Proceedings of LINAC 2004, Lübeck, Germany

DEVELOPMENT OF C-BAND HIGH-POWER MIX-MODE RF WINDOW
S. Michizono#, T. Matsumoto, K. Nakao, T. Takenaka, S. Fukuda, KEK, Tsukuba, JapanK. Yoshida, Mitsubishi Electric Corporation, Amagasaki, Japan
klystron failures. The electric field at the edge and centerof the ceramic disk should be less than that of the S-band
disk (84 mm in diameter and 3.2 mm in thickness). The
bandwidth of the rf window should be more than 100MHz, which is sufficiently wider than the performance ofthe klystron and/or the acceleration structure. The
calculated values are given in Table 1. The electric fields
at the edge and at the center of the ceramic are about 1.7
MV/m and 3.7 MV/m, respectively. In order to satisfy
these requirements, it is necessary to enlarge the diameterof the alumina ceramic disk compared with thewavelength of the C-band, which indicates thetransmission of the higher modes, such as a TM11 mode.
By mixing the TE11 mode and the TM11 mode, lower
electric fields can be accomplished, thus making a ‘mixmode window’. A high-purity alumina ceramic of HA997 (99.7% purity, NTK Co.) having a high durability forthe transmission of high power [5] has been adopted. A
diameter of 78 mm and a thickness of 4 mm were chosen
in order to avoid any resonant frequencies around theoperation frequency (5712 MHz).

Abstract
A high-power C-band (5712 MHz) rf system (40 MW,2 µs, 50 Hz) is under consideration for an electron-linac
upgrade aimed for the super KEKB project. An rf
window, which isolates the vacuum and passes rf power,
is one of the most important components for the rfsystem. The window consists of a ceramic disk and a pillbox housing. The mix-mode rf window is designed so as
to decrease the electric field on the periphery of theceramic disk. A resonant ring has been assembled in order
to examine a high-power transmission test. The window
was tested up to a transmission power of 300 MW. The rf
losses were also measured during rf operation.

INTRODUCTION
An upgrade of the KEKB injector linac is underconsideration for the SuperKEKB project [1]. C-band rf
sources (5712 MHz, 2 µs, maximum 50 MW) will be
installed in the upgrade for a higher acceleration gradientof more than 40 MV/m [2]. The rf power (~40 MW) willbe transmitted through an rf window and a pulse
compressor, and delivered to acceleration structures. Therf window consists of an alumina ceramic disk and pillbox housing, which enables us to transmit the rf andseparate the vacuum, which is one of the importantcomponents for this upgrade. A surface discharge due toelectron emission at the edge of the ceramic under high rf
fields results in excess surface heating, leading topunctures.
In this paper, the design of a mix-mode window [3],where the electric fields at the edge of the ceramic
decrease, is described. The results of high-power tests
using a resonant ring are also summarized.

Figure 1: Schematic of the C-band window. The
length of the first ring (P1), second ring (P2), thirdring (P3) and inner radius of second (P4) and thirdring (P5) are the parameters to be optimized.
E [MV/m @50 MW]

DESIGN OF THE RF WINDOWThe new rf window is required to transmit rf power of50 MW (2 µs, 50 pps). The criteria of the new C-band rf
window are determined based on the electric fields of theS-band rf window. The S-band window has a long life
with an MTBF of more than 100,000 hours [4] under an rf
transmission of 50 MW (2856 MHz, 4 µs, 50 pps), and isreliable for long-time rf operation, even though leakage ofthe klystrons is one of the reasons for3.7 [MV/m@50MW]
1.7 [MV/m@50MW]

Bandwidth （VSWR<1.2）

600 [MHz]S-band E//

5C-band Etotal
C-band E//

432
1001020y [mm]304050Figure 2: Electric fields on the ceramic disk
calculated for C-band and present S-band rf window.

Table 1: Electric properties of the S-band rf window
Center of the ceramic
Edge of the ceramic
Maximum electric fieldon the surface of the ceramic

S-band Etotal

6

The window is constructed with a combination of threerings. Five parameters (Figure 1), which are necessary tomatch the two different modes in a same length, areoptimized using HFSS. In order to avoid any volume
resonance of TE012-like and TM014-like modes, the rfwaves do not propagate as complete travelling waves inthe ceramic disk (quasi-travelling wave). However, the5.5 [MV/m@50MW]___________________________________________#shinichiro.michizono@kek.jp

1Proceedings of LINAC 2004, Lübeck, Germany

electric fields at the center and edge of the disk are about
20% and 50% lower than the present S-band window,
respectively, as shown in Figure 2. This indicates that thatthe electric fields at the power of 50 MW at C-bandcorrespond to those at a power of 35 MW at the S-band,so that a longer MTBF will be expected compared withthe present S-band window.

higher than the operation frequency, and the resonancefrequency is almost the same as the predicted one, basedon a calculation using HFSS. Figure 5 shows aphotograph of this C-band mix-mode window.

HIGH-POWER TEST USING ARESONANT RING
Resonant Ring

LOW LEVEL MEASUREMENTS

A resonant ring is designed for a high-power test of therf window. A schematic of the ring is shown in Figure 6.
The main characteristics of the ring are determined by thecoupling ratio of the hybrid [8]. In this resonant ring, acoupling ratio of 14 dB is adopted, and a maximum of 18times higher power than the klystron output is available inthe ring.
The total length of the ring should be an integer of thewavelength. A rough adjustment was carried out insertinga spacer in the ring with a preciseness of 10 mm. Finetuning was done by adjusting the operation frequency,
which was 5710.2 MHz at a high-power test of the rfwindow. The resonant ring is evacuated by two nonevaporating getter pumps (NEGs) and two ion pumps
located on both sides of the rf window. The base pressure
was less than 10-7 Pa.

