A high-current emittance-compensated RF photoinjector is a key enabling technology for a high-power CW FEL. The design presently under way is a 100-mA 2.5-cell {pi}-mode, 700-MHz, normal conducting demonstration CW RF photoinjector. This photoinjector will be capable of accelerating 3 nC per bunch with an emittance at the wiggler less than 10 mm-mrad. The paper presents results for the RF coupling from ridged wave guides to hte photoinjector RF cavity. The LEDA and SNS couplers inspired this 'dog-bone' design. Electromagnetic modeling of the coupler-cavity system has been performed using both 2-D and 3-D frequency-domain calculations, and a novel time-domain approach with MicroWave Studio. These simulations were used to adjust the coupling coefficient and calculate the power-loss distribution on the coupling slot. The cooling of this slot is a rather challenging thermal management project

Kurennoy, S. (Sergey)

Young, L. M. (Lloyd M.)

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Approved forpublic release; I distribution is unlimitod. Title. Author($ Submitted to: RF COUPLER FOR HIGH-POWER CW FEL PHOT’OINJECTOR Sergey S. Kurennoy, LANSCE-1 Lloyd M. Young, SNS-DO 2003 Particle Accelerator Conference (PAC2003) 3ortland, Oregon Vlay ‘I 2-1 6,2003 N A T I O N A L  L A B O R A T O R Y  Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the US. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the US. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for US. Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. Los Aiamos National Laboratory strongly supports academic freedom and a researcher’s right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Form 836 (8/00) Ahst M C ~  A high-currcnt emittaoce-cciiTlpr:rIsaEerl RP photo- injeclor lis a key enabling technology far a high .power CW FEL. The design premitly u n k t  way is a 100-trrA, 2.5-cell n-rnodi:, 700 MEIz, otrrtnal coawlucting demonstration CW 1-37 photoirljectol: This photoinjector will be capable of accelerating 7 nC pa. bunch with an emiltance: at the wiggler le+,s than 10 mm-mrad. Thc paper presents rewits for the RF: coitpliug from ridged wavegrnicles to the plioloii+x%w la7 chvity. 'rho 1,EDA and SNS couplers inspir et1 this "dog-bone" design. Electromagnetic rncudelirrg of the coupleixavity system has bean perfosnicd using bc ,kh 2-P) anti 3-D frc:qlaency- domain calculalions, and ;i reovel tima-domain approach with MicruWavc: Studio. These simulations were used to adjust the couplirig c~aeflicient atid colculde the power- loss distribution on the coupling slot. The c:ooling rsf this slot i s  a rather challenging thei mal mmnagement p i i ~ j ~ t .  The project is tnnclerway to build a detrmnstratibrr 700- MEIz RF ernitlanr:e-i:omperxsated CW photo-injei:tor (PI) capable crl' accelerating 101) nriA of the electron bcarrr (3 nC per buncAr :it 3:J.3-1Ulk repetition rate) to abluirt 5 MeV with the transvC:rse cmiitikrlce ilt the wiggler MOW 10 mna-mrail, sei? I I ]  itntl references therein. The PI consists of a 7,2k:ell cavity with the accelcrating gradient E0=7 M V h  (tht: PI propei) that bring:; the electrou beam energy to 2.7 MeV, followed by a 4-(:ell booster with l&=4.5 MV/m, where thl: brmi is ac:celeratd to 5-5 MeV. This grdient choici: is a ti a&-off he! ween the.: liearn- dynamics rer#uin;marrts aud challenges of the cavity thernml ~manngerrucsnt 12,1]. The injector is designed to be scal:ible to even higher beam cui~ent4;, up to 1 A, arid therefore, to higher h a m  power by ixicrwsiing the tttimh repetition irate. 'I'he high beam power plus a. signillcant wall ~IOWLX loss in the normal -cornducting 1Y cavil y 1md to sckoms challe.ng,es for cavity corrling. lrrr addition to a laigc total Rli power needed, t h a c  are. mgion4 ot rather high loss power densities on the cavity walls. 