Objectives Approach Accomplishments & Future  Experimentally demonstrated RPM oscillation  Simulations predict Mode-Control-Cathode (MCC) phase-locks linear arrays together  MCC reduces mode competition  Future research will optimize microwave output- coupling techniques  Investigate UM Recirculating Planar Magnetron (RPM) invention (patent pending)  Utilize two linear magnetrons connected by recirculating sections  Large-area cathode for high currents  Scalable to higher powers  Reduce magnetic field volume (V) scaling vs # cavities  Explore innovative relativistic magnetrons for HPM generation  Scale to higher microwave power levels  Exploit higher cathode currents  Extend HPM pulselength by lower cathode-anode thermal loading  Increase HPM pulse energy University of Michigan Relativistic Magnetron Research Recirculating Planar Magnetron: simulation and Experiment

Microwave Oscillation in a Recirculating Planar Magnetron Matthew Franzi, Ronald M. Gilgenbach, Y.Y. Lau David Chalenski, David Simon, Geoff Greening Plasma, Pulsed Power and Microwave Lab, Nuclear Engineering and Radiological Sciences Dept., University of Michigan, Ann Arbor Brad W. Hoff, David French Air Force Research Laboratory, Directed Energy Directorate, Kirtland AFB, NM John. W. Luginsland Air Force Office of Scientific Research, Arlington, VA Supported by, AFOSR (grant#: FA ), AFRL, L-3 Communications Electron Devices, and Northrop-Grumman

Recirculating Planar Magnetrons 1 Motivation: RPM Advantages Conventional 12 vane magnetron Anode Cathode Larger cathode surface area provides higher current Larger anode area allows for faster heat dissipation RPM allows for nearly full electron beam recirculation Planar cavities are decoupled from the anode- cathode and spacing Magnetic field volume (V) scales linearly with # cavities (N) instead of (N 2 ) as with cylindrical magnetron Conventional-polarity 20-vane RPM Anode Cathode 1- Patent Pending

Recirculating Planar Magnetrons Motivation-RPM Embodiments Radial magnetic field (r) Axial electric field between anode and cathode (Z) ExB drift ( θ ) Cylindrical cavity array Radial magnetic field (r) Axial electric field between anode and cathode (Z) ExB drift ~( θ ) Planar cavity array Axial magnetic field (z) Electric field between anode and cathode (x) ExB drift ~(y) Planar cavity array 123 z r z r y x z

Recirculating Planar Magnetrons Design Parameters Model: RPM-12A Design: 12 Cavities- 2 planar oscillators 6 cavities each Operating Frequency:1 GHz π -mode Guided Wavelength: 7.69 cm Phase Velocity: 0.26c 3.85 cm 4.45 cm 6.3 cm 2.54 cm

Recirculating Planar Magnetrons Simulated Results: Even-Odd modes HFSS Even π -mode GHz Odd π -mode GHz Closely spaced mode structure Same planar guided wavelength λg = 7.68 cm ~3Mhz separation

Recirculating Planar Magnetrons Experimental: Cold Test Mode 5 π /6 odd 5 π /6 even π odd π even HFSS (GHz) Antenna (GHz) : π -even 2: π -odd 3: 5 π /6-even 4: 5 π /6-odd

Recirculating Planar Magnetrons Simulated Results: Operating Modes Top and bottom oscillator operate in π -mode Each side operates at an offset frequency 8 MHz separation Slight competition

Recirculating Planar Magnetrons Simulated Results: Cavity Signals Oscillator voltage signal of symmetric cavities Phase drifting Slightly offset π -mode Drifts from in phase to out of phase in 40 ns 30 ns later

Experiment (Equipment Diagram) Recirculating Planar Magnetrons

Heterodyne(V) RPM Shot Axial Magnetic Field: 0.20 T Microwave Pulse Length: ~250ns Oscillation Frequency: GHz Anticipated Mode: π -even Current at Startup: 2.35kA Peak Current: 3.3kA

