BEAM COMMISSIONING OF THE J-PARC LINAC MEDIUM-ENERGY
BEAM-TRANSPORT AT KEK – IIS. WangA)* , S. Fu A)*, T. Kato A), M. Ikegami A), S. Anami A), S. Arai A), K. Ikegami A), Z. Igarashi A),A. Ueno A),
M. Okada A), E. Kadokura A), N. Kamikubota A), M. Kawamura A), H. Kobayashi A),C. Kubota A), E. Takasaki A),
H. Tanaka A), J. Chiba A), F. Naito A), S. Noguchi A), Y.Fukui A),S. Yamaguchi A), K. Yoshino A), T. Ito B), J. Kishiro B),T. Kobayashi B), Y. Kondo B), T. Tomizawa B), K. Hasegawa B), F. Hiroki B), Y. Yamazaki B), H. Yoshikawa B)
A) High energy Accelerator Research Organization, KEKB) Japan Atomic Energy Research Institute, JAERI
Abstract
A Medium-Energy Beam-Transport line (MEBT) of JPARC was installed in KEK for beam test. An rf
deflector, as a chopper, was constructed and test. During
the test of the rf deflector, some deviation between theresonant frequency and the operating frequency wasfound. By designing new coupling loops for the two rf
deflectors, the resonant frequency of two deflectors are
all changed to 324MHz, while the bandwidth were kept
unchanged. The rise time of the RF deflectors is studied
by both high power test and beam test. Somemeasurements with the bending system of MEBT werealso done, and the beam energy and related measured
results are given.
1 INTRODUCTION
A Medium Energy Beam Transport line (MEBT) of
Japan Proton Accelerator Research Complex (J-PARC)
was installed in KEK for beam test. The layout of theMEBT is shown in figure 1. As a key component ofMEBT, an rf deflector was proposed[1] and designed[2]as a chopper, because of its merits of high deflecting
field, compact structure and easy to manufacture. An rf
chopper, consists of two RFD (RFD-A and RFD-B), hasbeen successfully developed. Because of the very low
loaded Q of the RFD, a coupled RFD system wasadopted in operation for decreasing the demanded rf
power by half[2]. Some frequency deviation was foundafter the construction of the RFDs. The difference
between resonant frequency and operating frequency isabout 2MHz, and this can not be compensated by thetuner. Although the bandwidth of the RFD is very large,
the frequency deviation still induces some negativeeffects: additional reflection and mismatch between twoRFDs. The modifications of the RFDs were made fortuning the resonant frequency to the operation frequencyof 324 MHz. The measured results indicate the good
performance of the modifications. The rise time of theRF deflectors is studied by both high power test andbeam test.

*On leave from Institute of High Energy Physics, Beijing

Some measurements with the bending system ofMEBT were also done, and the beam energy and related
measured results are given.
Much more detailed descriptions on the beam study ofthe chopper cavity are presented in a separated paper inthis proceeding [3].
Buncher-1 RFD-A RFD-B
Q-1 Q-2
SCT-1
FCT-1
RFQ PRF

Q-3

BEND
Q-4

SCT-2
FCT-2
PRF
ST

ST ST

Buncher-2
Q-5

Q-6

Q-7 Q-8

SCT-3
FCT-3
PRF
ST
ST TEST BENCHSCRAPEREMFARADAYCUP

BPM BPM

BPM

BPM

2.97 mBPM

BPM BPM BPM
EM
FARADAY CUP

Figure: 1 The layout of the MEBT
2 THE DEVELOPMENT OF THE RF CHOPPER
2.1 Design of the new coupling loops
The RFD cavity consists of two parts: the cavity bodyand the two end plates with large coupling loops. Tuning
the resonant frequency by modifying the coupling loop
has two merits: easy to fabrication and take less cost. The
difference between resonant frequency and operation
frequency is about 2MHz. To tune the resonant
frequency to 324 MHz, one idea is to move the large
coupling loops about 3 mm towards the electrode. In
practice, just about a 3mm gasket is needed between theloop and the end plate, then the resonant frequency istuned to 324 MHz. But in this case the bandwidth of theRFD cavity is increased to 36 MHz, this means much
more input power is needed than design. To tune theresonant frequency to 324 MHz and keep the bandwidth
and the other parameters unchanged, a new coupling
loop is proposed. The RF simulation code HFSS is usedin the design of a new coupling loop.
Figure 2 shows the shape and parameters of thecoupling loop. The thickness of the loop is 3mm. In
design calculations by HFSS, the resonant frequencies
are firstly tuned to the measured values by changing thegap distance, for RFD-A and RFD-B respectively tomodified respectively to tune the resonant frequency tooperating frequency of 324MHz. The tuner effect with
15mm insertion is included in calculations, and theinsertion of 15 mm is set as the default value.