'l7nt:refore, we need to with the lowcst possildc increase of llre surfacc power density. TIwe we present the design and ellectrornagmetic modeling of the RP power d:ouplers for the M cavlty. provide & ~ C ~ I I V C  Kl; power f ~ A i t ~ g  to the 1% C X V ~ ~ Y  RP C0UlBLF# The IRI; feeds are in the 3"' (the 2''' full.) cell of the 2.5- cell PI cavity; sce Fig, 1. Two symmolrically piacad ridge- loaded tapC:red waveguides ilic a;tinne&A to the cavity via "dog-bm" coupling irism -- corrsistimg of a Imp, narrow slot with two circular hoks at its m d s  - in a 0,5"-thick copper wall. This RF ccitipler dosil:n is based on the "Supported by the Doll I-& Ihiixgy Laser Joint Technology Ollice thoiigh a conlmct from NAVSEA. - __ - - - - - - experience for the LEDA RPQ and SNS high-power RF couplers, e.& see [3]. The required cavity-waveguide coupling is given by the coupling coefficient /I, =: (ev + < ) I t v  ~ 4 1 3 ,  where P,=780 kW is the CW wall power loss [l], and Pb=270 kW is the beam power of a 100-mA beam at the 2.5-cell cavity exit. The coupling is adjusted by changing the radius of the holes at the ends of the coupler slot. Figure 1: Schematic of 2.5-cell cavity with emittance- compensating magnets (left) and vacuum plenum (right). Coupler Model To simulate the waveguide-cavity system we use a simplified model shown in Fig. 2: a short pillbox cavity, which can be considered as a slice of the 3"' cell in the 2.5-cell PI cavity, with two attached short sections of the ridge-iuudecl waveguides. 'The picture shows only the internal (vacuum) part of the coupler model as used in simulations with the CST MicroWave Studio (MWS) [4]. Figure 2: Model of pillbox cavity with two waveguides. The pillbox radius is slightly adjusted to provide the correct resonant frequency 700 MFIz. Some model dimensions are listed in Tab. 1. I Pill-box cavity radius I 163.922 I 1 Pill-box lendh I 60 ___._I 1-1 I Coupler slot width t 1.434 Coupler slot length L 2.5 .- I Coupler end-hole radius __- Obviously, for the pillbox cavity 1 he waveguide-cavity coupling will be different than for the 2.5-cell PI cavity. It is related to the required as where Wi, Qi are thc field energies and unloaded quality factors of the pill-box TSvlolo mode and the operating n- mode in the 2.5-cell cavily. We assume that W,, is scalcd such that the surface rnagnetic field far from thc coupler iris in the transverse cross section at the coupler location is equal to one at the same location in the 31d PI cell (14.8 kNmj  for the nominal PI gradient. From (I)  the required waveguide-pillbox coupling is fi,,(z6,1. Time-Domaiul Simulations with MWS The waveguidc-pillbox coupling is calculated directly with MWS time-domain simulations: the fields excited in the cavity with perfectly conducting walls decay due to radiation into the open waveguides. Due to the model symmetry, we restrict our sirnulatioils to 1/8 of the model, imposing the electric boundary conditions in xy-planc, and magnetic ones in xz- and yz-planes, cf. Pig. 2. --4 0 500 low IMU T i m  I l is Figure 3: Electric fjeld decay due to radiation into open waveguides as calculated by MWS. The decay of the on-axis electric field excited in the cavity center by a short pulse from the waveguide is illustrated in Fig. 3. This particular computation on a mesh of about 50K points took 3.5 hours on a dual 1-GHz Pentium I11 Xeon PC. Inverting the calculated decay time constant gives us the external quality factor Qe of the system, and then we find ppb=QplJQe [4]. Starting from large slot-end holes (shorter computation times), we found that the required coupling &=6.l is achieved for a hole radius of 2.5 mm, cf. Tab. 1 and Fig. 3. An exact resonance frequency of the pillbox-waveguide system is conveniently calculated with the auto regressive (AR) filter feature in MWS time-domain simulations. It allows one to analyze spectra of time signals that have not reached a steady state yet. With this technique we found the pillbox-waveguide resonance atfr=699.575 MHz. To calculate the power density distribution around the coupler during its nominal operation, we simulate the RF feeding of the model via the waveguides with a TE-mode at frequency fr. The waveguide-mode fields are mostly concentrated in the narrow gap between the waveguide ridges, as shown in Fig. 4. The field scale corresponds to 1 W of the peak RF power fed through the waveguide. V I .  2. Set083 Z 1Se*OB3 1.05e+003 1.54st003 1.Z3et003 '323 616 300 0 ni. 5 33 t 72 4.0) 3.37 Z 63 2 . 