UPGRADE DESIGN OF INJECTION KICKERS FOR JPARC MAIN RING
K.Fan#, K. Ishii, T. Sugimoto, S. Fukuoka, H. Matsumoto
KEK, 1-1 OHO tsukuba Ibaraki, Japan.
Abstract
The JPARC main ring injection kicker system consistsof four identical lumped inductance kicker magnets. Thesystem can inject 8 bunches into the ring to fill 9 buckets
with spacing of 300 ns. It requires the kickers to produce
fast field pulse with rise time less than 300 ns and noreflection. However, the total inductance in the circuit
restricts the rise speed of the kicker field. A matchingcircuit installed in the air is used to reduce the reflection,
but it creates a serious problem of high voltage discharge.
In order to resolve the existing problem a new structure of
kicker magnets has been studied. The new kickers havesmall self-inductance that can make the kicker field fasterand reduce the reflection. To keep the temperature of thematching resistors constant, the new structure enables thematching circuit be installed in a container filled withinsulation oil, which not only cools the resistors
efficiently but also reduces the risks of HV discharge.

the existing problems but also can deal with theanticipated problems.

PROBLEMS NEEDS TO BE RESOLVED
Kicker large self-inductanceThe basic circuit is shown in Fig. 2, in which the kicker
is represented as an inductor. The kicker current responseis heavily dependent on the kicker coil self-inductance. In
order to eliminate the reflection due to the mismatch
between the kicker coil and the transmission cable, amatching circuit is used which compose of Cmch, Rmch andRld. The actual coil inductance consists of 3 parts LA, L0and LB as shown in Fig. 3. In view of kicker field
generation, LA and LB should be reduced as much as
possible.

INTRODUCTION
The J-PARC main ring (MR) injection systemcomprises two septa (SM1/2), four kickers (K1-4) andthree bump magnets (BP1~3), which is shown in Fig. 1.
The four lumped inductance type kicker magnets weredeveloped during the period of big earthquake shutdown[1,2]. Lumped inductance kicker has the merits ofstructure simple and operation stable. But the kicker field
waveform is heavily depends on the total inductance inthe circuit, particularly the coil self-inductance.
BP1

QFP

SM1

SM2

BP2 QDT

K1~4

QFR

BP33-50BT

Figure 2: Basic circuit

A perfect matching circuit can be constructed toeliminate the reflection in the kicker field. However, due
to the large self-inductance the rise time becomes too long
to be acceptable. Currently, the actual inductance isrelatively large (1100 nH) and a mismatched circuit had
to be adopted. The resulting kicker field waveform (seeFig. 4) has 3 defects (A, B and C) that will cause injection
errors.
1.0

Figure 1: Layout of MR injection system____________________________________________+81-298645272

Normalized amplitude

AOne big problem of the kicker is that the coil selfinductance is very large, which is about 1100 nH. Large
coil inductance results in long rise time of the kicker field.
A matching circuit is needed to reduce the reflection
caused by the mismatch between the kicker and thetransmission cable. However, the matching circuit
elements are installed in a matching box filled with aironly. This configuration creates another big problem ofhigh voltage discharge in the matching resistors. Potential
problems due to high beam intensity and high operation
repetition rate in future may arise also.
The injection kickers need to be upgraded to realize
high power operation. The upgrade will not only resolve#kj.fan@kek.jp

Figure 3: Kicker structure

0.8

0.6

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BC

0.0-0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2 4.54.8x10-6
Time, sFigure 4: Kicker field waveform

High voltage discharge of matching resistor
The kickers are installed inside the vacuum chamberwhile the matching resistors are installed in a matchingbox, which is separated from the vacuum chamber by
feed-throughes. In order to protect the vacuum systemfrom contamination in the case of a broken feed through,the matching box is filled with air only.
The voltage across the resistors is about 25 kV, and thetotal current is about 2.5 A. Fifteen resistors are installed- 407 -heat generated by excitation current increases linearlywith the repetition rate, while the image current heatgeneration depends on beam intensity as shown in Fig. 8.
Suppose repetition rate is 1 Hz and beam intensity is
2.6×1013 PPP, the resulting heat generation in the resistors
is about 420 W and the temperature is expected higher
than 200 oC, which might break the resistors.
Total, Win parallel to decrease the current density. However, thepipe structure and the rough contact surface of the resistor
reduce the effective contact area greatly, which leads tovery high "local" current density and high "local" electricfield between the resistor and the metal cap. A large
number of micro-discharge occurs easily and may damage
resistors [3]. Fig. 5 shows the pipe structure of resistors.

500

All power dissipation in matching resistors

400
300
200

Present status:
beam power 200kw, rep. 2.56s
Image current power

Power dissipation, W

4400

Excitation current power

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Excitation current
0300

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Excitation current

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Time, s
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Time, s

0.4

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0.93.54.0x10-6

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Repetition rate, Hz

Figure 8: Power dissipation in resistors

Figure 5: Matching resistor structure

Potential HV discharge inside kicker

High temperature of matching resistors
The excitation current and the image current will
generate significant joule heat in the terminator resistors.
Because cooling water can only be installed at earth
potential as shown in Fig. 6, the cooling efficiency isrelatively low.

