Geng, R

Padamsee, H

Shemelin, Valery D

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MULTIPACTING IN A RECTANGULAR WAVEGUIDE  R. L. Geng, H. Padamsee, V. ShemelinLaboratory of Nuclear Studies, Cornell University, Ithaca, NY 14850, USAPresented at the 2001 Particle Accelerator Conference, Chicago, USA, June 18-22, 2001 SRF010723-07AbstractMultipacting in the reduced-height waveguide couplerused in CESR III superconducting RF system is studiedthrough numerical and simplified analytical calculations.An improved model for secondary electron emission is de-veloped for numerical calculations. We present in this pa-per a new method to counter against multipacting by open-ing slots on broad walls along the center plane of a rectan-gular waveguide. The simplified analytical approach pro-vides a fast survey of multipacting characteristics for vari-ous VSWR’s in the waveguide.1 INTRODUCTIONThe rectangular waveguide coupler has been adopted inthe normal conducting RF system of PEP-II and supercon-ducting RF system (SRF) of CEBAF and CESR-III (foran excellent review on RF couplers, see Champion [1]).These couplers have been performing well to deliver de-sired power into charged particle beams. However, whenhigher power regimes are explored, great care must be ex-ercised to avoid some performance limiting mechanism,among which multipacting is a main culprit.2 A COMPARATIVE REVIEWMultipacting in rectangular waveguides emerges as aproblem when over 100 kW CW power is required to trans-fer in high current colliding beam machines at the fre-quency range of 300-500 MHz [2]. This problem is par-ticularly pronounced in an SRF system because of the en-hanced secondary emission coefficient of the cryosorbedgas layers on the coupler inner surface. Other types ofwaveguides like coaxial lines adopted in RF couplers ofLEP-II and KEK-B SRF systems have also been baffledby multipacting. Numerical calculations by Somersalo et.al. [3] have provided tremendous insights for multipactingin coaxial lines. Increasing the line impedance was foundvery helpful. A DC electric bias technique was developedby Tu¨ckmantel et. al. [4]. Coaxial RF couplers in LEP-IIand KEK-B have both benefited from this technique andachieved excellent performance [5][6].Multipacting in hollow rectangular waveguides has notreceived much attention in the accelerator community.This is not surprise because couplers with rectangularwaveguides had been operating far away from multipact-ing regimes until the installation of the CESR-III SRF sys-tem with a reduced-height waveguide in its coupler. The   waveguide used in CEBAF input coupler operates	Work supported by the National Science Foundationrg58@cornell.eduat a power level of only 5 kW at 1500 MHz. Extrapolat-ing the 100 kW barrier for CESR-III, CEBAF waveguideshould be free of multipacting harassment up to a powerlevel of 50 kW, according to the simplified scaling rule1    by Hatch and Williams [7]. By the same to-ken, the    waveguide used in PEP-II 476 MHz RFsystem will not suffer from multipacting until a power levelof above 460 kW. The room temperature environment pro-vides further safety enforcement against multipacting forthe PEP-II waveguide.In the CESR-III SRF system, a reduced-height wave-guide ( ﬀ   ﬂﬁ   ), instead of a full height WR1800 wave-guide, is used between the ceramic window and couplertongue. Moreover, this section is cooled by cryogens, re-sulting in an enhanced secondary emission coefficient forinner waveguide surfaces. These two factors, the reducedheight and enhanced secondary emission coefficient, are re-sponsible for making this waveguide more prone to multi-pacting. As remedy measures, RF processing of the cou-pler and sometimes warming-up of the whole system wereresorted to keep a low RF trip rate.Pursuing more active counter measures, Geng andPadamsee [8] proposed a DC magnetic bias technique. Thistechnique has been implemented in the spare RF modulesfor CESR-III (see Fig.1). It will be tested after the installa-tion of the spare moudle in 2001. Chojnacki [9] proposedFigure 1: Solenoid coils are installed on the reduced-heightelbow waveguide to provide bias magnetic field for sup-pressing multipacting in the spare RF system for CESR-III.the wedge-guide concept which is revisited in section 4.3.In this paper, we present a new method to counter againstmultipacting by opening a slot on broad walls along thecenter plane of a rectangular waveguide.3 IMPROVED SIMULATION CODEThe multipacting code developed in [8] is used in thispaper with improvement in the following two aspects.1P is the multipacting onset threshold, f is RF frequency, and d is thenarrow dimension of the waveguide.