w

2.2 The measured results with new coupling loops

hThe above designed loops were fabricated andinstalled in the RFD cavities. The cold model test andhigh power test show the good performance of themodification. Table 1 show the designed and measuredFigure 2: The shape and parameters of coupling loopfrequencies of RFD-A and RFD-B with new couplingloops. The measured frequencies have a good agreementcompensate the manufacture error of the cavities, thenwith design values, and the resonant frequency was
the width w and height h of the loop of the two RFDs arechanged to 324MHz for two single RFD cavities.
Table 1 The measured frequency with new coupling loop (for RFD-A)
Res. Freq.(MHz)
Bandwidth(MHz)
Tuner=0mm
Tuner=15mm
RFD-A
RFD-B
RFD-A
RFD-B
RFD-A
RFD-B
Old loop
322.30321.9030
30
New loop (design value)323.75323.75
324.00324.0030
30
New loop (Measured)323.81323.88324.00324.153030
Figure 3 and figure4 indicate the waveforms of theAnother method for measuring the rise time of theRFD-A in coupled RFD system in high power test. With
RFD is using the BPM signals of beam test. Figure 6
the new coupling loops in figure 4, the reflection isshows the fifth BPM signals under the case of only RFDsmaller than that of the old coupling loops, the matching
B is excited and no scraper. In this case, no deflected
between two cavities are much better in comparison
beam is stopped, and the signals of the fifth BPM reflectwith figure 3.
the beam position, which is proportional to the deflecting
field during the rise time of the RFD. It shows that the0.3
rise time is about 9 rf periods, which is same as that
0.2
obtained in the high power test.
0.1
It should be noticed that the above measured results
0arefor investigating the rise time of an RFD. Because of-0.1
thelarge input power and adoption of the coupled RFD-0.2
system,the rise time of beam is about 10 nesc[3]-0.3Field level

a-0.2

0.1

0.4t(microsec)

0.7

1

Figure 3: The waveforms of the RFD-A with old loop
0.3

Field level

0.2
0.1

2.5
2
1.5
1
0.5
0-0.5
-1
-1.5
-2-2.525050

75100t(nsec)-0.1

Figure 5: The measured rise time of rf pulse

-0.2
-0.3
0.5

0.7

0.9

1.1

1.3

0.05

1.5

0.04t(microsec)

2.3 The measurement of the rise time of RFD
Short rise time is an essential requirement for clean
chopping. The Figure 5 indicates the measured rise timeof the rf pulse, which is obtained from the rf monitor ofRFD. The directly measured rise time, the field levelfrom 10% to 90%, is about 27 nsec, or 9 rf periods. Thisis much longer than _= 10.6nsec which is obtained fromthe measured loaded Q value. The reason is that the risetime of 27 nsec includes the effects of the rise time of thepower amplifier, which is about 15 nsec, 5 rf period.

0.03

Amplitude

Figure 4: The waveforms of the RFD-A with new loops

0.02
0.01
0-0.01 1

501

1001

-0.02
-0.03
-0.04tFigure 6: The measured rise time by fifth BPM
3 MEASUREMENTS WITH THE BENDING
SYSTEM
One of the main purposes of the MEBT is to measure
the characteristics of a 3-MeV beam: a beam energy, anenergy width and transverse emittances. The measured)

beam properties are utilized for achieving the matchingof the MEBT beam to the DTL acceptances, and alsoutilized as the fundamental parameters of the simulationin the linac. Therefore, a bending magnet system wasinstalled between the second chopper cavity and thebeam scraper. Since a maximum beam current is 50 mA,
a space-charge effect is very strong in the MEBT. One of
the authors showed that the detailed three-dimensional
simulation with space-charge effects in a double slit
system combined with a bending magnet of 45 degrees
gives sufficient information for the beam properties. In
the MEBT system, the energy width of the beam can bechanged by using the first buncher. This procedureprovides a lot of useful information in a non-acromatic
bending system. The preliminary experiments by using
the bending system and the two-slit emittance monitor
were performed. Both measured and calculated results
are described.

