The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

A Broad Band RF Cavity for JHF Synchrotrons
Miho FUJIEDA*, Yoshiharu MORI, Hitoshi NAKAYAMA, Chihiro OHMOIU,Kazuyoshi SAITOt, Shinya SAWADA, Yoshio TAN ABEt, Tomonori UESUGI§
and Masanobu YAMAMOTO,, KEK~Tanashi, 3-2-1 Midori-cho, Tanashi 188, JAPANEizi EZURA, Akira TAKAGI, Makoto TODA and Msahito YOSHII,KEK, 1-1 Oho, Tsukuba 305, JAPANAbstmct

2A broad band RF cavity in which FINEMET cores
are loaded has been developed for JHF (Japanese HadronFacility) synchrotrons. The FINEMET is a Fe-based
soft magnetic alloy composed of amorphous and ultrafine. grain. It has a low Q value, high shunt impedance
and permeability. Acceleration voltage of 8.5kV per gap
has been achieved with 12 cores. Acceleration voltage ofIOkV per gap has been achieved with an isolated RF pulse
for a barrier bucket scheme.

1

INTRODUCTION

The main parameters of the two synchrotrons in JHF{Japanese Hadron Facility)[!] are listed in Table l.

2.1

200p.A

9.6p.A

7A25 or 50Hz
420kV
42"'3.4MHz

7A
0.3Hz
270kV
173.4"'3.5MHz

Chamcteristics of FINEMET ·The test cavity was loaded with 24 pieces ofFINEMET. They are Fe-based soft magnetic alloys composed of amorphous and ultra-fine grain. They are madeof very thin tape which is coated with Si0 2 for insulation.
The FINEMET has very high permeability and a low Q
value[4]. The-Curie temperature ofthe FINEMET is highenough, as 570 °C, for stable at high temperature. The
shunt impedance of the FINEMET cores remains constantover a wide. range of RF magnetic field strength. On theother hand that of ferrite decrease above about 80 gauss.
Typical parameters of a FINEMET core are listed in Table 2.
Table 2 : Parameters of a FINEMET core
300mm ·inner diameter
670mm
outer diamete~,:thickness
25mm
15.50---permeability@3.4MHz
Q @3.4MHz
0.63 ~"
Rp @3.4MHz
83f!
"'

Table 1 · Main parameters ·3GeV booster 50GeV ring
averagebeam current
averagecirculating current
repetition rate
accelerating voltage
harmonic number
RF frequency

TEST CAVITY\2.2 · Components of the Test Cavity

The requirements for the JHF RF cavity are as follows.
Because of the space limitation for the RF apparatus, acceleration voltage of more than IOkV /m is required forthe 50GeV ring. About 13kV/m is needed for the 3GeVbooster. For the stable acceleration under the high beam
current, the shunt impedance seen by the beam should beless than lkf!Jm. In order to sweep RF frequency tuned
to momentum of proton, the shunt impedance should behigh enough over the wide frequency range. Because thegrowth rate of the coupled bunch instability is very fast
for a high Q parasitic resonance[2), there should be nohigh Q resonances. A test cavity[3] has been fabricatedso as to study the advantages of the broad band cavity.
This paper presents the results of the measurements withthe test cavity.
• also : Nuclear Science Research Facility, Institute for Chemical
Research, Kyoto University.
t also : HITACHI Co. Ltd.
f also : TOSHIBA Co. Ltd.
§also : CNS, University of Tokyo•also : RCNP, Osaka University.

The schematic view of the test cavity and the RF
amplifier is shown in Fig. 1. The length of the test cavity is 1.5 meter. The test cavity have two acceleration
gaps which are connected with each other by bus bars.
It consists of four cells of FINEMET cores. Each cell ismade of six cores. The RF power is inductively coupledwith a one-turn-loop wound around 12 cores. The shunt
impedance of the cavity is relatively high for the wide frequency range from 2MHz to 4MHz. Therefore it is notnecessary to have a tuning loop. The RF amplifier has atetrode, 4CW30,000A. Each piece of the FINEMET hasa water pipe on the inner fringe for cooling. Also fourfans and couple of water jacket attach on the cover of thecavity.

33.1

MEASUREMENTS

Impedance

We have tried to feed RF with a one-turn-loop anda two-turn-loop. The one-turn-loop was wound around-206-

The 11th Symposium on Accelerator Science and Technology, Harima ScienceoGarden City, 1997

I.OkV

G 2 .___ _ _ _ _ _ _ _ _ _ _ ____J

Figure 1: Schematic view of 30kW amplifier and test cav.:.
ity.
12 cores and the two-turn-loop was surrounded 6 cores
twice. The one-turn-loop was superior to the two-turnloop because the frequency of a parasitic resonance washigher and its amplitude was less.
The impedance of the cavity was measured with anetwork analyzer. The measured impedance as a functionof the frequency are shown in Fig. 2. The impedance seenat the gap point and the power feed point are presented
in Fig. 2(a) and (b), respectively. The solid curve shows
the impedance measured with the one-turn-loop and thedotted one shows the impedance measured with the twoturn-loop. There was a huge impedance peak seen fromthe power feed point. But no such peak was observed in
the impedance curve seen from the gap point. This was aparasitic resonance originating in the coupling loop itself.
It was hard to dump by putting parallel resistances tothe coupling loops. The beam sees the gap impedance,
therefore there is no harmful resonances up to 30MHz.