( a ) ( b ) ( c )Figure 2: Improved model for secondary electron emission.(a)Emission with an accelerating field. (b)Emission with adecelerating field. (c)A secondary electron experiences anextended period of deceleration after emission. It is drivenback to the emission surface and be absorbed there.First, a more realistic model for secondary electron emis-sion is implemented. In previous models, secondary elec-tron emission is prohibited if the electric field has a deceler-ating polarization at the time of emission. In the improvedmodel, secondary electrons are allowed to emit regardlessof the electric field polarization. Because secondary elec-trons are born with a finite amount of kinetic energy (2-5eV), even those emitted with a decelerating field have achance to run away from the surface (see Fig.2). However,if a secondary electron experiences an extended period ofdeceleration after emission, it will be driven back to theemission surface and be absorbed there.Secondly, new secondary electron emission coefficientdata by Bojko, Hilleret, and Scheuerlein [10] are incorpo-rated to reflect the nature of the waveguide surface, whichis Copper plated Stainless Steel with vacuum baking at 200ﬃC. The new secondary emission coefficient values are no-ticeably higher.With these two improvements, the multipacting bandsbecome broader and stronger (band positions are the sameas that predicted by the previous model [8]). Fig.3 showsthe multipacting bands for the CESR-III reduced-heightwaveguide operated in the traveling wave mode.1e+001e+011e+021e+031e+041e+051e+061e+071e+080 50 100 150 200 250 300 350 400 450 500e30Forward Power [kW]Figure 3: Multipacting bands of the CESR-III reduced-height waveguide in the traveling wave mode. The orderof the band near 100 kW is 21.4 COUNTER MEASURESTo fight against multipacting in the waveguide, moreactive counter measures than in situ RF processing areneeded. Here are a few examples for this purpose.4.1 DC Magnetic BiasThe DC magnetic field bias technique was proposed byGeng and Padamsee [8] and has been implemented in thespare RF module for CESR-III. The effectiveness of thisconcept is borne out again from the repeated simulationswith the improved numerical code.4.2 Opening Slot on Broad WallsHere we present a new concept - a waveguide structurethat is free of multipacting by design. The idea is to open aslot on the broad wall along the center plane of the waveg-uide. The slot is narrow enough that no RF field is leakinginto it. Fig.4 shows the multipacting bands of a reduced-height waveguide with a slot opened up on its top wall. Theslot has a dimension of 5mm in width and 5 mm in depth.As we can see, multipacting is suppressed to a great extent.This concept works by letting electrons drift in a field freeregion and hence provides a perturbation to resonance.1e+001e+011e+021e+031e+041e+051e+061e+071e+080 50 100 150 200 250 300 350 400 450 500e30Forward Power [kW]Figure 4: Multipacting bands of a slotted CESR-IIIreduced-height waveguide in the traveling wave mode.The slot can be further made to have a 45 ﬃ tilted ceil-ing as illustrated in Fig.5. According to the cosine law forsecondary electron emission, those excited by residual mul-5 mm5 mmelectrons being absorbed by side wall of slotresidual mutipacting betweem top and bottom walls433 mm102 mmFigure 5: Suppressing multipacting by opening a slot onthe top wall along the center plane. Titling the ceiling ofthe slot by 45ﬃprovides further suppression by deviatingthe