Proceedings of the 10th Annual Meeting of Particle Accelerator Society of Japan (August 3-5, 2013, Nagoya, Japan)

UPGRADE STUDY OF J-PARC MAIN RING INJECTION SYSTEM
K. Fan#, K. Ishii, T. Sugimoto, S. Fukuoka, N. Matsumoto, H. Matsmoto
High energy accelerator research organization (KEK)
Abstract
The J-PARC main ring has the ambitious goal ofaccelerating proton beam with power of 750 kW. To
realize the target, a high performance injection system iscrucial. However, several problems due to the imperfectperformance of the main elements have been observed inthe present injection system, which deteriorate theinjection beam orbit and cause beam losses. The problems
will become big road blocks to high power operation.
Thus, the injection system needs to be upgraded to correct
the difficulties. The upgrade includes two new designed
septa to reduce the leakage field, an optimized the kicker
system to improve the kicker field, and a compensation
kicker system to correct the injection errors. Careful 3Delectromagnetic field simulations are performed to ensure
the accuracy of magnets design. The 3D particle tracks inthe entire injection region are studied thoroughly toguarantee the injection beam pass through the injection
system without beam loss.

INTRODUCTION
The J-PARC MR injection system consists of two
deflecting septum magnets - a high field septum SM1 anda low field septum SM2, four kickers (K1-4) and threebump magnets (BP1~3). The schematic plan view of theinjection region is given in Fig. 1.

From the start of operation many technical problems
have been encountered. Some problems have been
resolved completely, but some of them are resolved partly[1]. The remaining problems may become worse with theincrease of beam intensity and the operation repetition
rate, which might become a big roadblock toward thehigh power operation. Presently, the operation problems
involve high field septum, the low field septum and thekicker magnets. The whole system need to be upgraded torealize the high beam power operation.

PROBLEMS NEED TO BE RESOLVED
Injection septum-I
The present injection septum-I has a very large physicalaperture, which leads to significant end fringe fields that
may interfere the circulating beam. The septum-I consistsof two parts SM1a and SM1b as shown in Fig. 3. Thisconfiguration makes the situation worse because thedownstream part SM1b tilt to the circulating beam line,
and its end fringe field can deteriorate the circulating
beam significantly.

Figure 1: Layout of MR injection system.

Figure 3: Injection septum-I structure.

The beam to be injected, from the 3-50 BT transport
line, passes through two septum magnets with a total
deflection angle of 260 mrad, and then the four kicker
magnets kick the beam onto the closed orbit with an angleof 8 mrad. A QDT magnet is in the between of the septaand the kicker, which also provide deflection angle to theinjection beam and minimize the required strength of thekickers.

Since the septum works in pulse mode, the leakage
field is time dependent that modulates the Closed Orbit
Distortion (COD) with time as shown in Fig. 4.

Circulating beam, 81 π

Bp1
QFP

Horizontal beam orbit, m0.1

Bp2

K1, ... K4

QDT

QFR

0.8

0.2
0.0

10

Bp3

SM2
SM1b-0.3

02

4

6

8

10

12

14

16

1820

Longitudinal coordinate s, mFigure 2: Injection beam orbit.

22240100

200Time, ms300

400500After reinforce the new magnetic shield plates, theleakage field have been reduced to nearly half. It is very
difficult to suppress the leakage field effects more____________________________________________#kj.fan@kek.jp

0-5

Figure 4: Leakage field effects on COD.

Injection beam
Circulating beam

Injection beam, 54 π
-0.4

COD modulation
Before correction
After correction

5-100

SM1a-0.2SM1 excitation current

0.4

0.0-0.1

Start injection

0.6

Amplitude, mm

0.2

1.0

Tel: +81-29-8645272- 54 -Proceedings of the 10th Annual Meeting of Particle Accelerator Society of Japan (August 3-5, 2013, Nagoya, Japan)

because the original design restricts further modification.
The leakage field effects on the COD before and after themodification of the septum are compared in Fig. 4. With
the increase of repetition rate the leakage field will
increase because of the induced eddy current. Therefore
its effects on COD become severer and may cause beamloss particularly for high intensity beam with significant
beam halo. In addition, the present power supply cannot
support the future high repetition operation, which needs
to be upgraded also.

