Goldberg, D A

Rimmer, R A

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Lawrence Berkeley National LaboratoryLBL PublicationsTitleMeasurements of Higher-Order Mode Damping in the PEP-II Low-Power Test CavityPermalinkhttps://escholarship.org/uc/item/2q82z4ndAuthorsRimmer, R AGoldberg, D APublication Date1993-05-01Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, available at https://creativecommons.org/licenses/by/4.0/eScholarship.org Powered by the California Digital LibraryUniversity of California,. . LBL-33312 UC-414 Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA Accelerator & Fusion Research Division Presented at the IEEE Particle Accelerator Conference, Washington, D. C., May 17-20, 1993, and to be published in the Proceedings Measurements of Higher-Order Mode Damping in the PEP-II Low-Power Test Cavity R. A. Rimmer and D. A. Goldberg May 1993 Prepared for the U.S. Department of Energy under Contract Number DE-AC03-76SF00098 ---'"'II(") 0 ..... , '"1 '"1 0 (') > ~s:: z ..... :E Ill (") Ill t+O Ill Ill "0 ';JI!("VI -< VI ---OJ ..... a. (Q t.n lSI r r OJ ..... r C"(") I '"1 0 w QI"'O w '"1'< w '< ...... . N N DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. Measurements of Higher-Order Mode Damping in the PEP-II Low-Power Test Cavity R. A. Rimmer and D. A. Goldberg Accelerator and Fusion Research Division Lawrence Berkeley Laboratory University of California _ Berkeley, California 94720 May 1993 LBL-33312 UC-414 This work was supported by the Director, Office of Energy Research, Office of High Nuclear Physics, High Energy Physics Division, of the U.S. Deparunent of Energy under Contract No. DE-AC03-76SF00098. .. ' LBL-33312 ESG-240 Measurements of Higher-Order Mode Damping in the PEP-II Low-Power Test Cavity R. A. Rimmer and D. A. Goldberg* Lawrence Berkeley LaboratOry 1 Cyclotron Road, Berkeley, CA 94720, USA Abstract The paper describes the results of measurements of the Higher-Order Mode (HOM) spectrum of the low-power test model of the PEP-II RF cavity and the reduction in the Q's of the modes achieved by the addition of dedicated damping waveguides. All the longitudinal (monopole) and deflecting (dipole) modes below the beam pipe cut-off are identified by comparing their measured frequencies and field distributions with calculations using the URMEL code [1]. Field configurations were determined using a perturbation method with an automated bead positioning system [2]. The loaded Q's agree well with the calculated values reported previously, and the strongest HOMs are damped by more than three orders of Iilagnitude. . This is sufficient to reduce the coupled-bunch growth rates to within the capability of a reasonable feedback system. A high-power test cavity will now be built to validate the thermal design at the 150 kW nominal operating level, as described elsewhere at this conference [3]. I. INTRODUCTION Impedances of higher order modes in the RF cavities can drive coupled-bunch instabilities in high-current storage rings. For the PEP-II B factory [4] we have taken the approach that the HOMs must be damped to a point where a practical broad-band feedback system can control the coupled-bunch growth rates. At the same time it is important to maximize the fundamental-mode impedance of the cavities to make most efficient use of RF power and to minimize the number of cavities and RF stations. The R&D effort, shared among LBL, SLAC and LLNL, has produced a normal-conducting cavity design [5,6] which uses a trio of dedicated damping waveguides to reduce the HOM impedances. The target was to reduce the longitudinal impedances to the order of a few kQ, and the transverse impedances to a few hundred kntm (comparable with the effect of the resistive-wall). The single-bunch parameters are typical of existing machines and not in the regime where microwave instabilities would be expected. As a result of the ongoing R&D program some of the lattice and RF parameters have changed compared with those _given in references 4,5 and 6. All of the changes make the RF design more conservative. Table 1 shows the current PEP-II RF system parameters. *This work was supported by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, High Energy Physics Division of the U.S. Department of Energy, under Contracts DE-AC03-76SF00098 and DE-AC03-76SF00515. 