The present circuit makes the kicker coil maintain highvoltage during the beam injection as shown in Fig. 9. For
high intensity beam injection beam halo can be large
enough to strike the ferrite generating secondary particles.
These particles can, in turn, ignite discharge inside thekicker magnet damaging the ferrite core
Kicker voltage
Injection beam

Figure 9: HV discharge inside kicker
Figure 6: Cooling of matching resistor

UPGRADE DESIGNFor long term stable operation the temperature of thematching resistors must keep constant. In practice,
however, the operation may stop occasionally and thenrestart that might change the temperature of the resistor.
The resistors’ temperature variation will change thecircuit parameters, which lead to different kicker angle ofsubsequent injection beam as shown in Fig. 7.
X, mm

1.6
1.5
1.4

Circuit optimization
If change the kicker and the resistor position, the kicker
coil potential becomes high only at the start and at the endof excitation. The kicker can keep earth potential during
moment when the injection beam pass though the kicker ,which is illustrated in Fig. 10. Therefore, even if highintensity injection beam strikes the ferrite and generate alarge number of secondary particles, discharge cannot
occur, which protects the kicker.

1.3
1.2
45.5

100200300400500600

Injection beamDegree

45.0
44.5
Temperature of matching box(air)44.0Resistors temperature is about100 oC

43.5
100200300

400500600

Kicker voltage

Time, min (Apr.18, 2012/ 17:40~24:00)

Figure 7: Temperature dependent injection errors
Figure 10: Low potential kicker

Potential high temperature in resistors
To realize the beam power of 1MW, both operation
repetition rate and beam intensity will increase a lot. The- 408 -Structure optimization
The new circuit enables the coil to connect to grounddirectly so that the kicker configuration can be optimized
to reduce the total inductance. Fig. 11 illustrates theprinciple.

flatten and lengthen the beam to reduce the space between
bunches. Narrow bunch spacing requires even fasterkicker field beyond what the present kicker can provid
We cannot improve the rise time by reducing the kicker
inductance further because of the restriction of the kicker
power supply. One solution is using a speed-up circuit(Cacc/Racc) and a tail bite circuit (Sw2)which, together, cangenerate required kicker field as shown in Fig. 14.
‐ Present‐ New kicker‐New kicker+ tail biter +speed

Figure 11: Simple kicker structure.
The total inductance of present kicker can be reduced
from 1100 nH to 710 nH. In this case, a perfect matchingcircuit can be realized to eliminate the reflection while therise time can also satisfy the requirement of beam
injection as shown in Fig. 12.
1.30x10-6

1.25‐ Present‐ New kicker

1.20Inductance, H

1.15
1.10
1.05

Present kicker
New kicker

1.00
0.95

Figure 14: Speed-up and tail biter circuit.

Coupling impedance reduction
For future high-intensity beam operation, beamcoupling impedance of the kicker is an importantparameter. The new simple kicker structure reduces thecoupling impedance significantly. Fig. 15 compares thelongitudinal coupling impedance.

0.90

600

0.85
0.80
0.75

Present kicker
New kicker

500
012345678

9Longitudinal impedance, 0.70
10Frequency, MHz

A: Inductance comparison B: Field wave comparison
Figure 12: Small inductance and resulting kicker field.

Cooling efficiency and insulation improvement

400300200100For high beam power operation in the future, the highpower dissipation in the matching resistor may increase
the temperature in excess of 200 oC. So it is necessary toimmerse the resistors in the cooling oil. To prevent
vacuum system contamination from a break in the feedthrough, the kicker magnet system must be modified. The
oil container is separated from the vacuum chamber by 2
feed-throughs. Fig. 13 illustrates the principle.

002040

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Frequency, GHz

Figure 15: Longitudinal coupling impedance comparison

Kicker pulse generator upgrade
The stray inductance in the pulse generator needs to beminimized also. New thyratron and its housing structure
will be redesigned. The absorber circuit will be modified
also to reduce the stray inductance.

SUMMARY
The biggest problem in the present injection system isthe kicker magnets that might be a big road block to highpower operation. To realize full power operation of the JPARC main ring the injection system needs to be upgrade.
In addition to the corrective actions on the existing
problems, we need to develop new technologies to deal
with the potential problems of high power operation.
Figure 13: Kicker assembly with cooling fluid.

REFERENCES

The insulation cooling fluid (silicone oil, flourinert) not
only improves the cooling ability but also helps to reduce
the HV discharge of the resistor.

[1] K. Fan, et al., “Design study of lumped injection
kickers for JPARC main ring,” PASJ 11.
[2] K. Fan, et al., “Design and test of injection kickers for

JPARC main ring,” IPAC12.
[3] T. Sugimoto, et al., “Performance of the Injection

Tail biter and speed-up
To mitigate the strong space charge effects of a highintensity beam, second harmonic cavities are used to- 409 -Kicker Magnet for the J-PARC Main Ring,” This
proceedings.