0 2  1.35 0.5?* e Figure 4: Electric (left) and magnetic (right) fields of the waveguide RF input mode. The model symmetry means that the system is driven by two waveguides in phase. Calculated normalized time signals are plotted in Fig. 5. This plot, as well as one in Fig. 3, includes hundreds of oscillation periods; we see only signal envelopes. While the waveguide input remains constant, the output decreases reaching a point (t=956.6 ns) where it vanishes, and increases again after that. The output decrease is due to a destructive interference of two waves: one is reflected from the coupler aperture, and the other is radiated into the waveguides from inside the cavily. The reflected-wave amplitude remains constant when the input is constant, while the radiated-wave amplitude increases as the cavity field increases. These two waves are always in opposite phases [5]. As a result, at certain moment the two waves cancel each other, so that the reflected power vanishes at that particular moment. This situation corresponds to an exact match; Iherefore, the field snapshots at that moment give us field distributions for the matched case. Tim Signals Surface magnetic field on the wall in the 3rd ccll far from the coupler” Power density on the smooth wall in 3Id cell far from the coupler Maximal surface magnetic field near the coupler slot ends Maximal power density near the coupler slot ends Maximal power density elsewhere in 2.5- cell PI cavity w/o W couplers”” 200 400 m 800 T i m  I n s  Figure 5: Input (red) and output (green) waveguide signals from MWS time-domain simulations. The peak surface current magnitude at this moment is shown in Fig. 6. ‘This picture was obtained using field inonitors in MWS time-domain solver. The field scaling here is the same as for Fig. 4, i s .  1-W peak BF; power input into each of two waveguides. Value 14.8 kA/m 76 W/cm2 24.4 kA/m 205 Wkm’ 107 Wkm2 Figure 6: Peak surface current distribution at the moment of zero reflected power (vicw cut in xy-plane). One can see that the surface power densities arc the highest near the ends of the slot aperture. Comparing the magnetic field on the cylindrical wall far from the coupler with one at the coupler location in the 2.5-cell cavity for the nominal field gradient, wc find a ficld-scaling factor. Some results for rescaled fields and power values are presented in Tab. 2. The rescaled avcraged R F  power transferred through each of the two waveguides is about 525 kW, as should follow from the energy balance for the matched waveguide-cavi ty coupling. CONCLUSIONS Electromagnetic modeling of the coupler-cavity system has been performed using 2-D (Superfish) and 3-D (MWS) frequency-domain calculations, plus a novel time- domain approach using the Microwave Studio. These simulations were applied to adjust the coupling coefficient and calculate the power-loss distribution on the coupling slot. The cooling of the regions around the coupling irises is a challenging thermal management issue due to the surface power density exceeding 200 W/cm2 at some points. However, since the hot spots are localized in small regions around the slot ends, they can be cooled ef€ectively with cooling channels placed properly around the slots in the thick cavity wall. The authors would like to acknowledge useful discussions with R Krawczyk, D. Schrage, and R. Wood (LAMd), as well as with J. Rathke and T. Schultheiss of Advanccd Energy Systems. REFERENCES 1-11 S.S. Kurennoy, et al. “Photoinjector RF Cavity Design For High Power CW EL”, these proceedings. I23 S.S. Kurennoy, et al. “CW RF Cavity Design for High- Average-Current Photoinjector for High Power FEL”, EL’O2, hrgonne, IL, Sep. 2002. [3] L.M. Young, et al. “High Power RF Conditioning of the LEDA RFQ”, Proceed. 1999 PAC, NY, p. 881. [41 Microwave Studio v. 4.2, CST GmbH (Darmstadt, 151 T.P. Wangler, RF Linear Accelerators, Wiley, NY, Germany), 2003. www.cst.de. 1998. 

RF coupler for high-power CW FEL photoinjector

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RF coupler for high-power CW FEL photoinjector

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