Several types of first and second rings having different
P1 and P2 parameters were prepared for determining thefinal dimensions. The electric fields were measured by aperturbation method [6] using this low-level model. Acubic (3 mm) of Rutile having high permittivity is used in
the measurements due to the low electric fields around theceramic surface. The measured fields are shown in Figure
3. The electric fields were measured from the center ofthe rectangular waveguide (z=-100 mm) to the center ofthe ceramic disk (z=0). The measured and calculated data
show good agreements.
Electric fields [arb. unit]

1210864High Power Tests

20-90-80-70-60-50-40
z [mm]-30-20-10

High power tests were carried out with a waveguideinstead of an rf window at first in order to progress theconditioning of the resonant ring. A maximum circulating
power of 400 MW (2 µs) was achieved after about 100Figure 3: Measured electric fields at the center of theaxis from the ceramic to the rectangular waveguide.
1.4
1.35

V SW R

1.3
1.25

210M H z

1.2
1.15
1.1
1.05
1
5.6

5.65

5.7
5.75
5.8
frequency [G H z]

Figure 5: Photograph of the C-band rf window.

5.85

Figure 4: Bandwidth of the C-band rf window.
Table 2: Volume resonances near to the operation
frequency
TM014-like [GHz]TE012-like [GHz]Calculation
5.78
>5.85

Measured
5.77>5.8

A window for high-power usage was coated with TiNfilms for multipactor suppression [7]. The measured
band-width (VSWR < 1.2) is more than 200 MHz, as
shown in Figure 4, which agrees well with the calculation.
The volume resonances were also measured, as shown in
Table 2. The TM014-like mode locates at about 50 MHz

rf window

Upstream

Downstream

Figure 6: Schematic of the resonant ring.

2

Proceedings of LINAC 2004, Lübeck, Germanydays of operation. High power tests of the rf window wereexamined up to 300 MW (2 µs), finally. Figure 7 shows
the rf power and vacuum pressure during 8 hours ofoperation. The rf trips took place only 3-times during
operation.
The optical emission was measured during rf operation.
Emission took place between about 70 MW and about

1412P loss [W]10-4300-5200-6100-7C-Mixwinφ78 4t

64RR test on May 27

400

8200100

200300

400

500

600

700

0Vacuum [Pa]RR power [MW]0468P average [kW]101214Figure 9: Rf losses at the ceramic disk measured by thetemperature increase of the cooling water.

are smaller than that of the S-band window, whichindicates the lower rf fields in the ceramic disk.

-8800

SUMMARYTime [min.]Figure 7: Rf transmission power through the rf windowand vacuum pressure during 8 hours of operation.

400 ns2The C-band rf window was designed based on a mixmode structure. The electric fields at the edge of theceramic, which are related to electric breakdown, become
about half compared with that of the present S-bandwindow. High-power tests were carried out with aresonant ring. The ring was evacuated to less than 10-7 Pa
and was successfully operated up to 300 MW, which isabout 6-times higher than the specification.

opticalemission
rf power (forward)

ACKNOWLEDGEMENT
The authors wish to thank Dr. Sergei Kazakov of BINP
for discussions and suggestions about the calculation ofthe mix-mode window.

rf power (reflection)

REFERENCES

Figure 8: Rf transmission power together with theoptical emission.

[1] SuperKEKB Letter of Intent (LoI), KEK Report 044 ,August,2004, http://belle.kek.jp/superb/loi/[2] S.Michizono, “KEKB INJECTOR LINAC ANDUPGRADE FOR SUPERKEKB”, this conference.
[3] S.Yu.Kazakov, “A New Traveling-Wave MixedMode RF Window With a Low Electric Field in
Ceramic-Metal Brazing Area”, KEK preprint 98-140,
Aug. 1998.
[4] S. Michizono et al., App. Surf. Sci., 169-170 (2001)
742.
[5] S. Michizono et al., IEEE Trans. Electr. Insul. 28(1993) 692.
[6] C.W.Steele, IEEE Trans. on Microwave Theory andTech. 14 (1966)70.
[7] S. Michizono et al., J. Vac. Sci. Technol. A10 (1992)
1180.
[8] L.J.Milosevic and R.Vautey, Inst.Radio Engrs. Trans.,
MTT-6(1958)136.
[9] S. Michizono and Y. Saito, “SURFACE CHARGINGBY UV IRRADIATION AT THE ALUMINA RF
WINDOW”, Proc. of 20th ISDEIV, Tours, June, 2002.

150 MW. The region of the optical emission depends onthe past records, and it is considered that the surface
charging of the ceramic disk affects the emission [9]. The
spectrum of the optical emission was also measured, anditwas
confirmed
thattheemission
wascathodoluminescence (luminescence induced by electron
irradiation) from the alumina ceramic, caused by oxygen
defects such as the F+ center [5]. The optical emission wasobserved at a higher power than that of the present S-band
window, probably caused by the lower electric field at theedge of the ceramic disk. The emission took place at theend of the rf pulse, as shown in Figure 8. A longer time isprobably necessary to accumulate a sufficient number ofelectrons contributing to the optical emission.
The rf losses were measured by the increase in thetemperature of the cooling water. The results are shown inFigure 9. The loss was 10 W at a transmission power of
10 kW, which is almost the same as the present S-band
window. Since the ceramic thickness is 30% thicker thanthe S-band window, the effective rf losses per unit length3