Izawa, M

Kobayakawa, H

Sakanaka, S

Tokumoto, S

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Particle Accelerators, 1990, Vol. 33, pp. 111-116Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaSOME CHARACTERISTICS OF THE HIGHER-ORDER-MODES IN THEPHOTON FACTORY ACCELERATING CAVITYMASAAKI IZAWA, SHUICHI TOKUMOTO, SHYOGO SAKANAKAand HISASHI KOBAYAKAWAPhoton Factory, National Laboratory for High Energy Physics,Tsukuba-shi, Ibaraki, 305 JapanAbstract The coupling between the higher order mode of the cavityand a resonance of the tuner structure, which has important influenceon the characteristics of the cavity mode, is discussed.INTRODUCflONFour single-cell accelerating cavities 1 are operated in the Photon Factorystorage ring. Each cavity has a tuning plunger to control the resonancepoint of the accelerating mode (500 MHz). The plunger together with itshousing makes a coaxial structure, thus, the tuner has coaxial moderesonances. The coupling between the cavity mode and .the tuner resonancewas observed. Since such coupling changes the characteristics of the cavitymode which is one of the causes the coupled-bunch instability2, it wasinvestigated in detail. The results are presented.COUPLING BETWEEN CAVITY MODE AND TUNER MODEFigure 1 shows the resonance frequency dependence of the TMOll mode ofthe cavity on the position of the plunger (referred to as the tuner position inthis paper, which is defined as the intruded length from the surface of thecavity inner wall). The resonance frequency of this mode is expected todecrease monotonously with inserting the tuning plunger. However, asillustrated in the figure, it has a minimum value at some tuner position.Moreover, a rapid decrease of the Q-value with increase of the tuner positionwas also observed. The same behavior of the resonance frequency wasobserved with a test bench cavity which is the same type as the oneoperating in the ring, as shown in Fig. 2 (curve B). The data of curve A inthe same figure was obtained when half of the extra space due to the shape[1801]/111112/[1802] M. IZAWA ET AL.N~ 759.0+-c::oc::oV)Q,)0::758.0~ ~ ~ ---:!:~_-~24Tuner Position (em)FIGURE 1 Resonance frequency dependence of TMOll mode on tunerposition. 1-IV represent the cavity number.760.0 AN 759 o·%••• •~ •••~ ••• B758 .....(,) ...... •c:Q) ••j•• ••• •tr 757 ••• •! ..........u.•Q)756 •(,) •c•asc: •0 C •U) 755 •Q)•a:•7540 10 20 30 40Tuner Position (mm)FIGURE 2 Resonance frequency dependence of TMOll on tunerposition measured with a test bench cavity.of the vacuum flange (Fig. 3) was filled up with metal. On the other hand,curve C was obtained when whole of the extra space was filled up. Thus, it isclear that the unexpected behavior of the resonance frequency of the TMO 11is related to the tuner structure. Figure 4 shows the output of SUPERFISH3 byFIGURE 3HIGHER-ORDER-MODESsuscavity wallCross sectional view of tuner.CooIinQ waterChannel[1803]1113which the coaxial mode resonance (A/2 resonance) of the tuner for twodifferent structures are computed. As shown in Fig. 4, when the tuner hasthe extra space in its housing, its resonance frequency is around 700 MHzwhich is close to that of TMOll, while it becomes around 1000 MHz when theextra space is removed. Also, with increasing the tuner position, theresonance frequency of this coaxial mode becomes high since the length ofthe coaxial part becomes short. The measured frequency variation of the-----------FRED= 678. 