Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons

1. Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons New measurement system and first results Andreas Schlaich1,2; Gerd Gantenbein1; Stefan Kern1; Manfred Thumm1,2 1Karlsruhe Institute of Technology (KIT), Germany – Institute for Pulsed Power and Microwave Technology (IHM) 2Karlsruhe Institute of Technology (KIT), Germany – InstitutfürHochfrequenztechnik und Elektronik (IHE)

2. Content • Introduction • Hardware Setup • System • Signal path • Data processing • System Summary • Measurements • Typicalstart-up • Cavitymodecompetition • Low-frequencyoscillation • Parasiticoscillation • Conclusion Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

3. Introduction • IHM/KIT Fusion gyrotrons • ITER: 2 MW at 170 GHz (TE34,19), coaxial • W7-X: 1 MW at 140 GHz (TE28,8), conventional(cylindrical) • Frequencystep-tunable Gyrotron (105-155 GHz) • Gyrotron testing: • Operation modes: • Short-Pulse operation (100µs .. 10 ms) • Long-pulse operation (100 ms … „CW“) • Typicalmonitoringequipment: • Filterbanks (8 channelswith 2 GHz width, ~24 dB dynamicrange) • Modulation domainanalyzer (2 GHz bandwidth, 20 dB dynamicrange) • Spectrumanalyzersystem (<500 MHz BW, >50dB dynamicrange) Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

4. Introduction Differenceto ECE radiometry etc.: - nohard real-time functionalityneccessary - „thin“ spectra • Problem: Transient phenomena in gyrotrons • Parasiticoscillations • Cavitymodecompetition / switching • Power orfrequencyvariationbased on internalorexternalparameters • Desirablesystempropertiesfor proper observation • High bandwidth (> 2 GHz) • High dynamicrange (>> 30 dB) • Time resolution (< 100 µs) • Fullcoverage (110-170 GHz) • Short-Pulse compatibility (100µs - 5 ms) • Not feasablewithfilterbanks, modulationdomainorspectrumanalysis  time domaindetection, butfrequencydomainanalysis Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

5. Hardware setup Localoscillator 1 Horn antennapickup (main / reliefwindow)  Strayradiation Computer Isolator Data processing Level control Mixer 1 Power split Oscilloscope Mixer 2 Detection (A/D) Overmoded (low-power) waveguidetransmissionline Down- conversion Isolator Localoscillator 2 > 90 GHz 0-5 GHz RF IF Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

6. System:Downconversion IF RF hn·fLO1 h(n+1)·fLO1 … P f Receiver bandwidth Upperside band || Low sideinjection Lowerside band || High sideinjection • RF: 110-170 GHz  harmonicmixingnecessary • Conceptofharmonicmixing: • Ambuguitysources: • Convertingharmonich • Injectionside (HSI/LSI) • Reception in twochannels, bothhaveindependentharmonicandinjectionsideambiguity Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

7. System: A/D conversion fN AFDR aliasing AFDR = 60dB Aliasing Aliasing (1) NADC=7.2 bit, ΔfFFT=1MHz AFDR: aliasing-freedynamicrange • VerticalresolutionSNR = 6.02NADC + 1.76dB + 10log(NFFT/2) ≈ 82 dB (1) • NyquistfrequencySR = 10 GS/s  fN= 5 GHz • Aliasing: • Limits usabledynamicrange / BW • Reducedbymixerdiplexers • Result: • BW = 3 GHz • AFDR = 60 dB Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

8. Data processing: STFT T1 T2 T3 t Spectrogram e.g.: 2 ms* 4 GHz (fres=1 MHz, xOV=0.5) 3000 * 4000pixel f • Short-time Fourier Transform • Extractsegments • Performsegment FFT • Concatenate in time direction Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