function of the buncher phase. The measured energy
shifts due to the bumcher phase are shown in figure 7d,
in which the results measured from two different
methods are plotted: energy analyzing system with thebending magnet and time-of-flight methods with twophase monitors on the straight beam line.
3.2 Transverse emittance measurements
There are large effects due to the momentum spreadon the measured transverse emittance in the horizontal
phase space after the non-acromatic bending system(dispersion), while there is no such effect on the verticaltransverse emittance. Figure 8a shows the measured
transverse emittances in the both phase spaces by using
the bending beam line as a function of the energy spread,changed by the applied buncher voltage. Figure 8b shows
the ratio of the measured horizontal emittance to thevertical one. The calculated result on the equal beam
parameters is also plotted. It should be noted that themomentum spread varies rapidly along the beam linefrom the position of the first buncher to that of theemittance monitor. Thus, careful comparison between
the measured data and the results of simulation isrequired for obtaining the meaningful results.

3.1 Energy measurement
The relative energy and the energy width weremeasured as a function of the buncher voltage. Figure 7ashows the raw data. Figure 7b shows the measured rms
energy width as a function of the buncher voltage.
a)
Figure7c indicates the measured rms energy width as ab)

30

432
1030

Energy width (keV, rms)

5352520151050-1050

100

150200250

300

0350

2520-0.02c)-0.0620

Bending current (arbitrary unit)

40

60

80

100

Buncher voltage (kV)d)

-0.08

15
10-0.1-0.12

50

Dw-bend
Dw-FCT-0.04

Energy shift (MeV)

6Energy width (rms, keV)

Beam intensity (arbitrary unit)

7

0-200

-0.14

-180-160-140-120

Buncher phase (deg)

-100

-80

-0.16
-200

-180-160-140-120

-100

-80

Buncher phase (deg)

(a)
(b)
(c)
(d)
Figure 7: Results of energy measurements: I=23mA: a) beam signal,b) rms energy width vs. the buncher voltage, c) rmsenergy width as a function of the buncher phase, d) average-energy shift vs. buncher phase by both the time-of-flightmethod and the bending system
the different size of the coupling loops for first chopper
cavity RFD-A and second chopper cavity RFD-B, the1.4
resonant frequency of two chopper cavities are all
1.2changed to 324MHz, while the bandwidth and the other1
parameters are almost kept unchanged. The measured
0.8
0.6
results indicate the good performance of the0.4
modifications. The rise time of the RF deflectors is0.2
measured by both high power test and beam test, and the00
5 10 15 20 25 30 35 40
results have a good agreement with each other. A beam
Energy width (rms, keV)a)b)
energy, an energy width and transverse emittances are
Figure 8: Measurements of transverse emittances,measured at MEBT by bending system.
I=21mA: a)emittances vs. the energy width at thebuncher position, b)Ratio of the transverse emittances
REFERENCES(horizontal to vertical) as a function of the energy width[1] T. Kato, “New Design of an RF beam Chopper,” Proc.
at the buncher position. The calculated result is also
7th Symp. on Accelerator Science and Technology,228(1989).
plotted[2] S. Fu and T. Kato, “RF-chopper for JHF linac,” Nucl.
Instr. And Meth. A 440, 296(2000).
4 CONCLUSION[3] T. Kato, et. al. Beam Study with RF Choppers in theMEBT, in this proceeding
The modification of the chopper cavities weredone by designing new coupling loops. By adoption ofex

measuredeysimulation

2.2

2.2

2

1.8

1.8?x/ ?y4?rms (? -mm-mrad)

2

1.6

1.6

1.4

1.4

1.2

11.20

510

15

20

25

? w (rms, keV)

30

35

40

1