Injection septum-II
Due to the narrow beam separation between theinjection beam and the circulating beam at the location of
the septum-II, an eddy current septum is used. It is
powered by a full sine wave current with a period of 600
μs. Two turn coils are selected to reduce the required
current of the power supply. Compare the conventional
direct drive septum, the septum can be made thinner. Theleakage field is reduced greatly by installing a thinmagnetic screen in front of the septum as shown in Fig. 5.

reduced to zero quickly before the subsequent circulating
beam come.
The present injection kicker is lumped inductance type,which has simple structure and low cost (Fig. 7).
However, the rise time of the field is dependent on the
total inductance in the circuit. The total inductance of thepresent kicker is nearly 1000 nH that might lead to very
longer rise time beyond acceptable. Thus, a missmatching circuit has to be adopted to speed up the risetime to 350 ns, but the reflection field increase. Inaddition, the present pulsed power supply uses the old big
thyratron, which has large conduction impedance at low
voltage that enlarge the reflection field about 5%.

Figure 7: Lumped inductance injection kicker.
The long rise time limits the maximum beam length ofthe circulating beam 125 ns, which prevent theimplementation of the second harmonic cavity to reduce
the space charge effects in the high beam power
operation. The large tail field can be seen by the previous
injection beam and increase that will beam emittance andcausing beam losses. Fig. 8 shows the reflection field
effects on the circulating beam.

Figure 5: Eddy current septum.
However, the large physical aperture provide
considerable end fringe field that can be seen by thecirculating beam and causing beam losses. Fig. 6 shows
the beam losses recorded by a DCCT monitor during theinjection period: the stored beam decrease a little
whenever the eddy current septum is fired.

Figure 7: Kicker field waveform.
The matching circuit elements are installed in the airand HV discharge between the end of resistors is a serious
problem, which degrades the kicker field waveform andenlarge the deterioration on the beam.

UPGRADE STUDY
Figure 6: Kicker field waveform.

Kicker system
The MR injection is single-turn injection, which repeatsfour times to inject 8 bunches to the MR during theinjection period. The kicker field must increase from zeroto the full during the time interval between the circulating
beam and the start of the injection beam, and must beIn order to increase the beam power to the design limitof 750 kW, both the beam intensity and the repetition rateof the MR must be increased, which calls for the upgrade
the injection system. The upgrade includes, design new
injection septum magnets, modify the main kicker
structure and modify the pulsed power supply. In addition,compensation kickers are being studied in parallel forfurther improvement the kicker performance.

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Injection Septum-I
One of the significant challenges to design a large
aperture septum is to reduce the leakage field to avoid
affecting the circulating beam. To overcome the problemof the present injection septum, several configuration of
the new septum have been studied carefully foroptimization. The new septum must parallel to thecirculating beam pipe at downstream, which can reduce
the penetration of the end fringe field to the circulating
beam region. An end fringe field clamp is installed tosuppress the leakage field further. Fig. 8 show thestructure.

Figure 11: Beam parameters comparison (Blue: entrance,Red: exit).
With the end fringe field clamp, the leakage field canbe suppressed to negligible level. Fig. 12 compares theleakage field seen by the circulating beam with andwithout the end fringe field clamp.
leakage dipole @ Present (FFT)leakage dipole @ Rectangle (FFT)

Leakage field By, Gs

20Figure 8: New injection Septum-I Kicker.
Due to the large aperture, the end fringe fields havesignificant interference on the beam, which can be studied
carefully by expending the magnetic field along thereference trajectory to multipoles using spatial Fouriertransformation. Fig. 9 shows the multipole fieldcomponents along the reference injection beam trajectory.
1.6

FFT analysis
Quadrupole

FFT analysis
Dipole1.4

By(FFT), T

By (FFT), T

1.0
0.8
0.6

-0.05

-0.10

0.4
0.2

-0.15

0.00.0

0.4

0.8

1.2

1.6

2.0

2.4

0.0

0.4

0.8

Orbit length s, m
0.04
0.02

30x10

By(FFT), TBy(FFT), T

1.6

2.0

2.4

2.0

2.4

FFT analysis
Octupole

-3

20

0.00
-0.02
-0.04
-0.06

50-5-2.0

-1.6

-1.2

-0.8

-0.4

0.0

0.4

0.8

Circulating beam orbit s, mFigure 12: Leakage dipole along circulating beam orbit.