413 MHZ-------------------FRED= 938.328 MHZFIGURE 4 Results of computation of tuner mode by SUPERFISH.114/[1804] M. IZAWA ET AL.TMOll is explained as follows. If the TMOll does not couple the tuner mode,its resonance frequency coincides with that of the tuner mode at some tunerposition. In this case, the frequency dependence of the TMOll and the tunermode on the tuner position would be such as shown schematically with thestraight lines in Fig. 5. However, since there exists the coupling betweenthem, they can not have the same resonance frequency, as well known, butsplit into two different frequencies around the crossing point. In fact, themeasured data plotted in Fig. 5 shows an avoided crossing, thus, the minimumof the curve shown in Fig. 1 appears.The tuner and its housing are partly made of stainless steal. Carbon graph i teis also used as RF contactor. Therefore, the Q-value of the tuner resonance ismuch lower than the cavity mode. This is the reason why the rapid decreaseof Q-value was observed when the plunger was deeply inserted where theTMO 11 has no longer the character of the cavity mode but the tuner mode.780N:t:~ 770>.Co)&:CD 760:::JC'"!LLCD 750Co)&:«S&:0II) 740CDa:7300 10 20 30 40Tuner Position (mm)FIGURE 5 Resonance frequency dependence of TMOll on ·tunerposition measured with cavity II for wider frequency range than Fig. 1.IMPROVEMENT OF TUNER STRUCTUREAs mentioned above, if the extra space in the tuner port is removed, it isexpected that the tuner mode does not couple the TMO 11 since it has theresonance frequency too different to couple the TMOl1. However, the cavityhas the TMll1 mode with the resonance frequency of about 1070 MHz, whichis known to be one of the causes of the horizontal instabilities2 .HIGHER-ORDER-MODES [1805]/115Its resonance frequency is close to that of the tuner without the extra spacein the port. With considering the field pattern of the TMlll, it has possibil ityto couple the tuner mode. In fact, after removing the extra space of the tunerport4 , the coupling between the TMlll and the tuner mode was observed asshown in Fig. 6 though the coupling between the TMO 11 and the tunerdisappeared as shown in Fig. 7.1072.0,---------r---~-----.----_TM111 (H)TV(a)Inmcoc~Q)a::~(.)cQ)::::JCTJ: 1071.01070.0oL-------L----2~----=------:Tuner Position (em)1072.0.--------,----~---~--____.cg 1071.0o(/)Q)a::(b)o 2 3Tuner Position (em)4FIGURE 6 Resonance frequency dependence of TMlll on tunerposition. (a) : before improvement of the tuner port, (b): afterimprovement.116/[1806] M. IZAWA ET AL.759.0r---m.......-----r------,-------.IVIN::x::::E>-<..)~;::]0""J: 758.0coc::0::Tuner Posit ion (em)FIGURE 7 Resonance frequency dependence of TMOll on tunerposi tion measured after the improvement of the tuner port.The cavities are operated normaly at the tuner pOSItIon of around 2 cm. InFigs. 1, 6 and 7, One can see the difference of the resonance frequencies ofthese cavity modes at this tuner position between before and after theimprovement. Note that the difference is caused by changing the resonancefrequency of the tuner structure. During summer shutdown in 1987, we trieda new method to avoid the coupled-bunch instabilities due to the higher-order-modes by carefully trimming their resonance frequencies so as not tocoincide with the frequency of the coupled-bunch mode4,5,6. Understandingthe coupling between the cavity mode and the tuner mode was veryimportant to carry out the frequency trimming.REFERENCES1. Y. Yamazaki and K. Takata, Proc. 3rd Symp. Accel. Sci. Tech. (1980), p. 125.2. H. Kobayakawa, Y. Yamazaki, Y. Kamiya and M. Kihara, lpn. I. Appl.~, 2.5., 864(1986).3. K. Haibach and R. F. Holsinger, Part. Accel., 1, 213(1976).4. S. Tokumoto, M. Izawa, H. Kobayakawa and S. Sakanaka, KEK Internal 88-6,(1988).5. H. Kobayakawa, M. Izawa, S. Sakanaka and S. Tokumoto, to be published inProc. 3rd Int. Conf. Syn. Rad. Instr. (1988).6. M. Izawa, H. Kobayakawa, S. Sakanaka and S. Tokumoto, KEK Internal 88-7,(1988).

Some Characteristics Of The Higher-order-modes In The Photon factory accelerating Cavity

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