The 9th Symposium on Acceleralor Science and Technology, Tsukuba, Japan 1993

Development of L-band Pillbox RF Window
Y.Takeuchi,S.Fukuda,H.Hisamatsu and Y.Saito

National Laboratory for High Energy Physics (KEK)
1-1 Oho, Tsukuba-shi,Ibaraki-ken,305,Japan
A. Takahashi
Mitsubishi Heavy Industries, Ltd. Mihara Machinery Works
5007ltozaki -cho.Mi hara -shi,Hi roshima-ken, 729-03.1apan
Abstract
A pillbox RF output window was developed for the L-band pulsed
klystron for the Japanese Hadron Project (JHP) I-Ge V proton linac. Thewindow was designed to withstand a peak RF power of 6 MW, where
the pulse width is 600 11sec and the repetition rate is 50 Hz. A high powermodel was fabricated using an alumina ceramic which has a low loss
tangent of 2.5xl o-s. A high power test was successfully performed up
to a 113 kW RF average power with a 4 MW peak power, a 565 11sec
pulse width and a 50 Hz repetition rate. By extrapolating the data of thishigh power test, the temperature rise of the ceramic is estimated low
enough at the full RF power of 6 MW. Thus this RF window is expectedto satisfy the specifications of the L-band Klystron.

Introductio nSeveral R&D studies have been being carried out for the 1-GeV
proton linac of the JHP. There will be 36 L-band pulsed klystrons(f= 1296MHz) used in the high-P section of the JHP linac. The klystron
will be required to supply a RF peak power of 6 MW (600 115ec, 50 Hz)[1]. A prototype of the klystron is being developed at KEK [2]. As a partof this work we have developed an L-band pillbox RF output windowfor the klystron. This window can be used also as an input RF window
for the high-P coupled-cell cavity.
Pillbox-type RF windows are widely used for high power klystrons.
Figure 1 shows a schematic drawing of a pillbox RF window. We madetwo types of high power test windows which have different alumina
ceramics. Two windows of type A and two windows of type B weremade with the alumina ceramic HA95 and HA997, respectively. Theseceramics were supplied by NTK co ..
HA95 was adopted for the first model (type A) to establish thefabrication techniques such as brazing between a ceramic disk and acopper sleeve. because the cost of this ceramic was much lower than thehigh purity alumina ceramic HA997. Butthe type-A window had a large
temperature rise of78.5 de g. at an average power of93.75 kW with a RF
peak power of 5 MW, (375 11sec, 50 Hz). And the temperature rise ofceramic at full RF power was estimated to exceed 100 deg .. Since thethermal shock fracture resistance of the alumina ceramic decreases
abruptly when the thermal shock exceeds 200 -300 deg., the alumina
ceramic should not be used over 200 oc_ When we take into account thesafety factor, the temperature rise of the ceramic should be less than I00
deg .. Therefore, we had to reduce the temperature rise of the ceramic
disk.
For the second model (type B), we adopted HA997 ceramic disk, theloss tangent of which is 2.5x I0-5and about 1/!0of that ofHA95 (3x!04). Thetype-B windows were tested up to the average power of 113 kW.
The design procedures of these two types of windows and theperforrnance ofthe high power test models of the type-A windows werealready reported precisely [3,4]. In this paper we mainly discuss theperformance of the type-B windows.

