Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 TUP35

DEVELOPMENT OF A FOLDED MAGIC TEE FOR THE ILC POWER
DISTRIBUTION SYSTEM
Prakash Joshi𝑎, Toshihiro Matsumoto𝑎,𝑏, Shinichiro Michizono𝑎,𝑏𝑎
SOKENDAI (The Graduate University for Advanced Studies), 240-0193, Hayama, Japan𝑏
KEK (High Energy Accelerator Research Organization), 305-0801, Tsukuba, Japan
Abstract
The folded magic tee (FMT) is a component of the variable power divider (VPD). It is used in the radio frequency(RF) power distribution system of the International LinearCollider (ILC). A unique characteristic of this device is thatthe phase and coupling ratios can be adjusted. The fixedphase shifter and U-bend with actuator are two other components of the VPD. The VPD is completed by assembling twoFMTs, four fixed-phase shifters, and two U-bends. The FMT
was designed and manufactured. Simulated and measured
S parameters of the FMTs satisfy the requirements. TwoFMTs were tested in an L-band 1.3 GHz resonant ring with arepetition rate of 5 Hz, where a maximum of 5.5 MW transmission is possible. The FMTs were pressurized to 0.2 MPa(29 psig) using nitrogen gas. The test was conducted by
gradually increasing the power at various pulse widths.

INTRODUCTIONA VPD is a four-port network that can adjust the coupling
ratio and phase. This device can be pressurized to 0.2 MPa(29 psig), which is useful for power distribution in the pressurized waveguide system. Each VPD consists of two folded
magic tees (FMTs), one on the input side and another on
the output side. The FMT on the input side splits power
equally into two ports of the collinear arm when the input
is provided at the H-arm, as shown in Fig. 2. The FMT on
the output side distributes RF power to the E- and H-arm
according to the phase relationship between two inputs at its
collinear arm.

REQUIREMENTS
The FMT is a component of a VPD. Each VPD has twoFMTs, as shown in Fig. 3. The return loss and isolated port
losses at the H-arm and two ports of the collinear arm are lessthan -30 dB. The RF power is divided equally between twoports when input is provided at any port. The requirementsof the 5 MW FMT are summarized in Table 1.

The International Linear Collider (ILC) is a future
electron-positron collider with a center of mass of energy
250 GeV and luminosity of 1.35 × 1034 𝑐𝑚−2 𝑠−1 withthe option to upgrade in the future [1]. According to theILC-Technical Design Report (TDR) electrons and positrons
are accelerated from 15 to 125 GeV in the opposite directionin the superconducting (SC) 9-cell 1.3 GHz tesla-type
cavities of main linac [2]. The average accelerating gradientof the cavities is 31.5 MV/m with a variation of ±20%.
This has been addressed by adjusting the coupling ratioof the variable hybrid to ±25% [3]. A 10 MW multibeam
klystron drives 39 SC cavities through three local power
distribution systems (LPDSs), as shown in Fig. 1. The 13
cavities of LPDS are grouped into three sub-LPDS. The firsttwo sub-LPDSs contain four cavities and the final one has
five cavities, as shown in Fig. 1. The klystron emits radio
frequency (RF) power through two 5 MW ports that are distributed into three LPDSs using two 5 MW variable power
dividers (VPDs). The use of 5 MW VPDs is now proposed
instead of the 5 MW variable hybrid mentioned in ILC-TDR.

Table 1: Requirements of 5 MW Folded Magic TeeParameters

Value and Unit

RF power
Operational frequency
Pulse width
Repetition rate
𝑁2 gas pressure
Electric field|𝑆11 |,|𝑆41 |,|𝑆22 |, |𝑆32 |,|𝑆23 |, and |𝑆33 ||𝑆21 |,|𝑆31 |,|𝑆12 |, |𝑆42 |,|𝑆13 |, and |𝑆43 |5 MW
1.3 GHz1.65 ms
5 Hz
0.2 MPa [4]< 3 MV/m< −30 dB
-2.9 to -3.1 dB

Figure 2: Descriptive design of a folded magic tee.

Figure 1: Power distribution system for the ILC [1].

- 384 -Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 TUP35

change in coupling ratio between port-2 and port-3 of theVPD with the difference in height between the two U-bends.
The FMTs were tested in two optimal cases, as shown inFig. 5. The U-bends and fixed phase shifters’ dimensions
were optimized to get the electric field below 3 MV/m. Thesetwo components remain to be manufactured.

Figure 3: Descriptive design of a variable power divider [5].
In VPD, the RF power at port-2 ranges from 2.50 to
4.16 MW to satisfy the ±25% variation in the coupling ratioof the variable hybrid.

