A NEW C-BAND 50-MW CLASS SiC RF LOAD
S. S. Win, H. Matsumoto, KEK, Tsukuba, Japan
M. Yoshida, University of Tokyo, Tokyo, JapanAbstract
A new high power rf dummy-load is being developedin this year as part of the R&D for the e+e- linear collider(GLC). In this rf dummy-load, sintered ceramic Silicon
Carbide (SiC) is used as the microwave absorbing material. Each SiC disk is 8-mm in diameter, and 3.3-mm high.
A total of 40 pieces of SiC per dummy-load can absorb
enough microwave power to meet the requirements
needed in a system with 50-MW of peak rf power at apulse width of 1-µsec and pulse repetition rate of 50-pps.
The main body of the load will be composed of cylindrical TM011 mode chained cavities for the ease of manufacturing. Simulating at 50-MW, 1-µsec and 100-pps, (therequired specification of the 500-GeV C.M. energy e+elinear collider), the calculated steady state temperaturerise of the SiC is only ~90 °C when the load is cooledwith 30 °C water.

1 INTRODUCTION
The first SiC-dummy-load, which was originally developed for the KEK 2.5-GeV injector linac at an S-band(2856-MHz) frequency in 1980, used direct water-coolingmethod [1]. Thermally this was very efficient, but corrosion problems risked water leaks into the vacuum. To
increase the safety of operation, the dummy-load wasmodified to use an indirect cooling method with brazed
rod shaped SiC pieces in 1995 [2]. 250 of the improved
SiC dummy-loads has now been in daily operation on theKEKB 8-GeV injector linac since 1998 [2].
The aim of the first phase ofthe Linear Collider project(GLC-I) in Japan is to build a
new high energy e+e- linear collider for the 300-500 GeV C.M.
energy region. For the GLC-I,
the klystrons will be of the 50100 MW class, and the accelerator will have to employ morethan 7,000 of them [3] and also
more than 10,000 high power rf
dummy-loads will be used.
When compared with accelerators currently in operation, thissort of large-scale machine requires an almost astronomical
total component count; certainly
heretofore no laboratory has any
experience in fabricating, or operating, so many accelerator devices. Therefore, manufacturability, reliability and maintainability

are all critical considerations.
There are no 50-MW class rf loads available for the Cband (5712-MHz) frequencies commercially in the world.
Therefore we have developed a new type rf load using
SiC ceramics with an indirect water-cooling structure. We
decided to use very simple cylindrical cavity of TM011
chained cavities. As can be seen in it can be manufactured
easily using a conventional turning lath. There are no special structural features required in the main body. Each
TM011 cell is of same physical dimension, with four SiC
disks of 8-mm in diameter and 3.3-mm in height brazed
symmetrically about the loading-disk. Thus, from the reliability point of view, it is greatly advantageous over theprevious rf dummy-loads. Furthermore, the new type rf
dummy-load allows elimination of 7,000 of the E-bendhigh power waveguide components, also contributing tocost reduction and system simplicity.
This paper will discuss the detailed design of the newtype rf dummy-load, especially using the HFSS (HighFrequency Structure Simulator) code.

2 OVERVIEW
The first model of the 50-MW SiC C-band rf dummyload is shown in Figure 1. The rf dummy-load consists ofthree main parts; an impedance matching section, a mode
filter and a main body. The impedance matching section
matches the impedance between the TE01 rectangular
waveguide from the power inlet and the TM011 mode cylindrical cavity.
As can be seen in Figure 1, we decided to use the simple impedance matching method (the so called door knob

Figure 1: A conceptual drawing of 50-MW class SiC rf dummy-load. (A cut away view)