Matching solutions of pillbox RF window
A pillbox window shown Fig. I can be expressed by an equivalentcircuit such as Fig. 2, where B I is a susceptance provided by the stepfrom the rectangular waveguide to the circular one; Yl,Y2 and Y3 arethe characteristic admittances of the WR650 rectangular waveguide, thecircular waveguide and the circular waveguide filled with the ceramicmaterial, respectively. It is assumed that only the TE10 mode canpropagate in the rectangular waveguide and only the TE 11 mode in thecircular waveguide.
From the graphical analysis of this equivalent circuit on a Smith chartusing typical parameters determined by measurements of a cold test
model, we have obtained the following characteristics of the matchingsolutions [5]. 1) When T<"Ag/2 and II ,l2<"Agl2, lJ must be equal to 12 ,where "Ag is the guide wavelength. Here we define lO as lo=l]=l2. 2)
When Ts;A.g/4, lo<A-g/2 and Tis small enough, two values of to satisfy
the impedance matching. Figure 3 shows these solutions. Here wedefine Tl as TJ =T$;"Ag/4. 3) When "Ag/4$;T<"Ag/2 ,lO<"Ag/2 and Tis large
enough, two values of lO satisfy tlie impecfance matching. Here wedefine T2 as "Ag/4$;T2=T<"Ag/2. 4) There exist the maximum value of Tland the minimum value of T2 which satisfy the impedance matching.
Then Tl +T2="Ag/2. If TJ goes to the maximum value, two solutions ofJomerge into one solution. Figure4 shows this solution. Because the line
Ll is tangent to the circle C2 when Tis the maximum value in Fig. 4, theVSWR is not sensitive to the small deviation of lO from the solution.
Therefore, this solution makes it possible to relax the tolerance for l. 5)
When T="Ag/2 and l] J2<"Agl2, there exist solutions (usuallylt#2). 6)
Each of the solutions ofl] ,12 and Thas the periodicity of'Ag/2. This meansthat if Tis a solution, T+Nx"Ag/2 is a solution too (where N=O,l,2, .... ).
From the results of this analysis, the solution which has the maximum
value of TJ (Tl <"Ag/4) was adopted for the windows of type B to relax
the tolerance for l.

