Cohn, D.

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DOE/ET-51013-227Concepts of Millimeter/Submillimeter Wave Cavities,Mode Converters and Waveguides Using HighTemperature Superconducting MaterialD.R Chon; L. Bromberg; W. Halverson*B. Lax** ; P. WoskovJuly 22, 1987Plasma Fusion CenterMassachusetts Institute of TechnologyCambridge, Massachusetts 02148 USA*Spire Corporation, Bedford, MA**Physics Department, M.I.T. Cambridge, MAThis work was supported by Department of Energy No. ET-51013-227.Reproduction, translation, publication, use and disposal in whole or in part, by or for theUnited States government is permitted.PFC/RR-87-10ABSTRACTConcepts for possible use of high temperature superconductorsin millimeter/submillimeter wave cavities, mode convertors andwaveguides are described. Reduction of the ohmic loss in the cavitiescould remove a major constraint on the power produced by highfrequency gyrotrons. Significant benefits could be obtained withoutthe need for ultra high values of cavity Q. Improved mode convertorperformance could be obtained by compact, high power densitydesigns.1. IntroductionOhmic power losses can be a major limitation inmillimeter/submillimeter wave technologies. These losses could begreatly reduced by use of superconducting materials. However,there are practical limitations involving the use of conventionalsuperconductors at liquid helium temperatures. 1  Furthermore, thesuperconducting energy gap of conventional superconductingmaterials can be in the millimeter/submillimeter wavelength range;photons could thus cause excitations across the gap, thereby reducingthe superconducting properties. Use of new high temperaturesuperconducting materials 2 ,3 such as Y-Ba-Cu-O could make possibleoperation at much more convenient temperatures as well asproviding high energy gaps. We describe concepts for improvedcavities, mode convertors, and waveguides based on this approach.These passive applications of high temperature superconductorsmight be achievable with presently available materials.2. Gyrotron Cavities and Mode Convertorsa) Effect of Superconducting MaterialsThe gyrotron produces high power millimeter wave radiationby bunching of an electron beam in a resonant cavity subjected toa magnetic field4 . When the electron cyclotron resonancefrequency is approximately equal to characteristic frequency of thecavity, energy can be transferred from the beam to cavityradiation. Ohmic heating of the cavity due to surface currentsinduced by the electromagnetic radiation can be the dominantlimitation on the amount of power that can be produced in a CWdevice. This problem is particularly severe in high frequency(>100 GHz) tubes which use compact cavities in order to provide asufficiently thin mode spectrum for operation in a desirable singlemode.1The use of superconducting material reduces the skin depth inthe cavity walls, resulting in a very high value of the cavity qualityfactor for ohmic losses, Qohmic. The high value of Qohmic reducesthe wall loading for a given amount of output power. The wallloading scales asW ~ ( Qdiff/Qohmic ( Pout/hiaLwhere the Pout is the gyrotron output power, a is the radius of thecavity, and L is its length. Qdiff is the diffractive cavity qualityfactor, which determines the fraction of radiation that is coupledout of the cavity.The allowed wall loading is constrained by heat transfer and isa fundamental limit on output power.Increasing Qohmic makes it possible to reduce the wall loadingwhile keeping other parameters constant. The cavity radius cannotbe increased without leading to a more dense cavity modespectrum and an increased potential for mode competition. Atypical cavity radius is on the order of 3 - 10 wavelengths.Increasing L does not decrease the wall loading because of the L2dependence of Qdiff.The problem of wall heating is particularly difficult to dealwith in very high frequency gyrotrons (>100 GHz) since the wallloading for a given resonator mode can be shown to scale as Pwallf2.5 poutput, where f is the frequency.In order to minimize the losses, Qohmic should be substantiallygreater than the value of Q for a copper-based cavity. However,extremely large values of Q are not necessary and might in fact bedetrimental; if the wall resistivity is very low, the peak currentdensity in the superconductor could be very high and couldexceed the critical current. Operation of the cavity at liquidnitrogen temperature would allow removal of high heat fluxes,obviating the need of eliminating all of the ohmic losses.2b) Mode ConvertorsMode convertors are generally required to convert source (e.g.gyrotron) output to a linearly polarized beam peaked on axis.Such spatial beam qualities are necessary for many applicationsincluding electron cyclotron resonance heating in plasmas, plasmadiagnostics, and possible application to radar and communication.Keeping the gyrotron resonator dimensions as small as possiblewith superconducting materials will facilitate mode convertordesign by minimizing source output mode order.Use of superconducting materials in the waveguide modeconvertors themselves can also lead to significant improvements.Eliminating or reducing the ohmic losses in these convertorswould make possible very compact designs at high frequencies.Efficiencies would be improved not only because of lower ohmiclosses, but also because mode conversion to unwanted