Septum 210
0-10

-0.08
-0.10
0.0

1.2

Orbit length s, m

FFT analysis
Sextupole

10The new eddy current septum has the similar structure
as the present one but with several modifications. The
excitation pulse width increases from 300 μs to 400 μs toreduce the power supply voltage, and to make it have theability to generate higher field. The septum consists of 2parts s shown in Fig. 11 to reduce the effective thickness
and to improve the gap field quality.

0.00

1.2

15

0.4

0.8

1.2

1.6

Orbit length s, m

2.0

2.4

-20
0.0

0.4

0.8

1.2

1.6

Orbit length s, mFigure 9: Multi-pole field of septum gap field.
To evaluate the gap field quality, 3D particles can belaunched to pass through the septum as shown in Fig. 10.
The beam parameters at any location can be obtained byinserting a patch perpendicular to the reference trajectory.

Figure 10: 3D particle trajectories.
Fig. 11 shows the beam parameter at before theentrance of septum and at the exit of the septum.

Figure 13: Eddy current septum structure.
Two end fringe clamps can reduce the effects of fringe
field on the circulating beam greatly. The circulating
beam pipe uses iron to replace the Aluminium pipe canreduce the leakage field further. Fig. 14 shows the end
configuration of the new injection septum-II. The twoturn coils are insulated using ceramic instead of thepresent organic materials-kapton so that the radiation
resistance increase greatly, which is very important inhigh beam power operation.

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Proceedings of the 10th Annual Meeting of Particle Accelerator Society of Japan (August 3-5, 2013, Nagoya, Japan)

Compensation kicker
To improve the kicker performance more, twocompensation methods using small compensating kicker
are studied. Fig. 17 shows that the exponential slow riseof the kicker excitation current can be speeded up by
superimposing a small current pulse, which is generated
by a simple LRC circuit in over-damped saturation [3].
1.0Before compensation
Compensation
After compensation

0.8
Normalized Kicker field

Figure 14: End configuration of new injection septum-II.
The basic parameters of the present septum and the new
septum are compared in table. 10.6

140ns
0.4

350ns
0.2

Table 1: Basic parameters of new septa.
Parameters
PresentNew
Magnet length, mm
1500
1500
Aperture height, mm8080
Septum thickness, mm
107
Power voltage, kV0.60.4300
Pulse width, μs
400Cir. Beam pipe
Al.
Iron
Insulation
KaptonCeramic

0.0
600

800

1000

1200

1400

1600

1800

2000

Time, ns

Figure 17: Rise time compensation.
Since the reflection tail field shape is irregular, a fastcompensation that can provide an arbitrary waveform isbeing studied [4]. The kicker is transmission line typeusing lumped capacitor to make the kicker compact. The
structure of the kicker is shown in fig. 18.

Kicker upgrade
The major problems of the kicker system, slow rise
speed and large reflection field, are associate with thelarge self-inductance of the kicker. Serious HV dischargeof resistor is because of the ionization of air molecule in
the atmosphere. The septum coil structure can be
modified so that the matching circuit can be immersed ininsulation fluid as shown in Fig. 15. Both the problem ofslow rise time and HV discharge can be resolved [2].

Figure 14: Transmission line compensation kicker.

CLUSION
The upgrade of the whole injection system is being
studied. Some of the results have been obtained proving
the correct of our prediction. More results will beobtained gradually.

ACKNOWLEDGMENTFigure 15: New kicker structure.
The circuit can be modified a little, parallel Rm, toreduce the reflection field as shown in Fig. 16.
Experimental result has just been proven to reduce thereflection from 5% to 2%. Further reduction can berealized in principle.

The authors wish to thank Dr. Wu Zhang (BNL) for
helpful discussing on the upgrade of our injection kickersystem, particularly on how to reduce the reflection field.

REFERENCES[1] K. Fan et al, “Low leakage field septa for J-PARC main
ring injection system”, IPAC’2010.
[2] K. Fan et al, “Upgrade design of injection kickers for
JPARC main ring”, JSPA’2012.
[3] K. Fan et al, this proceedings[4] S. Fukuoka, K. Fan et al, “Development of a fastcompensation kicker system for J-PARC maing ringinjection”, IPAC’2013.

Figure 16: Modification kicker power supply circuit.

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