3.2 RF Voltage
The RF voltage and the current of the cavity wasmeasured with a high voltage probe and a current transformer, respectively. The maximum voltage was achieved,
when the amplifier was operated in class B. We have obtained 8.5kV per gap, which corresponded to 17kV percavity. Figure 3 shows the waveform of the voltage between the gap. This operation was done in a burst modeof 10 pulses with the repetition rate of 10Hz because thecurrent in the anode power supply of the amplifier waslimited. In case of continuous wave operation, we got
the maximum voltage of llkV per cavity. The waveformof the voltage in class AB operation is presented in Fig.
4. This waveform looks more like a sine wave comparedwith that in class B operation. When the amplifier ismodified to a push-pull one, a sinusoidal waveform canbe obtained. We have planed to modify the amplifier us-Figure 2: Impedance curves of the test cavity.

Figure 3: Voltage between the gap in class B operation.
ing two tetrodes to class B push-pull drive. It is hopedthat the RF power is increased and the continuous wave
operation at· the maximum voltage becomes available.
The RF magnetic field strength becomes 143 gauss inthe case of the gap voltage of lOkV. The RF power density in the core is 525 W / cc when the supplied RF poweris 56kW.
The surface temperature of the cores at the several
points was monitored with thermocouples. When the RFpower of lOkW was fed, the maximum temperature of thesurface reached 145°C after 4.5 hours and saturated. TheRF voltag~ was very stable during the measurement evensuch high temperature.

4BARRIER BUCKETThe barrier bucket[5][6] operation at the injection ofthe 50GeV ring has been considered to increase the beam-207-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997(a)

Figure 4: Waveform of the voltage in class AB operation.
intensity. It is necessary for the cavity to provide an isolated RF pulse so as to repulse the beam for the barrier
bucket scheme. A broad band cavity has advantages toprovide an isolated pulse:

Figure 6: (a) An isolated pulse of lOkV in class B operation. Each waveform shows the voltage between eithersides of the gap and earth.(b) An isolated pulse in class
AB operation.
The isolated pulse of lOk V per gap has been achieved
already in class B operation. The waveforms of voltage
between either sides of the gap and earth are given in Figure 6(a). Figure 6(b) shows an isolated pulse in class ABbperation.

• The required peak RF current for the broad band
cavity to drive the waveform is much lower than thatof the ferrite loaded cavity.
The cavity loaded with FINEMET cores is suited for thebarrier bucket.
·A barrier bucket experiment in the AGS is planed innext year. The test cavity will be modified to have fourgaps. Fig. 5 shows the cavity design for this purpose. The
length is about 2.6 meter. It has four gaps connected withbus bars and it is loaded with 48 FINEMET cores. It also
has no loops for tuning. A couple of the one-turn-loops
are used to feed RF from the push-pull amplifier.

(b)

5CONCLUSIONS

A test cavity loaded with the FINEMET cores has
been developed. An acceleration voltage of 8.5kV per gap
was obtained with class B operation of the tetrode andlOkV per gap for the barrier bucket. The impedance ofthe cavity was high enough over the range from 2MHz to4MHz. There was no parasitic resonance up to 30MHz.
The test cavity and amplifier are modified for the barrier
bucket experiment in the AGS.

6REFERENCES(1) Y. Mori et al., "The Japanese Hadron Facility", Proc. 1997
Part. Accel. Conf., Vancouver B.C., 1997.
[2) T. Uesugi eta!., "Longitudinal Coupled Bunch Instability
in JHF 50GeV Main Ring", Proc. 1997 Part. Accel. Conf.,
Vancouver B.C., 1997.

2610(3) C. Ohmori et al., "A Broad Band Cavity for JHF Synchrotrons", Proc. 1997 Part. Accel. Conf., Vancouver B.C.,
1997.

Figure 5: Schematic cross section of the modified cavity.
The maximum voltage of 40kV per cavity is necessary.
The RF frequency is 2MHz and the isolated RF pulse
is generated at the repetition rate of the revolution frequency of 357kHz. A peak voltage of 40kV allows abucket height of ~E = 7.7MeV and momentum deviation D..pfp = 0.4%. The beam instabilities originating inthe emittance growth will be studied. The beam acceleration after accumulation with the barrier bucket will beconsidered.

(4) M. Fujieda et al., "Studies of Magnetic Cores for JHF Synchrotrons", Proc. 1997 Part. Accel. Conf., Vancouver B.C.,
1997.
(5] M. Blaskiewicz, J.M. Brennan, "A Barrier Bucket Experiment for Accumulating De-bunched Beam in the AGS",
Proc. 1996 Euro. Part. Accel. Con£., Barcelona, 1996.
(6) J. F. Griffin et al., "Isolated Bucket RF System in theFermilab Antiproton Facility", IEEE NS-30, No.4(1983)
pp.3802-3804.

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