residual secondary electrons to the side wall of the slot.tipacting electrons will be deviated to the side wall of theslot and then be absorbed there, thus providing further mul-tipacting suppression. In practice, cleaning the slot may bedifficult, but can be taken care of by new cleaning tech-niques, such as High Pressure Water Rinsing.4.3 Revisit the Wedge-Guide ConceptThe wedge-guide concept proposed by Chojnacki [9] isrevisited in this paper. Fig.6 shows the multipacting bandsof a wedge-guide with a 5 ﬃ tilting angle of the top wall withrespect to the bottom wall. As we can see, multipacting isstill quite appreciable. We found that even the tilting angleis increased to 13.2ﬃ, the wedge-guide concept is still lesseffective in suppressing multipacting than the DC magneticbias technique.1e+001e+011e+021e+031e+041e+051e+061e+071e+0850 100 150 200 250 300e30Forward Power [kW]Figure 6: Multipacting bands of a wedge-shaped waveg-uide with a tilting angle of 5 ﬃ of its top wall.5 MULTIPACTING MAPSFor operational purposes, RF input couplers usuallywork in a mode that part of the input power is reflected.Multipacting bands will shift as a result of reflected power.For diagnostic purposes, a multipacting map, multipactingsusceptibility as a function of input power and power re-flection ratio, is usually very helpful. Fig.7 shows such amap for the CESR-III reduced-height waveguide for the in-put power range from 250 kW to 350 kW. Both simulationsand the past experience from CESR-III indicated that themultipacting susceptible region shown in Fig.7 is a ratherhard barrier.Neglecting the effect of RF magnetic fields, a simpli-fied analytical approach has been developed, which givesresults agreeing well with numerical results (for more de-tails see Shemelin [11]). The analytical approach is veryefficient in drawing a multipacting map, as shown in Fig.8.6 CONCLUDING REMARKSMultipacting in a rectangular waveguide can be an an-noying or even limiting problem when multi-hundred kWpower is required. Great care should be exercised in order20%25%30%35%40%260 280 300 320 340Pr/PfPf [kW]working curvePb=200kWworking curvePb=220kWworking curvePb=240kWMajor MP Susceptibility Zone(order=9)Scattered minor MP susceptibility islands(orders in the neighborhood of 9)W1 Work point0.7A CESR currentPf: Forward Power      Pr: Reflected power      Pb: Beam powerextended MP zone with the relaxed trapping criterionFigure 7: Multipacting susceptibility of the reduced-heightwaveguide for input power range from 250kW to 350kW.0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 80000.20.40.60.81Forward Power [kW]Pr/PfFigure 8: Multipacting susceptibility for an extended inputpower range, provided by the analytical approach.to have a waveguide structure that is free of multipacting bydesign. In this paper, the slotted waveguide concept is pre-sented for this purpose. In case multipacting is encounteredin the post-design stage, active counter measures, such asthe DC magnetic bias technique, should be implementedto fight against this problem. A better designed waveguidegeometry and effective multipacting suppression measurewill not only reduce the time for excessive RF process-ing but also guarantee better coupler performance at higherpower levels.7 REFERENCES[1] M.S. Champion,   ! RFSC [12], 1995, P. 195.[2] S. Belmoestnykh, PAC [13], 1999, P. 272.[3] E. Somersalo et. al., PAC [13], 1995, P. 1500.[4] J. Tu¨ckmantel et. al., PAC [13], 1995, P. 1642.[5] P. Brown et. al., "  ! RFSC [12], 1999, P.1.[6] T. Furuya et. al., "  ! RFSC [12], 1999, P.31.[7] A.J. Hatch and H.B. Williams, Phys. Rev., 112(3), 681(1958).[8] R.L. Geng and H. Padasmee, PAC [13], 1999, P. 429.[9] E. Chojnacki, Phys. Rev. ST Accel. Beams 3, 032001 (2000).[10] I. Bojko, N. Hilleret, and C. Scheuerlein, J. Vac. Sci. Tech-nol. A 18(3), 972(2000).[11] V. Shemelin, LNS Report, SRF-010322-03, Cornell Univ..[12] Workshop on RF Superconductivity Proceedings.[13] Proceedings of Particle Accelerator Conference.

Multipacting in a Rectangular Waveguide

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