1 Table 1 PEP-II RF system parameters (including the effect of the 5% gao in the beam) PARAMETER HER LER RF frequency (MHz) 476 476 Beam current (A) 1.03 2.25 Number of bunches 1658 ~1658 Number of cavities 20 10 Shunt Impedance Rs (MQ) a 3.5 3.5 Gap Voltage (MV) 0.93 - 0.60 Accelerating gradient (MV /m) 4.1 2.7 Wall loss/cavity (kW) 122 51 Coupling factor without beam (~) 7.5 7.5 Unloaded 0 of cavityb >30000 >30000 a Rs =V2/2P b withports,at40"C I 11 LOW-POWER TEST CAVITY A low-power test cavity (LPTC) has been fabricated, see figure 1, which was designed to show the loading of the HOM Q's and, therefore, the impedance reduction that could be. obtained with the waveguide damping scheme. Figure 1. Low-power test cavity. The rectangular ports are for the HOM damping waveguides, the two large circular ports are alternative locations for a loop coupler. To save cost, the quality of construction, though good, was not intended to show the ultimate Q of the fundamental mode that could be expected in the final cavities. Low-power mock-ups of the waveguide loads were made using ferrite powder in epoxy, cast into long tapered wedges. The load material was placed at the edges of the waveguides at least 36 inches from the cavity, as would be the case in a high-power design, to avoid coupling to the evanescent fundamental mode. Ill. MEASUREMENTS The cavity was excited with small electric-field antennas inserted into the beam pipes and a Hewlett-Packard 8510C network analyzer was used to measure the transmission response (S21) between them. With all the. apertures plugged or sealed, approximately flush with the cavity surface, the undamped modes of the cavity were seen as distinct peaks. The resonant frequencies were very close to those predicted by URMEL allowing easy identification of the modes. CH1 Szt lag MAG 10 dB;' REF 0 dB Ca.-0•1 A"S 128 .. I c - -:-:2· II) r-'--""ili '---- ""6· --- ·S!!- -···- --··- --··-- ·-0 - ~~ -- --I v "~ -START • -400 DOD ODD GHz 9 ·-re- .---:::!:- cL f--10 :::E ...:_!!! .... ~- ili ~ E 0 w I -c- ···-- ----·-· ..... -- --.... 0 .lit .... - -~ -- .... ~ - t:-I _w 1-:::E ~ -- '--·-1/\ -.,... T"" 1---w .... / ~ k" ........ __,... STOP • 1100 ODD ODD GHz Figure 2. Fundamental mode and first three higher order modes in the LPTC without damping. CHl Szl lag HAG 10 dB/ ~EF D dB Ca.-Del A.,g lDD .. 0 .... 0 ~ 1-I v- 1"-START • -400 DOO DD:J CHx lQ I 0 ,.... .... 0 ~ -~ / "'-. -v STOP • 1100 000 ODD GHz Figure 3. Fundamental mode and first higher order modes in the LPTC with three damping waveguides. 2 Figure 2 shows the spectrum of modes from below the fundamental (fMoto) mode up to 900 MHz without damping. The lowest 1E mode (TE111-like), is visible, as are the TM011-like longitudinal mode (the strongest monopole . HOM), at about 770 MHz, and the TMuo-like deflecting (dipole) mode, at about 795 MHz. Figuie 3 shows the same frequency span with the three damping waveguides attached. The fundamental mode is still strong while the HOMs are substantially damped. By changing the azimuthal position of the probes around the edges of the beam-pipes it is possible tO enhance or reject the dipole modes relative to the monopole modes, and thereby measure the loaded Q' s of overlapping monopole and dipole modes. It is also possible to discriminate between the two orientations of the dipole modes when there is a strong perturbation to the cavity such as the addition of the loop coupler. Figure 4 shows more of the HOMs, between 900 MHz and 1.4 GHz, without any damping. The monopole and dipole modes are labeled. Figure 5 shows the same span with the CHI Szl lag MAG 1D dB' REF D dB .. c ~!- E .lit ~-~---~ c ... .,... 1--- 1-::::!: r-C f--v c c ::::!E C") ~ :L .::: ::E 1-0 r-eo 1--- --1-gj ~!;)- r---c<) N C') w 1-0 d ~-~ r-!LJ 0 1--- ili- ~C")- r---~ ::IE w ~ w 0 I ~-e-!-U .... - w ---:::E-- r-_ --! .... I I ~ T"" .,... I-\ J I \J I _,.. [7 v lr: ~ &TART • 800 ODD ODD GHz &TOP 1.400 DOD ODD GHz Figure 4. Higher order modes in the LPTC without damping, 0.9-1.4 GHz. CH1 Szl lag MAC lD ·da/ I'IEF 0 dll Car-Del ""SJ 128 1111 ~ I 0 ~ 0 ::::!E ..... A i/\ \ I / [7 1\: &TART .SJDD ODD ODD CHa ,.. " ~ 8 J 0 ~ ::IE :z ~- 1--1-- w .... I I -0 ili :::e ~ / ~ v If'. r---... SITDP I •• DD DOD DOD GHz Figure 5. Higher order modes in the LPTC with three damping waveguides, 0.9-1 .. 