DESIGN AND SIMULATIONAn FMT was designed and manufactured. The crosssectional area of the E-arm was enlarged to suppress theelectric field. Two posts of appropriate dimensions wereinstalled such that the electric field remains below 3 MV/mand S parameters satisfied the requirements. To address theproblem while installing the horizontal post, we prepareda separate E-arm and joined it to the main body. All fourarm dimensions were designed such that bolts can be usedfrom any direction to fix the arm with other components.
The height, length, and width of FMT were 360, 250, and
330 mm respectively as shown in Fig. 4.

Figure 5: Change in coupling ratio between port-2 and port3 in variable power divider owing to the difference in height
between two U-bends. The S parameters |𝑆11 | and |𝑆41 |remains below -30 dB.

TESTS
Low Power Test
A low-power test of the two FMTs was conducted by using
a four-port network analyzer. The return loss at port-1 of theFMT1 is close to -30.00 dB; therefore, it is used on the input
side. In FMT1, port-1 is the input, and port-2 and port-3
are the outputs. In FMT2, port-2, and port-3 are inputs,and port-1 and port-4 are outputs. The S parameters almost
satisfied the requirements listed in Table 2.
Table 2: Comparison among Required, Measured, and Simulated S-parameters of Folded Magic Tees

Figure 4: (a) Side view of the designed folded magic tee.
(b) Front view of the designed folded magic tee.
The VPD was designed with a height, length, and widthof 700, 580, and 330 mm respectively. The coupling ratio
can be adjusted by sliding one U-bend inside the fixed phaseshifter, and the phase can be adjusted by sliding both Ubends inside the fixed phase shifter in the same direction.
When the heights of both U bends are the same, all power
goes to port-2 and when height varies by 80 mm, then all
power goes to port-3, as shown in Fig. 5. Figure 5 shows theS Para.

Req. (dB)Sim. (dB)|𝑆11 ||𝑆21 ||𝑆31 ||𝑆41 ||𝑆12 ||𝑆22 ||𝑆32 ||𝑆42 ||𝑆13 ||𝑆23 ||𝑆33 ||𝑆43 |< −30-2.9 to 3.1-2.9 to -3.1
< −30
-2.9 to -3.1
< −30
< −30
-2.9 to -3.1
-2.9 to -3.1
< −30
< −30
-2.9 to -3.1

-43.01-3.01-3.01-66.83-3.01-42.10-36.85-3.01-3.01-36.86-42.52-3.01

Measured (dB)
FMT1 FMT2-29.63
-3.06
-3.06
-44.68-3.06-36.09-31.56-3.06-3.06-30.96-53.66-3.06-27.83-3.04-3.06-44.92-3.05-33.67-31.52-3.07-3.07-30.99-40.78-3.05

High Power Test
Resonant Ring: It is a waveguide loop that can amplify
the apparent power through the coupling of waves; therefore,
it is useful for high-power tests. A four-port FMT can be- 385 -Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 TUP35

tested in the resonant ring (RR) by connecting the collinear
arm of two FMTs with two H-corners to design a ring. Acombination of two FMT and four H-corner functions as
the VPD. When all power goes to port-1 of FMT2, it is
the same scenario as when all power goes to port-2 of theVPD (setup1). Furthermore, when all power goes to port-4of FMT2, it is similar to all power going to port-3 in theVPD (setup2). The coupling must be in the same phase as
the ring power to amplify the power. A 3 dB hybrid with aphase shifter is used to increase the power gain by adjusting
the phase [6], as shown in Fig. 6. The klystron output exhibits some fluctuation owing to the constant forward powerwith fluctuations in the gain. During the experiment, gain
fluctuated between 10.8 to 13.8 dB owing to fluctuations intemperature.

all power is transferred to port-2 in VPD. For each pulse
width, the power was gradually increased and maintainedfor one hour at 1, 2, 3, 4, 5, and 5.5 MW, as shown in Fig. 8.

