Hill, M E

Palmer, D T

Schwartzkopf, S

Siemann, R H

Witherspoon, R

CERN Document Server

MATERIALS RESEARCH RELATED TO W-BAND CAVITYCONSTRUCTIOND. T. PALMERz, M. HILLyz, S. SCHWARTZKOPF, R. H. SIEMANNz, AND R. WITHERSPOONzStanford Linear Accelerator Center, Stanford University, Stanford CA 94309yHarvard University, 42 Oxford Street, Cambridge MA 02138 Ron Witherspoon, Inc., 430 Industrial Street, Campbell, CA 95008AbstractLow power rf measurements, S11, of electro-discharge ma-chined (EDM) diffusion bonded mm-wave traveling waverf cavities were not in satisfactory agreement with electro-magnetic simulations. During subsequent mechanical in-spection, the cell-to-cell iris were found to be distorted.This led to a series of systematic experiments to studythe mechanical properties of oxygen free high conductiv-ity Copper (OFHC) and Glidcop AL-15. Results of thesestudies which include cell-to-cell iris distortion, EDM ma-chining accuracies, surface quality, and the results of dif-ferent bonding techniques are presented.The results of our mechanical studies are used to developa set of mechanical design constraints for a second series ofconstant impedance W-Band structures that also used wireEDM and high temperature bonding for their manufacture.1 INTRODUCTIONIn this paper we shall present the results of mechanicalstudies as it relates to issues of mechanical fabrication of90 GHz accelerating structures [1]. We shall present re-sults from our material tests that indicate Glidcop AL-15,which is a dispersion strength material, is the material ofchoice when these materials must be exposed to temper-ature cycles greater than 1000oC. We present results thatindicate that sinker EDM machining is capable of attain-ing the tolerances necessary to produce a tuned acceleratorstructure. Bonding studies are then presented. Finally, anoverview of the available technology to mechanical inspectfabrication structures are presented.These results have driven our efforts, to produce and testa W-band structures, away from bonded copper muffin tinstructures to a novel rf zipper structure [2], that have nobond joints located in regions of large rf currents, producedfrom dispersion strengthened materials.2 MATERIALS STUDIESVisual inspection of the first 25 cell constantimpedance(CI) traveling wave accelerating structuremechanical deformation of the cell to cell iris was ob-served, as seen in Figure 1. Two possible effects mighthave caused the iris deformation. The yield strength of theWork supported by the Department of Energy, contracts DE-AC03-76SF00515 and DE-FG03-97ER82470Figure 1: Visual inspection of the cell to cell iris revealeddistortion.base material was exceeded during the diffusion bondingcycle or during the beam pipe EDM machining step.To identify the cause of this distortion, we constructeda series of test structure made from OFHC copper andAL-15, using wire EDM milling technology. These sam-ple cavities were put through identical thermal cycles thatmimicked the diffusion bond process used to produce Fig-ure 1.Two subclasses of OFHC copper, annealed and workhardened, were investigated along with AL-15 as possiblecandidates for use as the base material for W-band struc-tures.Figure 2 shows a 30 m iris distortion that was intro-duced in the the annealed OFHC copper after wire EDMmilling of the test structure. It is thought that thermal in-duced crystal growth during the wire EDM milling processis responsible for this distortion.A identical series of test were also conducted on AL-15and work harden OFHC copper, see Figure 2. To withinthe optical resolution of our microscope no distortion wasobserved in either the AL-15 or work hardened copper dueto the thermal cycle of the diffusion bonding process. Butour metallurgy analysis of the 25 cell work hardened diffu-sion bonded OFHC structure indicated that the OFHC basematerial plastically deformed during the bonding processand therefore is not a candidate as an acceptable materialfor thermal bonding.To summarize, annealed and work harden OFHC cop-per are unacceptable base materials for mm-wave accel-erating structures which are fabricate by thermal bondingand EDM wire machining technologies. This is due tothe deformation induced in the rf cavities during the wiremilling and diffusion bonding processes. AL-15 is an ac-ceptable material to fabricate, thermally bonded and wireEDM milled, mm-wave accelerating structures.0-7803-5573-3/99/$10.00@1999 IEEE. 545Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Seven Cell Test Structure Before Thermal CycleWork HardendGlidcopAnnealedOFHCOFHCAL-15 x = 30     md mFigure 2: View of material test structures before and afterthermal cycle.3 ELECTRO-DISCHARGE MACHINING(EDM)In this section we studied the surface finish