The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

ULTRA-HIGH FIELD GRADIENT RF SYSTEMFOR PRISM-MUON BUNCH ROTATION
C. Ohmori, Y. Mori, KEK, Tsukuba，JapanY. Iwashita，Kyoto Univ. Kyoto, Japan
S. N inom iya ，RC NP，Osaka U n iv ，Osaka，Japan
M. Aoki, Y. Arimoto, Y. Kuno, A. Sato, M. Y oshida，Osaka U n iv. ，Osaka, Japan

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 achieve the field
gradient o f 200-300 kV/m at 5 MHz is designed andunder construction.

advantage o f saw tooth RF is that it has larger acceptancethan an ordinary sinusoidal wave. Figure 3 shows apreliminary result for the bunch rotation in the phasespace. It shows that the particles which have ±20 %spread will be rotated and the final momentum after 4
turns is less than ±5 %. In case o f the optimum energy
and RF voltage shape, the final momentum spread o f ±3%
will be obtained for initial spread o f± 30 %.

INTRODUCTIONPRISM [ 1 ] is an experimental proposal for constructiono f a highly intense muon source based on a novel idea o fphase rotation in a FFAG synchrotron. PRISM stands for
Phase Rotated Intense Slow Muon source. It would
provide a muon beam having an intensity o f about 10111012 |Li/sec, a narrow momentum width o f a few % and nopion contamination. The energy o f the PRISM beam isrelatively low (20 MeV in kinetic energy), since it isaimed to be used for stopped muon experiments ingeneral.
PRISM will use a pulsed proton beam from the J-PARC
50 GeV proton synchrotron [2] by fast extraction.
Experimental apparatus and PRISM will be placed at a
proposed pulsed proton beam facility which is notincluded in the Phase I plan o f J-PARC.
PRISM consists o f•a pulsed proton beam to produce a short pion
pulsed beam,•a pion capture system with large-solid angle by ahigh selenoidal magnetic field,•a pion decay and muon transport system in a longsolenoid magnet o f about 10 m long,•and a phase rotation system which accelerates
slow muons and decelerate fast muons by an RFfield.
Schematic layout o f PRISM is shown in F ig .1 . One o ffeatures o f PRISM is to do phase rotation at a Fixed-Field
Alternating Gradient synchrotron (FFAG), which has
several advantages, such as a large momentum acceptance.
Because o f a short life time o f muons, the bunch rotation
should end in 5 turns (1 |Lis) and RF voltage o f 2-3 MV isrequired.
To obtain the beam which has a small momentum
spread after the rotation, the shape o f RF voltage isimportant. Figure 2 shows the proposed saw tooth like RFvoltage [3]. In case o f the saw tooth RF voltage, theparticle motion in the longitudinal phase space is linear
and angular speed o f rotation is constant. The otherP roiji 'CHon T arc^ : t
P rimary P rotoni I n j b ^i ion S vsti^

F ig .1 .Schematic layout o f PRISM.

Fig. 2. Saw-tooth-like RF voltage. The bold line is thesum o f three harmonics(thin lines), H = l ，Hニ2 and Hニ3 .

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

％( )
d/dv

0-10-2 0-100-5 0500100

phase(nsec)

Fig. 3. Bunch rotation in case o f saw-tooth-like RFvoltage. Test particles which have different initialmomentum and phase are tracked. The initial beamincludes the bunch width o f initial proton beam (土5 ns).

RF SYSTEM

is possible to design it as a wide band system [6]. By
choosing the optimum air gap o f cut core and minimising
the gap capacitance, it is possible to change the qualityfactor o f the cavity at the fixed frequency. We are also
considering the dual harmonic operation o f RF system byminimising the air gap.
The cavity will be driven by push-pull amplifiers using
tetrode tubes, 4CW150,000E. The tube can be used at thehigh DC voltage o f 40kV and RF current o f 60 A ispossible to generate. In case o f 1 kX2 cavity impedance, 60kV will be available as a gap voltage.
To achieve the required field gradient, we also adopt
the ultra-thin cavity design. The 1 m cavity consists o f 4gaps and each gap has 4 MA cores. Two sets o f cavity are
installed in a straight section as shown in Fig. 4. The sideand front views o f two cavities are shown in Fig. 5. Eachgap generates the RF voltage o f 土 25_37.5 kV and isdriven by two bus bars which are connected to a RF
amplifier. To anve 4 gaps in a cavity, 4 push-pull
ampimers are used. Two amplifiers are located above thecavities and others are set below it.
Table 1 :Parameters o f PRISM cavities

