L-band Resonant Ring for Testing RF Windows for ILC
1

B.T. DU†,1, T. MATSUMOTO1,2, S. MICHIZONO1,2, T. MIURA1,2, F. QIU1,2, N. LIU1
SOKENDAI (The Graduate University for Advanced Studies), 240-0193, Hayama, Japan
2
High Energy Accelerator Research Organization, 305-0801, Tsukuba, Japan

Abstract
A resonant ring is widely used for the breakdown test of
RF components under high power. It can reach power gainof more than 10 dB, which is limited by the attenuation ofthe ring. An L-band resonant ring is constructed for testing
RF components of International Linear Collider (ILC)
which is based on an RF frequency of 1.3 GHz. The targetof the high power test is 5 MW. We have finished the testof an input power of 500 W using a solid state amplifier,and the principle of the resonant ring is verified. The resonant ring is tuned to an optimal condition, which is preparation for high power operation. This paper details the principle, construction, and test of the L-band resonant ring.

INTRODUCTION
The International Linear Collider (ILC) is a 250 GeV linear electron-positron collider, based on the 1.3 GHz superconducting radio-frequency technology [1]. Figure 1
shows the power distribution system (PDS) for ILC. Every
PDS contains one 10 MW multi-beam klystron and thatdrives 39 cavities. Based on Fig. 1, the power on RF window, which is next to the variable power divider, is 1.1 MW.
Considering the maximal standing wave, the power on theRF window may be 4 times to theoretical value, which is4.4 MW. Therefore, it is necessary to test the RF window
under 5 MW using a long operating period.

can be achieved by the resonant ring with an input from an800 kW klystron.
Secondly, considering the high power test of 5 MW circular power, the ring is expected to be pressurized by SF6or N2 for preventing high power breakdown. The variablehybrid and phase shifter cannot be worked as pressurized
due to its construction. Therefore, a fixed 11 dB hybrid isused to replace the variable hybrid and the operating frequency is changed to adjust one-turn phase. Moreover, the500 kW circulator cannot be used at 5 MW due to thepower limit. Even a low reflection generated in the resonant ring will be accumulated in the high reflected power.
A 3-stub tuner is inserted in the ring to reduce the reflectionin the ring. After adjusting the depth of the stub, the reflection is reduced significantly and the resonant ring worked
at an optimal condition with a higher forward power gain.

THEORY AND STRUCTURE
The structure of the resonant ring is shown in Fig. 2. Itcontains a signal generator, hybrid, dummy load, and certain length of the waveguide system.

Figure 1: Power distribution system for ILC.

Figure 2: Schematic diagram of the resonant ring.

A resonant ring is a closed loop of a waveguide systemthat can amplify power, and it is widely used for the breakdown test of the RF components under high power [2]. In
the superconducting RF test facility (STF), the variable hybrid, the phase shifter and the 500kW circulator have already been developed [3]. Moreover, the 800 kW modulated-anode klystron is developed in the STF, which is employed in the distributed RF system for the ILC [4].
Firstly, we a construct resonant ring with a variable hybrid, a phase shifter and a circulator at a low input powerof 500 W from a solid state amplifier. The power gain iscompared with one-turn phase of the ring and the coupling
factor of the hybrid is tested [5]. The measured resultshows that the maximum power gain can reach 10 ~13 dB,
and is matched with simulation. A circular power of 5 MW

If the hybrid is assumed to be lossless and symmetrical,
with no reflection, then the power gain (G) will be as given
in equation (1), where k1 and k2 are transmitted and coupling coefficients of the hybrid at voltage respectively; α,β, and L are the attenuation coefficient, phase coefficient,and length of the resonant ring respectively.
2bk22
G 32 2 2 L
1  k1 e
 2k1e  L cos  La1(1)It is obvious that the power gain reaches a maximum
when βL=2nπ, where n is an integer. This means that thewave in the ring will be superposed at an optimum condition [6].

Gmax k 22
1  k121  k12 e 2 L  2k1e  L 1  k e  L 2(2)If the transmitted coefficient k1 is a variable that corresponds to the variable hybrid, then the eventual maximal
power gain is obtained using equation (3) when k1=e-αL.
1'Gmax1  e 2 L(3)1SIMULATION AND MEASUREMENT OF
THE RESONANT RING
The resonant ring is constructed with a phase shifter, variable hybrid, 500 kW circulator, and an input of 500 W toverify the principle. Figure 3 is the structural diagram ofthe resonant ring. The maximal power gain relative to k1
and one-turn phase is measured. Further, the simulation iscompared with the measurement.

