Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

Development of RF for PRISM-FFAG
Y.Kuriyama1,A), M.AokiA), Y.ArimotoA), Y.KunoA), A.Sato A), S.Nakaoka A), K.Nakahara A), T. Matsushima A),
M.Yoshida A), S.NinomiyaB), Y.Iwashita C), M.Aiba D), C.Ohmori D), S.Machida D), Y.Mori D), T.Yokoi D),
K.Yoshimura D), M.Yoshimoto D)
A)
Graduate School of Sciense, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043
B)
RCNP, Osaka University, 10-1 Mihogaoka, Ibaraki, Osaka 567-0047
C)
Institute for Chemical Research, Kyoto University, Gokasho Uji, Kyoto 611-0011
D)High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-080

Abstract
PRISM is a project to produce a high quality, highpurity and high intensity muon beam by the bunchrotation technique in FFAG synchrotron using ultra-highfield gradient RF systems. The system to achive the field
gradient of ~200 kV/m at the low frequency (such as 4-5MHz) is made and tested using a test cavity.

1 INTRODUCTION

important. Figure 2 shows the proposed saw tooth like RF
voltage[3]. In case of the saw tooth RF voltage, theparticle motion in the longitudinal phase space is linear
and angular speed of rotation is constant. The otheradvantage of saw tooth RF is that it has larger momentum
acceptance than an ordinary sinusoidal wave.
Figure 3 shows computer simulation result of thebunch rotation in the phase space. It shows that theparticles which have ±20 % spread will be rotated and[1]

PRISM is an experimental proposal for constructionof a highly intense muon source based on a novel idea ofphase rotation in a FFAG synchrotron. PRISM stands for
Phase Rotation Intense Slow Muon source. It would
provide a muon beam having an intensity of about 10111012 μ/sec, a narrow momentum width of a few % andno pion contamination. The energy of the PRISM beam isrelatively low (20 MeV in kinetic energy), since it isaimed to be used for stopped muon beam experiment ingeneral.
PRISM will use a pulsed proton beam from the JPARC 50 GeV proton synchrotron[2] by fast extraction.
PRISM will be placed at a pulsed proton beam facility,
which is not included in the PhaseⅠplan of J-PARC, but
recently proposed.
PRISM consists ofa pion capture system with large acceptance by ahigh selenoidal magnetic field,a pion decay and muon transport system in a longsolenoid magnet of about 10 m long, anda phase rotation system which accelerates slow
muons and decelerate fast muons by an RF filed.
Schematic layout of PRISM is shown in Fig. 1. One offeatures of PRISM is to do phase rotation with a FixedField Alternating Gradient synchrotron ring(FFAG)
where FFAG has a large momentum acceptance. Becauseof a short life time of muons, the bunch rotation should
be completed in 5 turns (1 µs) and RF voltage of 2-3 MV
in total is required.
To obtain the beam which has a small momentum
spread after the rotation, the shape of RF voltage is1

Figure 1: Schematic layout of PRISM.
the final momentum after 4 turns is less than ±5 %. In
case of the optimum energy and RF voltage shape, thefinal momentum spread of ±3 % will be obtained for
initial spread of ±30 %.

E-mail: kuriyama@kuno-g.phys.sci.osaka-u.ac.jp683

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

Figure 2: Saw-tooth-like RF voltage. The bold lineis the sum of three harmonics (thin lines), H=1,H=2, H=3.

Figure 3: Bunch rotation in case of saw-tooth likeRF voltage. Test particles which have different
initial momentum and phase are tracked. The initial
beam includes the bunch width of initial proton
beam (±5 ns).

