INVESTIGATIONS ON SECOND HARMONIC GYROTRONS

Abstract

A varietyofsuccessfulgyrotronsarebeingdevelopedforvariousplasmaheatingand other industrial,scientificandmedical(ISM)applications.Theplasmaphysicscommu- nity isthefirsttoexploittheapplicationofgyrotronforpre-ionization(ignition),electron cyclotronresonanceheating(ECRH),electroncyclotroncurrentdrive(ECCD),stabiliza- tion ofneoclassicaltearingmodes(NTM).Also,gyrotronsarebeingusedforcollective thomson scattering(CTS)measurementsinexperimentsandplasmadiagnostics.Based on thefusionreactor’sdesign,thesedeviceswithoutputpowerslargerthanamegawatt are employedforECRHofplasmas.Gyrotronsalsofindapplicationsinindustrialor technological heating,materialprocessing,DynamicNuclearPolarization-NuclearMag- netic Resonance(DNP-NMR)inspectroscopy,advancedsinteringofceramics,medical imaging, etc.Similarly,gyrotronshavebeenoperatedatfundamentalcyclotronharmonic with outputpowersofaroundmega-wattsforECRHapplications.Themainconcernin the designofgyrotronsoperatingatfundamentalcyclotronharmonicisthemagneticfield requirement increaseswiththeoperatingfrequencyandtherebyincreasingtheoverall system costandsize.Thisbringstheneedofsecondharmonicgyrotronsintopicture. Forlowandmediumpoweroperationinsteadofoperatingthegyrotronatfundamental harmonic, itisspecificallyadvantageoustooperateatsecondharmonicofthecyclotron frequencywherethemagneticfieldrequirementcanbereducedbyhalf.Apartfromthe research beingcarriedoutonfundamentalgyrotrons,avarietyofsecondharmonicgy- rotrons arealsobeingdesignedanddevelopedfordifferentISMapplicationsrequiring powerlevelofaround100-200kW. Second harmonicoperationofgyrotronleadstothereductioninthesizeofthesuper- conducting magneticcoilsandtheassociatedcoolingsystems.Thus,theoverallvolume of thesecondharmonicgyrotronisgreatlyreducedincomparisonwithafirstharmonic gyrotron andtherebyreducestheoverallspaceandmaintenancecostofthesystemand also, thesystemcanbeusedformobileapplicationsaswell.Therefore,thesegyrotrons find applicationswherethecostandcompactnessofthesystemareofprimeconcern. However,thechallengeinthedesignofgyrotronsatsecondharmonicisthehighermode competition duetobothfundamentalandsecondharmonicneighbors.Themagneticfield requirement getsreducedbutatthecostofefficiencyascomparedtothefundamental operation. Forhigherfrequenciesofoperation,themodecompetitionseverityincreases evenmore.Therefore,themodeselectionproceduresarereallycriticalandhastobe carefully investigatedandstudied.Thesechallengesarekeptintoaccountwellwhileper- forming thedesignstudiesofsuchharmonicgyrotrons.Thisgivesthemotivationtotake up thecriticaldesignandassessmentofsuchdevicesasourdesignproblems.Anexten- siveresearchbyvariousgroupshaveperformedthesuccessfultechnologicaldevelopment i of conventionalcavityfundamentalandsecondharmonicgyrotronsovertheyears. Introduction tothelimitationsofconventionalelectronbeamdevicesandsolid-state devicesalongwiththebriefoverviewofdifferentclassesofgyro-devicesarediscussedin Chapter 1ofthisthesis.Thebasicelectroncyclotronmechanismofthegyrotronandthe major sub-componentsofthegyrotronarediscussedindetailinChapter2.Also,theliter- ature surveyontheconventionalcavityfundamentalandsecondharmonicgyrotronsde- velopedforvariousISMapplicationssuchasplasmaheating,materialprocessing,plasma diagnostics, CTSarealsopresentedinthischapter.