PROPERTIES OF THE PISK-ANP-WASHER CAVITY
S. Inagaki, T. Higo, K. Takata and H. Nakanishi
National Laboratory for High Energy Physics
Oho-machi, Tsukuba-gun, Ibaraki-ken, 305, JapanThe disk and washer (PAW) structure would possibly be superior
to any other structures like the alternating periodic structure or theside coupled structure in the effective shunt impedance and the coupling
constant in the high beta region. However, the field distribution ofthis cavity i.s so violated by the washer support that the expected valueof the shunt impedance or quality value can hardly be obtained. Ourmodel cavity experiments at 500 MHz showed that an effective shunt impedanceof 40.0 Mohm/m or 35.8 Mohm/m is obtained by a single radial stem or two
longitudinal stem scheme, while the calculated value by SUPERFISH is
44.8 Mohm/m. We expect an overall impedance to be hifher than 35 Mohm/m
for a 12-cell cavity for the TRISTAN accumulator ring). This geometry
is referred as Type A afterwards.
Further it is revealed that in the Type A cavity, the TM 11 passband
crosses the accelerating mode 1 • 2 • 3). The Brillouin diagram of the above
cavity is shown in Fig. 3(a). TMolTI resonance is crossed by TE31 andTM 11 dispersion curves, each of which splits into two modes with thetransverse field either parallel or perpendicular to the stem. In our
12-cell cavity, the model cavity measurement shows that the nearest TM11
mode will exist 3.7 MHz higher than the accelerating mode. LANL 4 )managed to make a stopband at the operating frequency by the biperiodic
four-T stem structure. We propose as an alternative way the geometry
Type B which has a considerably small disk diameter. It has a calculated
effective shunt impedance of 34.6 Mohm/m, which is still higher than any
other structures. The electric field of the accelerating mode, couplingmode, zero mode and 2TI mode are shown in Fig. 2. The bandwidth of TM 01
mode is as wide as Type A. The coupling cavity is so much isolated fromthe accelerating cavity that it rather looks like the annular coupledstructure. The electric field is concentrated between the washer rim
and the disk. Therefore, the effect of the radial stem is expected tobe small and the bandwidth of parasitic modes to be narrow compared toType A.
Figure 3(b) shows the dispersion of Type B cavity. Both TM 1 1 andTE 3 1 passband are moved above the accelerating mode. The modes withsmall cell-to-cell phase shift are moved upward because these frequencies
are mainly determined by the disk radius. Those with large phase shift
remain almost unaffected. The bandwidth of TE11, TE21, TM11 and TE 3 1
modes becomes narrower than those of Type A as expected.
The study of the third cavity with the intermediate di~ension (TypeC) is in progress.
Acknowledgmen t
We express our sincere thanks to Prs. P. Swenson and R. Jameson ofLANL for sending many informations.

-257-References
1)
K. Takata, S. Inagaki, H. Nakanishi, K. Kitagawa, H. Ishimaru,
Y. Yamazaki and P. Grand, ";Disk and Washer Structure for an
Electron Storage Ring", IEEE Trans. Nucl. Sci. NS-28, 2873 (1981).
2)
L. Young and J. Potter, "Deflecting Modes in the DAW Structure
for the Racetrack Microtron", LANL memorandum, AT-1:82-59 (1982).
3)
T. Higo, S. Inagaki, K. Takata, H. Nakanishi, K. Kitagawa, S. Noguchi
and T. Furuya, "Dispersion Characteri stics of Disk-and-W asher Model
Cavity for TRISTAN", Proc. 1982 Linac Conf. in Japan (to be publishedin Japanese).
4)
Y. !washita, "RF Cavity Mode Spectra Measuremen ts in the ;Disk-andWasher Linac Structure", LANL memorandum, AT-DO:YI-243 (1982).
Table I

Three Types of theDAW Cavity

TypeRc(cm)~5 .45

33.36

~0.0

Rd(cm)

~0.25

25 .o

33.0

Td(cm)

7.2

10.0

9. 25G(cm)

10.25

10.51

10.85
25.0Rw(cm)2~. ~525 .oTw(cm)0.951.0

1.0

Tr(cm)

0.95

2. 5

1.0

Rh(cm)

5,0

5.0

5.0L(cm)

15 .o

15.0

15.0

0. 7650. 759o. 75~Z(Nohm/m)

76.49

60.07

72.55

ZT 2 (Hohm/m)~4. 76

34.59

~1.19

Fig. 2

Electric Field of Type B Cavity.
0-mode, TI-accelera ting Mode, TICoupling Mode and 2TI-mode (1-R).
i..'.l'!lJ'I!

! I~;~#tMst~~~-+~,"~~7Ff[~~tcl"· J'-.L:U_0±;'ll'••Lf;il;:l HH'LH';·;I .. ;•i···i;t;L [·--j Tdk LVEi--:'' TE~,·~HI.

RcRd(a)Fig. 1 Geometrica l Notationof a Half-cell DAW Cavity.

Fig. 3(b)Dispersion Curves of (a) Type A
and (b) Type B Cavity.

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