E-mail: susuwin@post.kek.jp

type), which comprises of two rf elements. Although it isactually a distributed field situation, the two steps of door
knob dominate the effective capacitance C and the distance between the center of the door knob and the shortend of the waveguide dominates the reactance L. Thus
various impedances at the matching section can be effected easily through combinations of C and L easily.
The mode filter, which is used to excite only theTM011 mode into the chained cavities, and it can suppress
unwanted modes (at 83-mm cavity inner diameter) suchas TE11 (2116.6-MHz), TM01 (2764.9-MHz), TE21(3511.3-MHz), TM11 (4405.4-MHz), TE01 (4405.7-MHz),and TE31 (4830.0-MHz).
The main body includes a chain of 10 TM011 mode
cavities with 4 SiC disks per cavity, surrounded by a cooling water jacket. The cavities are separated by a 10-mm
thick loading-disk with a 55-mm diameter coupling iris.
The coupling iris aperture diameter is determined so as tokeep the transmitted rf power safely below a 300-W percc power absorbing density in each SiC disk. The loading
disk thickness of 10-mm was chosen to provide goodthermal conductivity for the SiC disks and to give mechanical strength.
Four SiC disks are brazed on one side of each loadingdisk; so there are 40 SiC disks in all which absorb the rf
power. We will use the same brazing method as was used
for the KEKB injector linac dummy-loads.
The 30 °C cooling water flow at the inlet is within
range of 10 to 20 L/min when running at the maximum rf
input power (50-MW, 1-µsec and 100-pps). In that case,the cooling water temperature rise at the outlet should be
only about 8 °C only. Therfore, we will use a very simple
water jaket, which has the pipe shape strucutre and one
way water flow system.

3 SIMULATION
We used the ANSOFT commercial FEM code, HFSS,
for simulation of the TM011 cylindrical cavity during thecavity design process. To understand the basic
characteristics of the TM011 cavity, we started with single
cavity, which is connected with the circular waveguide atboth sides as shown in Figure 2, then we added the SiC
disks to the simulation.
The loaded quality factor of cavity QL, which corresponds to the rf transmission loss was determined to findthe optimum diameter for the coupling iris between cavi-Waveguideties. To separate the TM01 mode from other unwanted
modes as well as to reduce the calculation time, the simulation was done for an idealized quarter-cavity assuming
perfect H boundary conditions at the truncation surfaces
as shown in Figure 2.

Figure 3: Typical resonance curves of TM011 cavity. Up: S11(reflection). Down: S21 (transmission).

Figure 3 shows the typical resonance curves of theTM011 mode for the reflection (up: S11) and the transmission (down: S21) power respectively. From the measurement of the plot, QL may be calculated by the equation QL= f/(2∆f) and the group velocity vg by the equation vg =1/2 kωL where k is the coupling factor when the cavities
are in series. Table 1 shows the QL, and vg as a function ofthe coupling iris diameter.
Table 1: Group velocity, vg, and loaded quality factor QL.

Diameter of coupling irisVg/cQL[mm]
400.07632
500.2171550.3329The coupling iris diameter was chosen to be 55-mm,
and then the power loss in each cavity was calculated with
the SiC disks mounted on the one side of the loading-disk
as shown in Figure 4. The power loss per cavity was calculated while varying the dimension of the SiC disks (seeTable 2). Finally the total cavity transmission loss taking
into account the coupling factor between the cavities for
5-cavity chain or 10-cavity chain were calculated again bycombining cavities in series.
Waveguide

CavityCavity
SiC

Perfect-H boundary

Perfect-H boundary
Figure 2: The drawing of single cavity for simulation.

Figure 4: The drawing of the cavity-chain for simulation.

The power loss per cavity is 0.05 if the SiC is 8-mm india and 3.3-mm in height. The loss per cavity is 0.16
when SiC in 8-mm in diameter with 3.8-mm in height.
Therefore, to achieve the power loss per cavity of 0.1,
choose the SiC to be between 3.3- and 3.8-mm high witha diameter of 8-mm.
Table 2: The various combinations of SiC dimensions andcavity loss.