4 GHz. '.! ( three damping waveguides. Still visible are the TMo2o-like monopole mode at about 1016 MHz, the TM021-like mode at about 1296 MHz and the dipole mode at 1313 MHz. The residual impedances of these modes are within the capabilities · of the feedback system. The results are similar for all the higher frequency ROMs up to the beam-pipe cut off. The residual impedances are shown in table 2 for the longitudinal modes and table 3 for the deflecting modes. A more detailed account of the results is available [7]. Table 2 t 489.6 108.8 769.8 44.97 1015.4 0.006 1291.0 7.68 no TM021 1295.6 6.57 1296 1585.5 5.06 1588 1711.6 4.75 no 0-EE-5 1821.9 0.06 1821 O-ME4 1891.0 1.68 no longer isible 0-EE-6 2103.4 3.52 2109 233 0.82 0-ME-5 2161.8 0.02 2168 201 0.004 0-EE-7 2252.2 1.21 2253 500 0.61 *note that model does not test ultimate fundamental-mode Q Table 3 1-ME-1 679.6 0.002 no longer 1-EE-1 795.5 15.263 779 122 1-ME-2 1064.8 27.590 no longer visible 1-EE-2 1133.2 0.243 1141 112 0.65 l-ME-3 1208.2 0.258 1203 1588 10.3 1-EE-3 1313.2 5.861 1311 498 80.1 1-ME4 1429.0 2.873 1435 3955 341 1-EE-4 1541.0 0.850 1554 59 1.62 1-EE-5 1586.2 2.045 1588 178 12.1 1-EE-6 1674.2 5.140 1674 2134 385 1-ME-5 1704.4 0.096 1704 444 1.52 1-ME-6 1761.9 0.104 1757 7129 27.3 * R.LL=RIQt(kr2 )x QL, where r in the beam pipe radius IV. MACHINE SIMULATIONS A simulation code has been used to study the growth rates of the longitudinal coupled-bunch instabilities that would be driven in PEP-II by the cavity impedances. The code includes a model of the beam dynamics, all of the residual HOM driving impedances from the cavities and a model of the proposed broad-band feedback system. Running the simulation fcir the longitudinal modes shows that the beam remains stable and that the large single-bunch transients that occur during injection are quickly damped down. A transverse simulation program is being developed which 3 is expected to show similar results for transverse beam motion since the deflecting impedances measured in the test cavity are reduced to the order of the resistive wall impedance. V. CONTINUING R&D Studies of the low-power test model are continuing, in particular to evaluate the effects of the loop coupler which may provide additional damping for some of the ROMs. The automated bead-puller is being used to measure the field . profiles of the modes and hence the RJQ' s, and to study the perturbations introduced by the various ports. The PEP-II R&D effort is now also focusing on building a high-power test model to prove that such a design can be conditioned and p(>wered with up to 150 kW wall dissipation. High-power versions of the HOM loads are also being developed. VI. CONCLUSIONS The low-power test model has shown that it is possible to achieve the HOM impedance reduction required for PEP-II with the broad-band waveguide damping scheme. The worst ROMs have been damped by more than three orders of magnitude and the loaded Q' s agree well with the calculated values reported previously. The detailed design of a high-power test cavity is well under way. Vll. ACKNOWLEDGMENTS Although all the measurements were performed in the Lambertson Beam Electrodynamics Laboratory of the Center for Beam Physics at LBL, the design of the cavity has been very much a collaborative effort with the other participants in the PEP-II project. Thanks are due John Byrd at LBL for running the machine simulations. VIII. REFERENCES [1] "Reports at the 1986 Stanford Linac Conference., Stanford, USA, June 2-6 1986," DESY M-86-07, June 86 [2] D. A. Goldberg, R. A. Rimmer, "Automated Bead-Positioning System for Measuring Impedances of RF Cavity Modes", this conference. [3] H. Schwarz et. al., "Design of a High-Power Test Model of the PEP-II RFCavity", this conference. [4] "An Asymmetric B-Factory based on PEP," Conceptual Design Rep<)rt, LBL PUB-5303, SLAC 372 [5] R. Rimmer et. al., "An RF Cavity for the B-Factory", Proc. PAC, San Francisco, May 6th-9th, 1991, pp819-21. [6] R. Rimmer et. al., "Higher Order Mode Damping Studies on the PEP-il B-Factory RF Cavity", Proc. 1992 Europ. Part. Accel. Conf., Berlin, Germany, March 24-28, 1992, pp 1289-1291. [7] R. A. Rimmer, "RF Cavity Development for the PEP-II B Factory", Proc. Int. Workshop on B-Factories, BFWS92, KEK, Japan, Nov. 17-20, 1992, (PEP-II ME note 2-93, LBL-33360, ESG-242). LA~NCEBERKELEYLABORATORY UNIVERSITY OF CALIFORNIA 1ECHNICAL INFORMATION DEPARTMENT BERKELEY, CALIFORNIA 94720 Cll =Q) l!l ·c ,....-~ l!) ;Q I .....J Ill .....J A' Cil ..... _.....J ' 

Measurements of Higher-Order Mode Damping in the PEP-II Low-Power Test Cavity

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Measurements of Higher-Order Mode Damping in the PEP-II Low-Power Test Cavity

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