Figure 7: Setup1 showing the installation of two folded
magic tees in a resonant ring for the high-power test. Thecomponent names are indicated in the figure.
Figure 6: Sketch of the resonant ring showing two folded
magic tees installed in it. The figure also shows the positionof the components such as RF window, 3-stub tuner, klystron,
directional couplers, circulator, phase shifter, 3 dB, and11 dB hybrid.
For the high-power test, an L-band 1.3 GHz RR driven by
an 800 kW klystron pulsed with a repetition rate of 5 Hz wasused. A 500 kW circulator was used to prevent the klystronfrom reflecting power. The FMTs section of the ring waspressurized to 0.2 MPa (29 psig) using nitrogen gas, and theremaining section of the RR was pressurized to 0.1 MPa
using 𝑆𝐹6 gas. An RF window is utilized to apply distinct
pressures and gases to the FMTs section and the remaining
section of the ring, as shown in Fig. 6. A three-stub tuner
was used to suppress the reflected power. Three directional
couplers were installed to measure the RF power, asshown in Fig. 6. Five acoustic sensors and 16 temperature
sensors were installed in the RR. The test was conductedfor a pulse width of 50 𝜇s, 200 𝜇s, 500 𝜇s, 1.00 ms, and1.65 ms. For each pulse, the RF power was increased gradually, but the test was not performed continuously; instead,
it was conducted at a favorable time and data were combined.
Folded Magic Tee Setup1: The lengths of the connections between FMT1 and FMT2 are the same for bothcollinear arms, as shown in Fig. 7. Therefore, the input
phases at port-2 and port-3 of FMT2 are the same, and all
power went to port-1 of the FMT2, and port-4 ends with theload, as shown in Fig. 7. This setup is similar to a VPD when

Figure 8: (a) One-hour testing of the folded magic tee at
1, 2, 3, 4, 5, and 5.5 MW with a pulse width of 50 𝜇s. (b)One-hour testing of the folded magic tee at 1, 2, 3, 4, 5, and
5.5 MW with a pulse width of 1.65 ms and an additional one
hour at 5.5 MW. The fluctuation in klystron output was due
to fluctuation in the gain.
Folded Magic Tee Setup2: The difference between thelengths of the connections between the collinear arms ofFMT1 and FMT2 is 160 mm, as shown in Fig. 9. Therefore,the input phase at port-2 and port-3 of FMT2 are opposite,- 386 -Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 TUP35

and all power goes to port-4 of the FMT2 and port-1 ends
with load, as shown in Fig. 9. This setup is similar to theVPD when all power is transferred to port-3 in the VPD. Our
test plan was to repeat the process in Setup1. The first arcing
was recorded in FMT1 at 3 MW for a pulse width of 200 𝜇sand solved through the conditioning effect. Owing to thearcing in the RR remaining test could not be completed.

a pulse width of 1.0 ms and 5.5 MW for 30 min with a pulse
width of 50 𝜇s, as shown in Fig. 10. During operation, an
average of 67% of power was transmitted to port-2, and 33%of power was transmitted to port-3 of the VPD. Therefore,
Setup1 is more realistic.
Table 3: High-power Test Results
Parameters

Requirement

Test Setup1

RF power
Pulse width
Repetition rate
𝑁2 gas pressure

5 MW
1.65 ms5 Hz
0.2 MPa

5.5 MW
1.65 ms
5 Hz
0.2 MPa

In the future, U-bends and fixed-phase shifters will bemanufactured. The low- and high-power tests of the Ubends with fixed-phase shifters will be conducted. All threecomponents will be assembled to construct VPD. Low- andhigh-power tests of the VPD will be conducted. Finally,
5 MW VPD will be demonstrated at the superconducting
RF test facility of KEK.
Figure 9: Setup2 for high-power test of the folded magic tee.
The installation of an 80 mm phase shifter distinguished itfrom Setup1.

SUMMARY
Two folded magic tees were designed, manufactured, andtested. The low-power test was completed using a four-port
network analyzer and the measured data were matched withrequirements and simulation. The high-power test was completed in two test setups using a resonant ring. Test Setup1
offers a more realistic approach with variable power dividers.
In Setup1 5.5 MW power with a pulse width of 1.65 ms wastransmitted for two hours; therefore, the evaluated folded
magic tees are suitable for use in a variable power divider.

ACKNOWLEDGEMENTS
I am grateful to Kazuya Ishimoto, Naoto Numata, andTaiga Hanawa of the NAT group for their help with experiment setup and measurements. I also extend my thanks toMizukoshi Corporation for manufacturing.

REFERENCES

Figure 10: (a) One-hour testing of the folded magic tee at 1,2, 3, 4, and 5 MW, and 30 min at 5.5 MW with a pulse widthof 50 𝜇s. (b) One-hour testing at 1, 2, 3, and 4 MW witha pulse width of 1.00 ms. The fluctuation in the klystron
output was due to fluctuation in the gain.
These two tests evaluate the reliability of the FMTs forthe following reasons. In Setup1, the test was completed
successfully at 5.5 MW, which is higher than the requirementof 5 MW. In Setup2, the test was completed for 4 MW with

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[4] Faya Wang et al., “Study of radio frequency breakdown inpressurized L-band waveguide for the ILC”, Applied Physics
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[5] Nantista Christopher et al., “Waveguide component R&D forthe ILC”, Proceeding IPAC-2013.
[6] Du Baiting, “Development of a compact Local Power Distribution System for the ILC”, Ph.D. Thesis, SOKENDAI, Japan,
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