SiC size
D (mm) × H (mm)Single cavity
6.0 × 3.0
8.0 × 3.0
5-cavity-chain
8.0 × 3.0
8.0 × 3.7
8.0 × 3.88.0 × 4.0
10-cavity-chain
8.0 × 3.3
8.0 × 3.8

ume corresponding to the maximum input rf power can becalculated as follow;
Q=Pave50[W ]
3== 7.53 [W / cm ]VSiC × N SiC 0.166[cm3 ] × 40

The temperature rise of a SiC disk is calculated from theequation;

Power loss/cavity

0.02
0.04
0.04
0.15
0.16
0.29
0.05
0.16

4 TEMPERATURE RISE OF SiC
To reduce the variation in the microwave loss-tangent,
we use β-crystallized SiC, which has a good uniformity ofpowder size due to the chemical reaction that occurs between silicon-dioxide (SiO2) and carbon-black (C) powders when processed at a temperature range of 1500 to1800 °C in an inert gas atmosphere. The resulting SiC
characteristics are listed in table 3.
Table 3: Basic characteristics of the SiC-ceramics.
3.14
Density (g/cm3)
Hardness2900
at RT1)
2(Knoop, kgf/mm )Thermal conductivity
0.19
at RT1)(cal/cm·sec·ºC)
0.14
at 600 ºC
Thermalexpansion 4.6 x 10-6 RT1) to 1200 ºC
coefficient (ºC-1)
Oxidation weight gain
0.015
at 1200 ºC for 24 hours(mg/cm2)
at RT1)5 x 105
DC Resistivity (Ω·cm)-1
7 x 10at 800 ºC
Dielectric constant
30~35
0.5 to 20 GHz2)
Loss tangent
0.3~0.5
0.5 to 20 GHz2)
Note: 1) RT: Ambient Room Temperature, 2) The measured
frequency range is limited by the network analyzer.

The heat flow through0L
the SiC at steady state isshown in Figure 5. FromQ
our design study, the size
Sof each SiC disk is 8-mm
in diameter and 3.3-mm inZheight so that the volume∆Zper one SiC disk, VSiC, isT0TL
0.166-cm3. Using the totalof 40 disks, NSiC, the therFigure 5: Heat flow in SiC
mal energy Q per unit volat steady state.

QL 

7.53  0.33 λ 2 0.8 2

∆T =



=2

 2  = 0.52 [°C ]where λ is 0.8 joules/cm-s-°C and L is the height of SiC
disk in cm.
Table 4: SiC temperature raise as a function of rf repetition rate.

rf repetition rate[pps]11050100∆T [°C]

Twater(30°C)+∆T [°C]0.52
5.13
25.63
51.25

30.52
35.13
55.63
81.25

5 SUMMARY
We have designed a new type 50-MW rf dummy-loadusing SiC absorber. From the simulation using the HFSS
code it was found that the use of the cylindrical TM011
mode cavity loaded with SiC disks provides a great advantage over existing rf dummy-loads. As the main partsof the rf dummy-load are axially symmetric and can be
manufactured by using conventional turning lathe, theresult is much better manufacturability in mass production
due to the ease of machining. In addition, it allows elimination 7,000 E-bend high power waveguide components;that will also contribute to overall cost reduction and system simplicity for the linear collider. Moreover, a very
simple jacket type water cooling system can be used as
the cooling water temperature rise at outlet is only ~8 °C
for an average power dissiapated is 5-kW.
From this work, for the next step we will develop anintegral rf dummy-load for the accelerating structure so as
to replace the output coupler. That would eliminate all
7,000 output couplers and thus reduce the costenormously. It will also increase the reliability especiallyin high gradient operation.

6 REFERENCES[1] H. Matsumoto, “Application of the SiC-ceramics for
Microwave absorber” (in Japanese), Proc. of the Linac Meeting in Japan, 1983.
[2] H. Matsumoto et. al., “Experience on the HighPower SiC Microwave Dummy load using SiC Absorber”, PAC99, March 29-April 2, 1999, New York
City, U.S.A.
[3] H. Matsumoto et. al., “KEK C-band RF System
R&D Status for Linear Collider”, Proc. of 28th
LINAC Meeting, July 30-August 1, 2003, Tokai, JPARK.