High Field Gradient
PRISM requires very high field gradient o f 200-300
kV/m at the low frequency (5 MHz). To realize such agradient, we have solved following problems;•Magnetic core:Magnetic Alloy•Way to drive the cavity: Tetrode tube which standfor 40 kV operation•
Length o f cavity : 25 m/gap.
Magnetic alloy (MA) cavity [4] has been employed.
One o f advantages o f MA is that the characteristics isstable at the high rf field. Figure 4 shows thecharacteristics o f magnetic cores [5]. Ferrites are thematerial which is used in some accelerators, however,they show the impedance change at the high RF field. Incase o f MA, the characteristics are constant at therequired magnetic field for PRISM (320-490 Gauss).
- - 令 - . ferrite A• • O - . ferrite B

Number o f cavities

12

Number o f gap per cavity

4Length o f cavity

1mCore material

Magnetic Alloy

Core size

1.4 m X 1.0 m X 3.5 cm

Core shape

Racetrack

Number o f cores16/cavity

RF frequency

5 MHz

Required field gradient

200 kV/m

Flux density in core

320 Gauss

Duty<0.1% (15jlisX 100/3.4sec)Racetrack し ore
Because o f a large horizontal physical aperture o f
FFAG, the racetrack shape for MA core is adopted. Incase o f racetrack and rectangular shapes, the shunt
impedance o f a core is given by the following equation,

R P = //0 〜 Q f 1球 2 / 1 ^⑴

where L2 and L! mean the inner and outer circumferenceso f the racetrack core.
Brf (Gauss)

Fig. 4. Characteristics o f magnetic cores. Solid lines mean
MA cores with different size. Dashed lines are ferritecores.
To resonate at 5 MHz, the cut core configuration [6]will be used to reduce the large core inductance to theoptimum value. Another advantage o f MA cavity is that it

Table 2: Parameters o f Racetrack core
Inner size

74 cm X 34 cm

Outer/Inner circumference

3942mm/1868 mm|iQ f @ 5MHz (design value)

5.5 X 109

Shunt Impedance> 180 H/coreThe 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Power AMP system
Because o f low duty factor, the push-pull operation o f
the tubes, 4CW150.000E, will generate the maximum RFpower o f 1.8 MW. Operation o f the power amplifier wasanalysed using a typical constant current characteristics ofthe tube. During the high power period, the cathode peakand DC currents will be 120 A and 40 A, respectively. To
generate the RF current o f 60 A, the required voltage
swing on the control grid is about 700 V. To drive thepower a m p lifie r ,1.5 kW solid state amplifier isconsidered. In case o f the narrow band operation togenerate only 5 MHz at the gap, the control grid circuit
will have a narrow band impedance and 1:3 step up
transformer will be used. In case o f the dual harmonic
operation, the driving circuit is under consideration.
The anode power supply is designed to be suitable forthe pulse power operation. The maximum current is 400
A for 15 \xs pulse duration to supply 4 sets o f theamplifiers. Although the peak power o f 12.8 MW isnecessary for 4 AMP systems, the average power is below
10 kW. The required power is stored in 4 high voltage
capacitors. To protect the tubes from the destruction bysparking, crowbar circuit is used.
The filament, control grid and screen grid power
supplies for 4 amplifiers are installed in an auxiliary unit.
The control grid power supply has a function to add thevoltage o f 150 V in short time. By this function, we canchoose the class AB operation although the idling tube
cathode current during off-operation is almost 0 A.
Table 3: Parameters o f AMP system
Operation mode o f tube

Push-pull, class AB

Number o f tubes per AMP

2Size o f AMP

1.35 m X 0.8 m X 0.7 m

Tetrode4CW150,000E

Max. RF current

60 A

Max. cathode current (DC)50 A

Max. plate voltage

40 kV

Max. RF power

1.8 MW

CONCLUSIONSThe RF system for the PRISM project has been designed.
The amplifier system is now under construction and will
be tested using a test cavity at the RCNP, Osaka
university in this winter. The ultra-thin RF cavity will betested in the next year.

Fig. 5. Side(left) and front(right) views o f RF system

REFERENCES[ 1 ] Letter o f Intent to the J-PARC 50 GeV Proton
Synchrotron Experiments, (LOI-24, 26)[2] The Joint Projct Team o f JAERI and KEK, KEKReport 99-4, JAERI Tech. 99-059.
[3] M. Yamamoto et a l, “Multi-Harmonic Acceleration
with High Gradient MA Loaded Cavity”，PAC99[4] C. Ohmori et al., “A Wide Band RF Cavity for JHF
Synchrotrons”，PAG97.
[5] M. Fujieda et al., “Studies o f Magnetic Cores for JHF
Synchrotrons”, PAC97.
[6] C. Ohmori et al., “High Field Gradient Cavity loadedwith MA for Synchrotrons”，PAC99 (Invited).