Heterodyne(V) Frequency(Hz) Time (ns) RPM Shot Axial Magnetic Field: 0.18T Microwave Pulse Length: ~200ns Oscillation Frequency: 1.01GHz 0.99GHz Anticipated Mode: Bimodal Current at Startup: 1.7kA Peak Current: 3.9kA

Recirculating Planar Magnetrons Experimental Results: B-dot probes Experimental Frequency and Phase Measurements 2 B-dot probes placed near symmetric cavities of the RPM Oriented to diagnose fringing H-field from cavity ~3.8 cm away from cavity ~0.2 cm 2 area

Recirculating Planar Magnetrons Experimental Results: B-dot probes Primary Frequency: GHz Microwave Oscillations measure by b-dots extend past diode signal Phase drift and offsets (between oscillators) exist throughout pulse duration

Recirculating Planar Magnetrons Simulated Results: MCC Concept Mode Control Cathode Geometrically similar to Transparent Cathode (Schamiloglu et al. ) Emission priming structure Acts as a resonant electromagnetic coupler Full electron circulation Cross oscillator self focusing Independent tuning mechanism y x z

Recirculating Planar Magnetrons Simulated Results: MCC Concept Even Mode Dispersion Solution: UV=-YZ β n = β 0 +2 π n/L Propagation Constants: Boundary Conditions: Phase Shift per Cavity

Recirculating Planar Magnetrons Simulated Results: MCC (Analytic Versus HFSS) h 1 = 6.3 cm b=2.4 cm h 2 =0.75 cm w 1 =1.92 cm w 2 =1.92 cm

Recirculating Planar Magnetrons Simulated Results: MCC in 2D PIC Odd π -mode Even π -mode

Recirculating Planar Magnetrons Simulated Results: MCC in 2D PIC MAGIC PIC Results: Re-entrant boundary conditions 0.18 T applied magnetic field Applied voltage varied from 150 to 500 kV depending on AK gap Phase locking occurred in as little as 28 ns for small AK gaps Simulations reached a locked state when the relative phase between top and bottom cavities did not vary mode than +- 2 degrees for 50+ ns

Recirculating Planar Magnetrons Recent Work: Mode Control Cathode Experimental MCC 5 periodically spaced 2.2cm OD rods 3.8 cm apart (matching vane-cavity spacing) 3.34cm AK gap Phase locking is evident in some cases

Recirculating Planar Magnetrons Recent Work: Mode Control Cathode Top B-dotBottom B-dot Locked π -mode π -mode – Raw Data π -mode – FFT

Future Work: Axial Extraction Recirculating Planar Magnetrons Based off the slotted waveguide antenna [greenwood patent] Ridged waveguides allow for smaller guided wavelengths and higher efficiency. Pulsed Bias : 300kV Axial Magnetic Field: 0.13T Diode Current: 3kA Operating Frequency: 2.2GHz Output power: 400MW Efficiency: 45% Axial Extraction (Hoff et. al)

RPM experiments are currently underway at UM on kV, 1-10 kA, microsecond, T ~100 recorded shots with observed microwave oscillation in the vicinity of π -mode frequencies were measured ( GHz ) ns microwave pulse length 2-10kA emitted beam current B-dot probes validated frequency/phase Mode Control Cathode has been shown, in simulation, to: Increase cross-oscillator communication Support phase locking Increase mode separation Reduce start-oscillation time Experiments on the Mode Control Cathode are underway Recirculating Planar Magnetrons Conclusions

Finish parameter sweeps of magnetic field for the MCC B-dot probes will diagnose locking between oscillators Adapt RPM slow wave structure and MCC design to optimize theoretical locking criteria Smaller AK-gap Rectangular cross section for MCC rods Extraction system design and fabrication are anticipated for Spring/Summer 2013 Future Work: Objectives Recirculating Planar Magnetrons