attained byEDM machining techniques, wire milling and sinker EDM,using non-destructive inspection techniques.During SEM analyses of the our first 25 cell structurezinc (Zn) was found to have contamination the structure.Clearly, this is a byproduct of the Brass coated EDM wireused during fabrication. It should be pointed out that therewas limited flushing due to the constrained geometry of thisstructure. Our future design and step by step manufacturingprocess eliminates the problem of limited flushing action.3.1 Wire EDMPrevious attenuation studies of EDM OFHC copper waveg-uide [3] measured an attenuation of -0.2 dB for Cu waveg-uide, manufactured using wire EDM technology. Since re-sults in Section 2 indicate that the base material should beAL-15, we have extended the previous work to include theeffect of different EDM wire base material used to fabricatestructures out of AL-15.In these studies, the surface finish of WR-10 waveg-uide, constructed out of AL-15, was investigated non-destructively, by measuring the attenuation of the rf power,in the frequency range of 85 to 95 GHz, as a function ofEDM wire base material. Three 2 inches long W-bandwaveguides were wire EDM milled out of AL-15 using aBrass coated wire, trade name Cobra Cut A, pure Copperwire, and a Molybdenum wire.Comparing the attenuation factor of pure copper, -0.13dB, to that of AL-15, -0.29 dB. We find that Al-15 is sat-isfactory base material for accelerator structure in the mm-wavelength. No variation in the AL-15 attenuation factorwas measured before or after cleaning.3.2 Sinker EDMPrevious planer structure design at W-band have limitedthemselves to only utilized wire EDM. We have investi-gated the use of sinker EDM machining to produce the cav-ity structure. This technique would eliminate the need toDimension (mm) Mean (mm)  (mm)1.000 0.99970 0.002320.250 0.24964 0.002002.000 1.99259 0.001362.400 2.39083 0.00170Table 1: Optical inspection results of a series of 6 SinkerEDM 7 cell test structures.bond the cavity structure in the region of large rf currents.This test was conducted with a single electrode, pro-duced by a precision wire EDM machine, with the nega-tive of the 7 cell cavity structure we wanted to produce. Inthis manner we were able to transfer the precision of thewire EDM machine to the sinker EDM machine and the 7cell test structure. Optical comparison techniques were uti-lized to inspect the surface finish of the cavity walls andbottom. The cavity walls and bottoms were found to have aRMS surface finish of 0.41 m and 0.82 m, respectively.Dimensional accuracy attained by the sinker EDM tech-nique are tabulated in Table 1. In conclusion, we find thatsinker the EDM machining technique can attain the dimen-sional tolerance required by mm-wave accelerating struc-tures. Improvements in the surface finish are possible withthe use of higher quality cathode material.4 BONDING STUDIESDuring the course of our studies into the cause of the irisdistortion, see Figure 1, the iris to cavity bond was studied.Metallurgy analysis indicated that they was no significantepitaxy crystal growth in the iris region of the bonding sur-face. This is the region where large rf currents flow per-pendicular to the bond joint. This has caused us to pursuea novel rf design [2] that does not necessitate any physi-cal bond in the regions of large rf currents. But it shouldbe noted that we are studying different bonding techniques,since the ability to bond metals together without physicallydistorting the structure allows for a larger range of rf struc-tures to be envisaged and fabricated.Therefore, we are investigating diffusion brazing withsilver plated alloy, ultra thin alloy brazing, and also diffu-sion bonding of diamond fly cut surfaces. We shall discusseach of these process in the following subsections.4.1 Diffusion BondingInitial diffusion bonding of W-band structures at SLAC uti-lizes hand lapping of all bonding surfaces, that are flat andparallel to 0.5 m. A pressure of 20 psi was applied to thestructure with a temperature plateau of 1020oC held for 1hour. The metallurgy analysis indicated that they was nosignificant epitaxy crystal growth across the bonding sur-face and in some case no physical contact in the region ofcoupling iris where rf current flow.The lack of bonding in the iris region could possible be546Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Machine cell width (m) cell length (m)DMM 12-106 837.04  0.43 2368.71 0.90M48 828.71  0.50 2363.04 0.81Apex 200 828.43  0.83 2361.54 0.64Table 2: 41 cell muffin tin structure baseline and opticalinspection results.due to the lose of surface flatness in the iris region dueto preferential chemical etching used to clean the surfacesprior to bonding, or roll off in iris