150 KW CW CIRCULATOR
Y. Takeuchi, S. Araki and H. Nakanishi
National Laboratory for High Energy Physics
1-1, Oho, Tsukuba-shi, Ibaraki-ken, 305, Japan
H. Baba, K. Shinohara and S. Tsuruoka
Nihon Koshuha Co., Ltd.
1119, Nakayama-ch o, Midori-ku, Yokohama, 226, Japan
Abstract
We developed Y -junction waveguide circulatorsfor. t~e the 508.6 MHz RF system for superconducting
cavttles of the TRISTAN main ring. Thirty-six circulators were constructed and thirty-two of them havebeen installed. Before installation, they were tested
at a power level of up to 150 kW CW with the output
port shorted. The results showed sufficient power
handling capability. In this paper, the high power
performance of these circulators is presented.
Introduction
We developed 150 kW CW Y -junction waveguide circulators in order to install in an RF system
for superconducting cavities of the TRISTAN mainring. A high power section of the RF system is shownin Fig. 1. Four cavities are driven by one klystron.
The output power of the klystron is divided into fourchannels by three magic-tees. The circulator is installed in each channel between the second magictees and the cavities. The reflected power from thecavity is dissipated at a 160 kW water-cooled dummyload. As thirty-two superconduct ing cavities havebeen installed, the same number of circulators areneeded. The reason for their installation is as follows.
cryostat

MT : magic-tee
OR : circulator
DL : 160kw dummy load

Fig. 1 High power section of the RF system

First, the circulators protect the klystron from thedestruction of the output window or the unstable operation due to the power reflected from the cavities.
Second, the cavities are desired to be isolated fromeach other by the circulators. Without them, a small
amount of the reflected power from a cavity might gothrough the magic-tee and reach the other cavities.
This will affect the stability of phase and amplitude of
the superconduct ing cavity.
The circulators should have the power handlingcapability of 150 kW. They should continue the operation even when the superconduct ing cavities aredetuned. The circulators should also fulfil the requirement that the phase shift in the circulator should
not change so much either at the low power conditionor the total reflecting condition of the full power, because this phase deviation makes the operation of thecavities unstable. Specifications of the circulator islisted in Table 1.
Table 1
Specifications of 150 kW Circulator
VSWR
Insertion Loss
Isolation
Input Power
Frequency
Cooling Water
Flow Rate
Pressure (max.)
Waveguide
Waveguide Flange< 1.15< 0.2 dB> 25 dB
150 kW
508.581 ± 1.5 MHz

40 1/min
10 kg/cm2
WR1500
UDR6

There are two types of waveguide circulators, adifferential phase-shift type and a junction type. Thedifferential phase-shift type circulator has high power
handling capability, but has a rather large insertion
loss and a bulky body. On the other hand, the junction type circulator, in general, has a better performance, but less power handling capability. Therefore,
the junction type is preferable if the circulator withthe sufficient power handling capability can be constructed.
Although the junction type is inferior at highpower handling, this type of circulator with the capability of 100 kW was already realized at 940 MHz.l)
Junction type circulators of 500 MHz are operated at a-112-

transmitted power level of 180 kW in the Photon
Factory of KEK.2)
To investigate the high power limit, we carriedout a preliminary test using a junction circulator of
the same structure as those used in the Photon
Factory, but tuned to 508.6 MHz. With a matchedload, we could operate it at the power level as high as
240 kW. Then, we tested it with the output port
shorted. In this case, at the power level of a slightly
below 150 kW, sparking occurred. From the check of
the interior structure, we concluded that we must
make the metallic corners rounder and electric contacts better to achieve higher power ratings.
We measured the phase shift for various power
levels with the output port terminated by a matchedload. The phase shift characteristics are affected bytemperature rise of the ferrite due to the insertion
loss. The power lost in the ferrite in the total reflecting condition is equal to the power loss when thepower of 300 kW is transmitted to the matched load.
The phase deviation at 300 kW, if extrapolated, is estimated to be about 5°. Although there might beother unknown factors which affect the phase deviation, we concluded that a circulator with small phasedeviation could be constructed by selection of aproper ferrite with a low insertion loss and good temperature properties.
Design and Construction
The circulator consists of a central junction section with the shape of deformed hexagon, and three
540 mm long waveguides attached to it, as shown inFig. 2. The interior of the junction section is divided
into three gaps by two metal plate, and there are six
ferrite disks, as shown in the cross-sectional view
given in Fig. 3. The ferrite disks are 4 mm thick andabout 25 em in diameter. Each disk is fixed on a copper plate, as shown in Fig. 3. Part of each waveguideis also divided by the metallic plates. Each port ismatched by posts inserted in every gap of the plates
in the waveguide section.
319