Performan ce of high power test model
Two windows oftypeB (d=l90mm, T=5.6mm and 1=38 mm) were
fabricated for the high power test. Figure 5 shows the VSWR curve forone of the windows.
A vacuum side of the ceramic disk was coated with titanium nitride
to suppress multipactoring. A 60 A-thick coating and a 100 A-thickcoating which were performed by ULCOAT co. were adopted, because
the optimum thickness of the TiN coating performed by ULCOAT wasfound to be60to 150 A for the window ceramic of the UHF klystron usedin TRISTAN [6].
The TiN coating by ULCOAT contains oxygen atoms and has acolumnar grain structure. These properties play an important role tomake the resistivity of this coating high [7]. We measured the Josstangent of the window ceramic by using an L-band cavity to confirm
these properties and estimate the dielectric loss in the ceramic. Table I
shows the results of these measurements. The coating did not affect theloss tangent. On the other hand the pure TiN coating has a low resistivity.
And the thickness of the pure TiN coating was optimized 5-15 A for theS-band RF window by measuring the effective loss tangent after TiN
coating [8]. In this case the coating with a low resistivity affected theeffective loss tangent. And this effect made the upper limit of theoptimum thickness. By the measurements of the loss tangent, theproperty differences between the TiN coating by ULCOA T and the pureTiN coating were clearly confirmed.
Figure 6 shows the layout of the high power test. Each window was-137-connected with an H-planecorner, the space between two windows wasConclusion
evacuated. The vacuum pressure was IQ-10 Torr after a 72-hour baking
at I 00 °C. Other spaces were filled with SF6 with a 1.5 kg/cm2 absolute
A pillbox RF window for the JHP L-band pulsed klystron has beenpressure. Three sapphire viewing ports were mounted to monitor glow developed. The type-B windows with HA997 ceramic disks wereemitted by discharge on the ceramic during the conditioning and to conditioned up to a5.0 MW peak power with a 375 J.lSeC pulse width andmeasure the ceramic temperature using an infrared thermometer.
a 50 Hz repetition rate. And these were operated up to a 113kW average
Two H-plane corners were cooled by water jackets to keep thepower with a peak power of 4 MW (565 J.lSec, 50 Hz). The power loss
waveguide at room temperature. This condition is necessary toin the pillbox window was measured and compared with the calculationcompensate the background emission when we measure the ceramic
by using MAFIA. From these data, the temperature rise of the ceramic
temperature using the infrared thermometer. (In the type-A windows
is extrapolated at the full RF powerto be less than 40 de g.. Therefore thistest, waveguides were not cooled thus the compensation of the backgroundwindow is expected to satisfy the specifications of the JHP L-band
emission has some errors and the measured temperature rise of ceramic
pulsed klystron.
may be larger than the actual temperature rise.) It is difficult to measure
the ceramic temperature accurately using the infrared thermometer in
Acknowledgments
this test system because I) temperature rise is small compared with anamount of inaccuracy which comes from the calibration of the effect by
We would like to thank Dr. S. Michizono for the measurement of thethe sapphire viewing ports and the effect by the background emission,
loss tangent. We also acknowledge valuable discussions with Dr. S.
2) a few of the infrared rays can penetrate the high purity alumina Yamaguchi.
ceramic disk such as HA997.
The sleeve of the ceramic disk was water-cooled. In measurements
Referencesof power loss in the ceramic the most reliable method in this case wasto measure the temperature rise and the flow rate of the cooling water in [1] Report of the design study on the proton linac of the Japanese Hadron
Project,No.l ,JHP-lO,KEK Internal 88-8,Sep. 1988.
the water jacket of the pillbox. The tern perature rise was measured using
the quartz thermometer and the flow rate was calculated measuring the[2] S.Fukuda eta!., "L-band Pulsed Klystron for the JHP", presented inthis symposium.
water volume which flowed during I minute. The both rectangular[3] Y.Takeuchi, "L-band output window", Report of the design study on
waveguide ports of the pillbox window were cooled by water jackets andthe proton linac of the Japanese Hadron Project,No.2,JHP-14,KEK
kept their temperature equal to that of the pillbox to cut off thermal flow
Internal 90-16,June 1990, pp. 259-277.
across the cross-sections of A-A' and B-B' in Fig. 6.
[4] Y.Takeuchi eta!., "Development of L-Band Pillbox RF Window"
RF power was fed by an L-band 5 MW klystron (THOMSON
Proc. of 6th symp. on accelerator science and technology, RIKEN,
TH21 04A). The windows were conditioned up to a peak power of 5MW
Wako, Saitama, Nov.,l991.
with a pulse width of 375 J.lSec and a repetition rate of 50 Hz, andoperated up to the average power of 113 kW with a RF peak power of [5] Y.Takeuchi, to be published.
4 MW (565 J.lsec, 50 Hz), which was the maximum average power that [6] S.Isagawa et al., "Development of high power CW klystrons for
TRISTAN", Proc. of the IEEE particle accelerator conference,
the klystron could drive at that time.
Washington, D.C., March,I987.
During the conditioning, sudden gas bursts and emissions of blue[7] S.Isagawa and Y.Takeuchi, "Suppression ofmultipactoring on highwhite glow were observed frequently. After the sufficient conditioning
power RF ceramic windows by TiN coating " , Proc. of annual
few gas bursts or few emissions of the glow were observed.
meeting of the surface finishing society of Japan, Nov. ,1991.
Figure 7 shows the dependence of the ceramic temperature and the[8] S.Michizono et al., "TiN film coatings on alumina radio frequency
power loss in the pillbox (the dielectric loss in the ceramic and the wallwindows", J. Vac. Sci. Techno!. A 10(4), Jul/Aug 1992, pp. 1180loss in the pillbox) on the transmitted power (375 J.!Sec, 50 Hz). The open
1184.
eire les represent the measured temperatures. The closed circles represent
the measured power loss. The temperature data by the infrareq [9] S.Yamaguchi, Private communication.
thermometer have some errors because these data are not calibrated yet.
Since these curves in Fig. 7 are almost linear to the RF power,
Table 1 Measurement of Loss Tangent
multipactoring was suppressed effectively by the TiN coating. We did
not observe a significant difference between a 60 A-thick coating and a
Loss Tangent
Loss Tangent
100 A-thick one.
(beforecoating)(aftercoating)Since a traveling wave in the pillbox window can be expressed as a55
2.36x 10· (lllOMHz)
60A
2.51 xI 0· (Ill OM Hz)
superposition of two standing waves with the even and odd parities, we
2.45x 10· 5 ( 1648MHz)
can estimate the dielectric loss in the ceramic and the wall loss in thecoating
2.37x 10·5 (1648MHz)
pillbox by calculating both modes using MARA [9]. Table 2 shows the
2.97xi0· 5 (lllOMHz)
100A
2. 73x I 0· 5 (lllOMHz)
estimated and measured data of the power loss in the pillbox and the
2.49x10· 5 (1648MHz)
coating
2.41xl0·5 (1648MHz)
temperature rise at the center of the ceramic at a peak power of 5 MW(375 J.lSec, 50 Hz). The estimated temperature rise in the Table 2 wasTable 2 Power loss in Pillbox Windows
calculated by assuming the uniform power loss distribution in the(Peak Power=5MW, PulseWidth=375J.lsec, Repetition Rate=50Hz.)
ceramic disk.
In this table, the measured power loss of the type-B window (I 00 ALoss
Dielctric TemperaturePower loss
TiN coated) is about 2.7 times as large as calculated one. The thermal
tangent loss
rise of ceramic in pillbox
flow from the heated SF6 gas was considered as a cause of thisdisagreement. However, if the measured power of 45.8 W was lost only Type A 3xl04
54.6 W
44.6 deg.
67.6W
in the ceramic in the worst case, estimated temperature rise would be(HA95) (Catalog) (Calculated) (Calculated) (Calculated)about 21 de g. and the extrapolated temperature rise at the full RF power
would be about40deg., which is much less than the criterion of lOOdeg.. TypeB 2.5xi0· 5 4.2 W
1.9 deg.
17.1W (Calculated)
Therefore, the window of type B has sufficient capability to withstand(HA997) (Measured) (Calculated) (Calculated) 45.8W (Measured)
the full RF power.