region induced duringthe lapping process.Due to the success of diffusion bonding of diamondturned surfaces for X-band structure development [4] weare pursuing this technology with respect to W-band struc-tures. To date, we have been able to utilize this technologywith a W-band cavity plate without physically damagingprecut irides. For this to be a viable technology for W-bandstructure, it must be applicable to AL-15.4.2 Diffusion BrazingTo overcome the drawbacks of the solid-state diffu-sion bonding process in fabricating mm-wave accelerat-ing structures, we began exploring an alternative bondingmethod called Liquid Interface Diffusion (LID) bonding.LID bonding is a metallurgical bonding technique com-monly used in the aerospace industry for applications asdiverse as the assembly of honeycomb structures and theassembly of jet turbines. In this technique, a low-meltingpoint metal is inserted as an interlayer between the twometals to be bonded. The interlayer is chosen from amongthe metals that readily alloy with the metals to be bonded;for LID bonding of copper, the optimum low-melting al-loy to use is silver. Upon heating, a liquid forms as theinterlayer melts. As a result of liquid formation, a com-bination of wetting and capillary attraction insure that themating surfaces are drawn closely together. By holdingat the liquid-forming temperature, inter-diffusion betweenthe interlayer and the higher-melting parent metal resultsin isothermal solidification of the bond line. When thebond-line is subsequently examined metallurgical, no ev-idence of a distinct low-melting phase can be observed.This is the principal difference between LID bonding andtraditional brazing. Processing variables such as interlayerthickness, masking to limit deposition to only the interfaceto be bonded, and selection of joining temperatures andpressures are currently under investigation. A major advan-tage of LID is its significantly lower brazing temperature of800oC versus that of 1020oC for conventional H2 Brazingwith Au-Cu alloys.4.3 Ultra Thin Alloy BrazingS-Band structure normally utilize 40 m thick Au-Cu al-loy to bond surfaces together. At higher frequency, the fil-let produced by this technique would significantly detunea resonant structure. We have been able to bond coppersurfaces together with 35-65 Au-Cu alloy that is 2.5 mthick. No detuning fillet was observed. This same processis being studied with AL-15, as the base material of thestructure.5 MECHANICAL INSPECTIONTECHNIQUESDimensional tolerance at W-band are on the order of 2.5m. Contact measurements are capable of attaining mea-surement tolerance of 0.25 m. It has been observed thatcontact measurement can cause significant damage to W-band structures constructed out of work hardened OFHCmaterial [5]. Similar damage was observed on a Al-15 testfixture. We report, the results of our investigation into opti-cal inspection techniques that would eliminate the surfacedamage induced by more invasive techniques.To provide a calibrated baseline, SLAC’s Leitz PMM 12-106 coordinate measuring machine was used to measure a41 cell muffin tin structure. These results are accurate towithin .25 m.The 41 cell muffin tin structure was then sent to San-dia National Laboratory and was inspected on a MooreM48 optical inspection machine. The 41 cell structure wasalso inspected on an Optical Gauging Products Apex 200machine. The results of these inspection are listed in Ta-ble 2. The optical measurement technique under estimatedthe cell physical dimensions by 10 m. This is due to theedge finding schemes getting confused by shadow effectsat the wall edge. Whereas the contact method depends onphysical force to find an edge of a cavity wall. It should benoted that during all optical inspection, the 41 cell structurewas back lit with low intensity white light.6 CONCLUSIONSWe find the Glidcop AL-15 is an acceptable base mate-rial for use in fabricating W-band accelerating structures.Problem with thermal bonding of planar structures must beovercome to increase the diversity of exotic structure thatmay be envisaged by accelerator physicists. We have by-passed this problem with a novel rf structure design thateliminates bonding metal surfaces together perpendicularto the rf current flow. Sinker EDM machining is capable ofachieving the tolerances necessary to fabricate mm-wavestructures. Optical inspection techniques under estimatethe absolute dimensions due to lighting and edge effects.7 REFERENCES[1] P.J. Chou et al., PAC97, page 464[2] Kroll et al., PAC99, FRA92[3] M. Hill, ARDB Technical Note 126[4] C. Pearson, Personal Communication[5] P.J. Chou, ARDB Technical Note 115547Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

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