Figure 5 shows the measured and the simulated power
gain compared with transmitted coefficient k12 . The optimal condition is to make k12 equal to A, and the power gain
will decrease faster when k12 is larger than A. Figure 6
shows the measured and simulated power gain comparedwith one-turn phase drift (k12 = A = -0.21 dB). The delta
phase of the x-axis is the phase difference obtained fromthe optimum condition. The maximal simulated power gain
is 13.26 dB with a one-turn loss of -0.21 dB and the maximal measured power gain is 14.12 dB, which corresponds
to the one-turn loss of -0.17 dB. This may due to the measuring error of one-turn loss. The 3 dB width is 5.4° for simulation and 4.0° for measurement. If the one-turn phasecannot be adjusted to be an integer times 2π precisely, it isexpected to be within the 3 dB width to ensure sufficient
power gain.

Figure 5: Measured and simulated power gain comparedwith transmitted coefficient k12 .
Figure 3: Structure diagram of the resonant ring.
Figure 4 shows the simulated power of the resonant ringcompared with time. The parameters for the simulation are
obtained from the measurement of the constructed resonant
ring, such as one-turn phase, on- turn loss (A) and the transmitted coefficients (k12 ) of the hybrid are set to 11 times 2π,-0.21 dB and -0.21 dB, respectively. The maximal power
gain is 21.18 times (13.26 dB) and the rising time of theforward power from 10% to 90% is 0.89 μs. This showedthat the power in the ring is accumulated to a high value by
many turns and reaches saturation when the one-turn loss
is equal to the coupling input.

Figure 6: Measured and simulated power gain comparedwith one-turn phase drift.

PREPARATION FOR HIGH POWER OPERATION

Figure 4: Simulated power with time.

To prevent breakdown at high power, the resonant ringshould be pressurized; however, the variable hybrid andphase shifter cannot be used in this condition. The operating frequency is shifted to change the circular phase to adjust the resonant ring to its optimal condition. The fixed
hybrid of the coupling coefficient, which is -11.15 dB at
1.3 GHz, replaces the variable hybrid. The reflection dueto the hybrid is typically higher than -20 dB around 1.3
GHz.
The circulator cannot be used for the 5 MW case. We
find that even small reflections generated in the resonant

ring accumulate to cause high reflected power. It is therefore challenging to measure the power of the device under
test owing to the serious standing waves generated by thehigh reflected power. Thus, it is necessary to suppress orcompensate this reflected power in the ring.
A 3-stub tuner is commonly used to match impedanceand decrease reflection [7]. Figure 7 is the schematic diagram of the resonant ring with a 3-stub tuner.

Figure 8: Measured and simulated responses of fully
tuned resonant ring with input of 500 W.

Figure 7: Resonant ring with 3-stub tuner.
For the process of tuning in the preliminary step, a network analyzer is inserted in place of the H-corner near the
3-stub tuner to pre-tune the ring. In the second step, theresonant ring is constructed integrally and an oscilloscopeis used to monitor the waveform of the reflected signal in
the ring.
Figure 8 shows the measured and simulated response of
the fully tuned resonant ring with an input of 500 W. The
optimal or peak frequency is found to be 1.3017 GHz. The
difference between the forward and reflected power valuesof the ring is 56.31 dB, as shown in Fig. 9. With the measured one-turn loss of ring A, transmitted coefficient k12 , andcoupling coefficient k 22 are -0.04 dB, -0.50 dB, and -11.16
dB, respectively, and the measured and simulated maximal
power gain values are 14.80 dB and 13.26 dB, respectively.
We consider two possible causes for this difference. First,the measured one-turn loss may be less than the true lossvalue. Second, if k1 is not changed and k 22 is equal to one
subtract k12 , the expected G will be 14.76 dB, which is
close to the measured G of 14.80 dB. This means that when
the resonant ring is tuned well, all the input power exceptthat delivered to the load will be accumulated as the forward power in the ring. This will make the estimated valueof k2 larger than the measured value, and the hybrid will beoperated as the ideal model. For the ideal model of the hybrid, power will only be delivered to the straight and coupling ports but not the isolated port.
Unloaded quality factor Qunload can be calculated using
equation (4), where vg is the group velocity [6]. Measured
and simulated Qunload values are 6505 and 3176, respectively. The reason for the difference here is that the measured G and power across load are higher than the simulatedvalues.
 f G (1  A2 )Qunload =(4) vg 1  Pload / Pinput

Figure 9: Measured responses of fully tuned resonant ringwith input of 500 W.

CONCLUSION
The maximal power gain reached 14.80 dB with an input
power of 500 W after the resonant ring was tuned by 3-stubtuner. An 800 kW klystron can satisfy a target of circularpower of 5 MW with this power gain. A high power test
was conducted after the preparation and a new RF window
for PDS of ILC was designed simultaneously.

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HLRF scheme for ILC”, in Proc. 22nd Linear Accelerator
Conf. (LINAC’10), Tsukuba, Japan, 2011, paper MOP027, pp.
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