1. All septa are design to have bipolar sothat the FX system has two functions: normal extraction
to the neutrino experimental target and abort extraction tothe garbage to protect the superconducting magnets of theneutrino beam line in case of hardware failures.

0.5

COD_X

0.0

Table 1: Basic parameters of low-field septa
Turns
Length,Current,
Gap field,
numbermATvalue 40.875
3250
0.23

+81-298645272- 410 -Field quality improvement

Figure 4: Low-field septa in vacuum chambers
The low-field septum is a four turns conventional
design septum with two “c” cores opposite each other togenerate bipolar dipolar field for both the extraction beamand the abortion beam, respectively.

To satisfy the above requirements, two long eddy current
type septum magnets are proposed to replace the present
four short septa. The cross section of the eddy current
septum magnet is shown in Fig. 7. Compared with thepresent low-field septum, an eddy current septum has
several merits, such as thinner septum thickness, stablemechanical structure, low leakage field and high gap field
uniformity. Since the coils are wound at the back-leg
region where there is no restrict space limitation, multiturn coils can be used so that the magnet has thecapability to provide higher field.

Figure 5: Coil structure of low-field septa (Left half side)

Figure 7: Bipolar eddy current septum

The coils are insulated by a ceramic layer, which isdeposited using a plasma deposition. In the original
design, the coils were fixed on the iron shield plate byskews insulated by ceramic washers. However, thewashers were broken due to the strong electromagnetic
force on the coils. An alternative method is fixing thecoils on the shield plate using stainless steel ribbon(30m) as shown in Fig. 5. The thin ribbon restricts themaximum gap field and the highest operational repetitionrate because of the weak strength.

A comparison of the field quality between the proposed
eddy current septum and the present septum SM1 isshown in Fig. 8. Eddy current septum can produce muchbetter field uniformity and much lower leakage field. Inaddition, the eddy current septum is excited at beam
extraction energy only, the influence of the leakage fieldon the 30 GeV beam can be neglected.

NEW LOW-FIELD SEPTUM

Gap field uniformity

10

Present septun SM1/2
New eddy current septum

15x10

Leakage field, T1550-5

Extraction beam-10Septum-15x10Present septum SM1/2
New eddy current septum-51050-5
Circulating beam

-10-15-3-12-11-10-9-8-7
Septum aperture_x, cm-6-3.0-5-2.0

-1.0

0.0

1.0

2.0

3.0

Transverse coordinate, cmGeneral requirements on low-field septa

Figure 8 Fields comparison between present/new septum

The new septa will not only replace the present septa
but also have capability to provide higher field for orbit
optimization. In order to preserve enough space for theupgrade of QFR and QDT in future, the new septa must
shorter than the present septa. Due to the narrow beam
separation, thinner septum is also required. Fig. 6 shows
the arrangement of the new elements and the beam orbit.
The new septa enable further studies to optimize the FX
beam orbit so that the requirements on high field septa at
downstream can be reduced.

Flatness of top field

0.20

Amplitude, m

0.15

QFR

SM11/12

SM21/22

New-SM1

New-SM2

In order to extract 8 bunches from the MR precisely, thesepta must provide flat top field (>6 s) with accuracy ofnearly 10-4. If the eddy current septum is excited by afundmental sinusoidal pulse only, the pulse duration must
be longer than 1 ms, which results in thicker septum andworse field uniformity. An alternative way is superimpose
a 3rd harmonic to a fundamental sinusoidal pulse, which
can reduce the pulse duration and then reduce the septum
thickness [1,2]. Fig. 9 shows the principle.
1.0

QDT

0.10

0.8

0.05

0.6

0.00

fundamental
10% 3rd harmonic
Required gap field

0.4

-0.05
0.2

-0.10
-0.15
73

0.074

757677787980
Insertion_C coordinate, m818283

050

100

150

200

250300Time, s350400

450 500x10-6

Figure 9: Long flattop field generation

Figure 6: New septa arrangement- 411 -

Two methods can be applied to realize the superposition of a 3rd harmonic on a fundamental harmonic.
One way is super position in pulse generator circuit, andother way is super position in magnetic field. Fig. 10
illustrate the principle.
Fundamental+
3rd harmonic

Fundamental+
3rd harmonic

Fundamental

3rdHarm.

Fundamental

SM1SM2

SM1

SM3SM2Figure 12: Eddy current with end clamps

A: super position in circuit
B: super position in field
Figure 10: Realization of superimpose of 3rd harmonic
Super position in magnetic field is easier, but it
requires at least three magnets and power supplies in thiscase. Three magnets not only occupy more space that may
required for Q magnet upgrade but also consume morecost. Super position in the pulse generator circuit creates
difficulties in the circuit design and waveform adjustment.
But it has the advantage of cost effective and can leave
more space for Q magnet upgrade in future. So, the study
will focus on the second method.

Fig. 13 compares the fringe leakage field with andwithout the end clamps during the acceleration period(t=0.2s), which is predicted by OPERA [3]. Since theleakage field works on the beam with 30 GeV, the effects
is negligible small.

Circuit design
In order to obtain a long flat top, a third harmonic
circuit can be added in series or in parallel on thefundamental harmonic circuit. The basic circuit with aparallel L/C resonant circuit is shown in Fig. 11. In theideal lossless case, the current flowing in the septum (Lm)is given by,isep (t )  k1 sin t  k 2 sin 3tWhere,  is determined by the inductance L1 and C1. To
optimize the third harmonic, it is necessary to adjust theparameters of the inductor L1.

Figure 13: Field comparison

Basic parameters of new septum
The main parameters of the new septum aresummarized in table 2.
Table 2: Basic parameters of new septa
Parameters
valueunit
Magnet length
1500mmAperture height
80mm
Aperture width
120mm
Septum thickness
7mm
Maximum field
0.6T
Maximum charging V300
VPulse duration
500s
Flat-top duration
10sFigure 11: basic circuit.

SUMMARY

The excitation current must be stable during the period
the extracted beams pass through the septa. To achieve
the very high stability and reproducibility, a digital
control loop must be applied

In order to provide high beam power for the neutrino
experimental station, high beam intensity injection andhigh repetition operation are necessary. However, theexisting and anticipated problems of the FX system may
have significant effects on the high power beam. Upgradeof the FX septa is necessary. The new eddy current septa
can not only resolve the present problems but also candeal with the anticipated problems.

Fringe leakage field shield
The new septa are designed with large physical aperturethat can accommodate the extracted with 70 mm.mrad.
As a consequence one of the detrimental effects is thelarge end fringe field that will impair the circulating beam.
The septum copper plate extends beyond the magnet ends
to help to reduce the end fringe field. In addition, four end
field clamps (see Fig. 12)are installed that can reduce thefringe leakage field greatly.

REFERENCES[1] G.H. Schroeder, et al., A novel eddy current septum
magnet for sps extraction towards LHC and CNGS.
[2] J.P. Royer, et al., High current with high precision flat-top
capacitor discharge power converters for pulsed septum
magnets.
[3] Opera user manual, Cobham Ltd.

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