2 PRISM RF DESIGNPRISM RF was designed to have the following features
magnetic alloy cavity,
wide band resonance,
thin cavity design: 35cm/gap.
tetrode tube amplifier with 30-40 kV DC voltage.
The magnetic field in the cores of PRISM RF system isestimated to be several hundreds gauss. The magnetic
alloy (MA) is one of materials whose characteristics arestable at this level of magnetic filed, and thus we employ
MA for the PRISM RF system[4]. To resonate at low
frequency (such as 5 MHz), the cut core configuration[5]will be used to reduce the large core inductance to theoptimum value. Another advantage of MA cavity is that it
is possible to design it as a wide band system[5]. By
choosing the optimum air gap of cut core and minimising
the gap capacitance, it is possible to change the quality
factor of the cavity at the fixed frequency. We areconsidering the dual harmonic operation of RF system byminimising the air gap.
Cores are all air-cooled since the RF power loss into
the core is very small due to small duty factor (about0.1 %) of the PRISM RF system operation. This enables
us to realize thin cavity configuration. In the current
design, six MA cores will be installed for each gaps,
whose size along the beam direction is only 35 cm. Fivegaps form a single cavity of 175 cm length. The thicknessof MA core is 35 mm, and closely packed each other tosecure more-than-9cm of gap size. The racetrack-shaped
core is adopted. Parameters of the RF system aresummarized in Table 1.
Number of gap per cavity
5
Length of cavity
1.75m
Number of core per gap
6
Core material
Magnetic AlloyCore shapeRacetrackCore size
1.4m X 1.0m X 3.5cm
Shunt impedance~159Ω/core@5MHz
RF frequency
4~5MHzFiled gradient~200kV/m
Flux density in core
320 Gauss
Tetrode4CW150,000E
Duty
<0.1%
Table 1: Parameters of PRISM-FFAG RF system
Each gap will be driven by push-pull amplifiers using
tetrode tubes, 4CW150,000E. The tube can be used at thehigh DC voltage of 40 kV and RF current of 60 A is
possible to generate. In case of 1 kΩ cavity impedance,
60 kV will be available as a gap voltage. Tetrodeamplifiers are installed either on-the-top of or underneaththe cavity to secure the total cavity length being less than
175 cm. A low duty factor enables the tubes to generate
the maximum RF power of 1.8 MW. Parameters of AMP
system are summarized in Table 2.
Operation mode of tube
Push-pull, class AB
Number of tubes per AMP
2Size of AMP
1.35m X 0.8m X 0.7m
Tetrode4CW150,000E
Max. RF current
60A
Max. cathode current (DC)50A
Max. plate voltage
40kV
Max. RF power
1.8MW
Table 2: Parameters of RF system684

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

3 A PROTOTYPE OF THE RF SYSTEM

5 ULTRA-THIN RF TEST CAVITY

A prototype of the RF system, which consists of anamplifier and an anode power supply and an auxiliary
power supply, was built and installed in the RCNP, Osakauniversity. A test cavity and a drive amplifier wereprovided by JAERI for the initial test of the RF system.
This cavity consists of ten finemet cut cores whose radius
is 900mm and has 8 cm gap. The pictures of thisprototype of the RF system are shown in Fig. 4.

Ultra-thin RF test cavity is being deigned. A single gap
model of to match race-track MA core. This cavity
consists of six MA cores with 9 cm gap, and whose size
along the beam direction is 35 cm.

6 CONCLUSION
RF test with the test cavity had started and we obtained
43 kV gap voltage at the 5 MHz frequency with 733 Ωcavity impedance. The ultra-thin RF test cavity is being
designed. The overall system test will be performed in thewinter of 2004.

REFFERENCES

Figure 4: A prototype of RF system in the RCNP,Osaka University.

[1] Letter of Intent to the J-PARC 50 GeV Proton SynchrotronExperiments, (LoI-24,26)[2] The Joint Project Team of JAERI and KEK, KEK-Report99-4, JAERI Tech, 99-059.
[3] M.Yamamoto et al., “Multi-Harmonic Acceleration withHigh Gradient MA Loaded Cavity”, PAC99.
[4] C.Ohmori et al., “ULTRA-HIGH FIELD GRADIENT RFSYSTEM FOR PRISM-MUON BUNCH ROTAION”,
proceedings of SAST03.
[5] C.Ohmori et al., “High Field Gradient Cavity loaded withMA for synchrotrons”, PAC99 (invited).

4 RF TEST WITH THE TEST CAVITYRF test with the test cavity had start at in the spring of
2004. Main purpose of this test is to test power AMP
system. Through this test, we obtained 43 kV gap voltage
at the 5 MHz frequency with 733 Ω cavity impedance.

Figure 5: The ultra-thin RF cavity

685