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COil 8C2.•

•

00

8HA.

IA EF•.c
1 29ll.

1.020100 0 0 .....

-MAR ER 1 __ - - - -- ---- 1 . 29E GH~-- --- 1---- - - r--- - - - --r--- - r--·--·~Figure 1 Structure of Pillbox RF Window~---r---·--r-- ---""" ~CENTER 1- -I - - ·........
aae. 000 000 HH:r.

L:..----- r-SPANr!500. 000 000 MHz

Figure 5 VSWR Curve of Type-B Window

YlMeasurement of the type-B #2 window (100 ATiN coating).
Horizontal range, 1046-1546 MHz. Vertical range 1.0-2.0.

Yl

I~ 12 ~I~ T ~I~ II ~I
Figure 2 Equivalent Circuit of Pillbox RF Window

Figure 6 Layout of High Power Test-e- Ceramic Temperature

Figure 3 Matching solutions on Smith Chart_....Power Loss in Pillbox
50Admittancechart.lJ=15.8,61.0mmwhered=190mm,T 1=3.0mm,
tr=9.9, B 1/Y1=0.318, Yl/Y2=1.97, and Y2/Y3= 0.2286.
2040E 19...
~.. 188.
E..... 17~
30 ~

::
ifc

20 -~
0..;l·..

...

"isf!

'"

10 ~.
u 16~

15

002
RFPower

Figure 4 Matching solution on Smith Chart
Admittance chart.IJ=29.8 mm where d= 190 mm, T 1=5.45 mm,
tr= 9.9, Bl/Y1= 0.318, Y1/Y2= 1.97, and Y2/Y3= 0.2286.

3456(MW)

Figure 7 Ceramic Temperature and Power Loss in Pillboxvs. RFPower
Measurements of the type-B #2 window (lOOA TiN coating).

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