1533

159.5

15

As pointed out in the previous section, the performance of the circulator is affected by the temperature rise of the ferrite due to the power loss in it.
Therefore, to satisfy the above requirements, it is important to select the ferrite material with low loss andgood temperature properties. For this purpose, small
stripline circulators were made with three kinds offerrite material, "Y-5" and "6E-3" supplied by TDK and"G-DF" supplied by Mitubishi Electric. Measurementsof the VSWR, insertion loss, isolation and phase deviation were carried out at room temperature and at60°C. The ferrite material which showed the least
temperature dependence was not selected, because ithas large insertion loss. The other two kinds of ferrite
had a loss half of the first sample. Therefore, we havechosen the one which showed the better temperaturedependence. The selected one is "G-DF". It is agadolinium and aluminum doped yttrum-iron garnet(Y ,Gd)3(Fe,Al )s0 12. which has the saturation magnetization 47tM = 650 Gauss, Curie temperature Tc =200°C, dielectric loss tan o < 3 x 10-4 and thermal conductivity 6.7 W/m°C.
Each ferrite disk consists of 16 small ferrite
plates which are fixed on the copper base by an epoxy
resin and teflon screws. Cooling water channels areembedded in the copper bases to remove the heatgenerated in the ferrite. The magnetic field of about1000 Gauss is generated by permanent magnets. Themagnets are set on the top and bottom sides of thejunction section as well as in the two partitionplates)) This has the good advantages of obtaining auniform magnetic field and making the circulator
body compact.
High Power Performance
Two different tests were carried out to investigate the high power behavior of the circulators; aforward power test and a power test with total reflection. All circulators were tested in the total reflecting condition. On the other hand, the forward
power test was carried out for five circulators only.
Forward power test
The performance at the power level of 150 kW
was measured with both the portc2 and port-3 terminated by matched loads. The results show that themagnet
ferriteCu plate

c:LFig. 2Fig. 3

150 kW cw circulator-113-

Cross-sectional view of the circulator

input VSWR is less than 1.12 and the isolation greaterthan 22 dB. Typical values of the insertion loss andthe phase deviation are 0.13 dB and 2.3°, respectively.
Power test with total reflection
This high power test was carried out with theoutput port shorted. Each circulator was tested withtwo different short conditions : L = 1374 mm and L =1604 mm, where L was the distance from the circulator center to the short plate position. We were afraidthat sparkings might occur around the contact part
between the central junction section' and the waveguide. Hence, one short condition (L = 1374 mm) waschosen so that the electric field takes its maximum at
this contact plane. In the other condition (L = 1604mm), the electric field takes its minimum there.
All circulators stand the total reflection of 150
kW input power for more than 30 minutt::s in both
short conditions.
Figures 4 and 5 show the input VSWR and theinsertion loss in two different short conditions, where
the horizontal axis represent temporal serial numbersof the circulators. The results on the phase deviation
are given in Fig. 6. Although the results are rather
large at L = 1374 mm, they are less than 4° at L =1604 mm. The average temperature rises of thecooling water were 2.2°C at L = 1374 mm and 1.1 °C at
L = 1604 mm. Calculated power losses using thesevalues are 0.18 dB and 0.09 dB, respectively. Thetemperature difference along the 4 mm ferrite thickness is estimated to be 6.1°C and 3.0°C, respectively.
The different behaviors of the phase deviation between the two short conditions are considered partly
because of this temperature difference.

· Conclusion
Thirty-six circulators were constructed for theRF system of the superconducting cavities. They
showed the power handling capability of up to 150
kW and the sufficiently small phase deviation, evenwhen the output port is shorted. Sixteen of themwere installed in summer of 1988, and other sixteen
in this summer. Since then, they have been in operation without any trouble.
References
1) F. Okada and K. Ohwi, IEEE Trans. Microwave
Theory Tech., vol. MTT-26, pp. 364-369, May 1978
2) S. Tokumoto, private communication
3) A patent of Nihon Koshuha Co., Ltd., PAT. 57-43363
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Serial Number of Circulators

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Serial Number of Circulators

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Serial Number of Circulators

40Fig. 6 Phase deviation at 150 kW-114-