higherorder modes would be reduced with small guide dimensions.Peak power handling capabilities could be maintained byincluding the compact convertor designs in the high vacuum of thegyrotron.The use of quasi-optical mode convertors could also befacilitated with superconducting gyrotron resonators. Quasi-optical mode convertors have been shown to work well intransforming gyrotron radiation generated in whispering gallerymodes, TEmi, where m>>1. Gyrotron operation in such modes isalso advantageous for minimizing mode competition since theelectron beam is propagated near the surface of the resonator anddoes not excite the more closely spaced volume modes. However,whispering gallery modes have ohmic losses with conventionalconductors that make such gyrotrons impractical at very highfrequencies. The new superconducting materials could makepractical this type of gyrotron into the submillimeter-wavelengthrange.3c) Use of Highly Conducting, Non-Superconducting PropertiesQohmic scales asQohmic ~ a/5 ~ af1 / 2 G1/ 2where 8 is the skin depth and a is the wall conductivity. Even ifthe cavity wall material is not completely superconducting, a largeincrease in a relative to acopper (- 3.6 x 107 Ql-1m-1) couldsubstantially reduce the wall loading and increase the allowedpower output. For example, an increase in a by 100 times relativeto copper would reduce the wall loading by a factor of ten.d) Cavity CoatingThe superconducting material could be deposited by sputteringor vapor deposition techniques, including laser evaporation.Polycrystalline coating may be sufficient if the wall currentdensities are sufficiently low. If not, single crystal material couldbe used.One method of making the cavity is to grow thesuperconducting ceramic on a small tube made of soluble material,deposit structural material around the superconductor and finallydissolve the tube material. - The tube material may have patternsthat would be passed on to the superconductor. Another approachis to make the cavity on sections.Single-crystal coatings may be obtained by depositing thesuperconductors on an etched substrate with well defined patternsand then shock heating the ceramic superconductor with a shortpulse laser5 .4e) Cavity CoolingCooling of the cavity could be done using liquid nitrogen if theproperties if the superconductor at 70-80K are acceptable. Theadvantages of cooling at this temperature is that large amounts ofheat can be removed by the liquid nitrogen at relatively highefficiencies. Other cooling fluids could be used if better propertiesare required (such as Ne, H or He) at the expense of decreasedefficiency.3. Microwave Cavities. The high temperature superconducting material could also beused in a variety of microwave cavities. One obvious application isthe use in RF linacs (This application would involve ultra high Qcavities). Another application would be for the development ofelectromagnetic wave wigglers for free electron lasers. Specialpurpose cavities might be constructed by using lithographictechniques to provide features that would allow for mode selection.The methods of cooling and coating that were described earlier couldbe applied.4. WaveguidesSuperconducting waveguides could also be developed using theapproaches described above. This would be particularly useful in themillimeter wavelength range where present low-power fundamentalmode guides are very lossy and high-power guides are usuallyoperated well over moded to reduce ohmic losses. Withsuperconducting materials the performance of these guides would besignificantly improved. Higher-powers could be transmitted insmaller guides because ohmic and mode conversion losses would bereduced. The availability of such waveguides might make possibletransmission over much larger distances and could significantlyimprove the front end performance of millimeter-wave receivers.55. ConclusionsHigh temperature superconductivity materials may beespecially well suited for very high frequency RF components. Thesesmall scale, high temperature applications might not requireextensive material development and might be achieved relativelynear term. However, there are a number of constraints that mayprevent the development of these applications. These issues includecritical RF current density, critical RF field and power limitations.6References-1. See, for example: W. H. Hartwig and C. Passow in "AppliedSuperconductivity," V. L. Newhouse, ed., Academic Press, NewYork, 19752. J. G. Bednorz and K. A. Muller, Z. Phys., B 64, 189 (1986)3. M. K. Wu, J. R. Ashburn, C. J. Torng, P. A. Hor, R. L. Meng, Z. J.Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev. Lett. 908 (1987)4. See, for example: A. V. Gaponov, A. L. Gol'denberg, D. P. Grigor'ev,T. B. Pankratova, M. I. Petelin and V. A. Flyagin, Radiophysics andQuantum Electron, 1.L No. 2, pp 204-211, 1975 (EnglishTranslation); K. E. Kreischer and R. J. Temkin, in "Infrared andMillimeter Waves," vol. 7, K.J. Button, ed., Academic Press, N.Y.(1983)5. H. Smith, M.I.T., Private Communication7

Concepts for millimeter/submillimeter wave cavities, mode convertors and waveguides using high temperature superconducting material

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Concepts for millimeter/submillimeter wave cavities, mode convertors and waveguides using high temperature superconducting material

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