9. Data processing: RF reconstruction … h1,S·fLO1 (h1,S -1)·fLO1 B receiver P Ch1 f f • Valid h? • HSI/LSI? h2,S·fLO2 (h2,S -1)·fLO2 ΔfS P Ch2 f Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating Redundancyinformationofsecondchannel: Defined LO frequencydifference: Correlationof IF spectrashiftedby +/-ΔfScanreconstruct RF spectrum! Observedsignal must haveB < ΔfS: 50-150 MHz typicallysuffiecient Suitablecorrelationoperator: Minimum()

10. Dataprocessing: RF reconstruction ΔfS= h1 fLO1-h2 fLO2 P / dBm S1,i (ch1 IF spectrum) S2,i (ch2 IF spectrum) Min[S1,i( f-ΔfS), S2,i( f )] Min[S1,i( f ), S2,i(f-ΔfS)] High side Injection (HSI) Low side Injection (LSI) f / GHz ΔfS „blind spot“ Examplebytwo IF spectra: Fullreconstructionof RF LSI & HSI forh1, h2 Achievable real-time bandwidth: BRF = BIF – ΔfS ≈ 2BIF = 6 GHz Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

11. System Summary • Extensionsalready • plannedfor 2012: • 2ndchannel pair • Memory extension Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

12. Measurements: Normal operation W7-X SN5 BIF (span) was chosen 4 GHz insteadof 3 GHz  sensitivity roll-off visible BIF (span) BIF (span) D < 60 dB D < 60 dB zoom Low sideinjection (upperside band) High sideinjection (lowerside band) 142.4 GHz 138.35 GHz 138.45 GHz 134.4 GHz Downsampledoverview: frequencyaxisbyfactor 29, time axisbyfactor 4 Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

13. Measurements: Normal operation W7-X SN5 400 MHz Spectrumposition Externalparameters: T2 T1 T1 Ib/ A Uk/ kV T2 t / ms xov= 0.25, Δf < 0.1 MHz TE28,8 at 140 GHzclearlyvisible Variation vs. Uk, Ib TE28,8detectablefromUk= 70 kV, Ib= 22 A Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

14. Measurements: Cavitymodeswitch (Zooms) 250 MHz 250 MHz 400 µs Pulse end TE27,8 TE28,8 xov= 0.25, Δf = 0.5 MHz 0 < T / ms < 1.93: Clean TE28,8 operationat ~140.250 GHz 1.93 < T / ms < 2.02: Competitionwith TE27,8, strong P andfvariation 2.02 < T / ms < 2.06: 40µs parallel operationtillregular pulse end Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

15. Measurements: Low-frequency (LF) modulation SpectrumatT2 = 1.0 ms T2 SpectrumatT1 = 0.4 ms T1 xov= 0.25, Δf = 0.25 MHz Strong LF modulationfor dominant TE27,8operation: Possibleexplanation: Long-lineeffect (strong reflectionatwindow) Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

16. Measurements:Parasiticfrequencyjumping MeasuredatUk=76 kV, Ib=43A Significantjumpsandtracesoflow-frequencymodulationvisible Justifiesearlierhypothesesaboutparasiticoscillationsjumpingbetweendiscretefrequencies Probable source: Dynamic after-cavityinteraction (ACI) Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

17. Conclusion • Summary: • New measurementsystem was developedforbroadband real time gyrotronoutputspectrumexamination • First measurements on W7-X tubes SN5 and SN6, also theKIT coaxial modular prototype (not shown) wereconducted • Transient phenomenalikefrequency/power variation vs. externalparametersormodeswitching etc. couldbeobserved • Outlook: • System is still underdevelopment, morefeatures (complexsignalanalysis, test stand integration, performanceoptimization..) tobedone • Full band characterizationincludingtransmissionline • 2ndchannel pair andhardwareimprovementalreadyplanned Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating

18. The authors would like to thank T. Rzesnicki, A. Samartsevand J. Jelonnek (IHM), and Francois Legrand, GéraldLietaer (TED) for their kind support. Acknowledgement: This work, supported by the European Communities under the contract of Association between EURATOM and KIT, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Thankyouforyourattention! Schlaich et al. – Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating