1. Second-Harmonic Fundamental Mode Slotted Peniotron Pulsed Power Plasma Science Conference, PPPS-2001 Las Vegas, NevadaJune 7-22, 2001 This work has been supported by AFOSR under Grant F49620-99-1-0297 (MURI MVE). Distribution Statement A: Approved for Public Release; Distribution is Unlimited L.J. Dressman*, D.B. McDermott, and N.C. Luhmann, Jr. University of California, Davis *Also NAVSEA, Crane D.A. Gallagher Northrop Grumman Corp. T.A. Spencer Air Force Research Lab. 1 of 221a

2. Abstract The harmonic peniotron has been demonstrated to be a highly efficient generator of millimeter-wave power [1]. Since a practical peniotron design must provide immunity to mode competition from gyrotron interactions as well as high device efficiency, the UC Davis peniotron design [2] employs an overcoupled interaction cavity for a predicted device efficiency of 47% at 34 GHz. Stability will be insured by operation in the lowest order mode of a slotted four-vane (magnetron type) circuit, the  /2 mode. The TE 11 -like  /2 mode couples well to the TE 11 mode of the circular output waveguide through the 2.5 mm radius iris at the end of the cavity. The output diffraction coupling configuration results in heavy loading of higher order axial modes and avoids mode conversion in the output waveguide. For diagnostic purposes, the experimental device will also incorporate side-wall coupling to the cavity. The peniotron will operate with a 70 kV, 3.5 A,  =1.5, axis-encircling electron beam generated by a recently developed Northrop Grumman Cusp gun [3]. Large-signal simulation of the interaction predicts an electronic efficiency of 58% and an extracted power output of 120 kW (47% device efficiency). The overall efficiency can be raised to 57% by use of a depressed collector. [1] T. Ishihara, et al., IEEE Trans. on Electron Devices 46, 798 (1999). [2] D. B. McDermott, et al., IEEE Trans. on Plasma Science 28, 953 (2000). [3] D. Gallagher, et al., IEEE Trans. on Plasma Science 28, 695 (2000). 1b2 of 22

3. Second-Harmonic Fundamental Mode Slotted Peniotron Objectives Approach Accomplishments Received two Northrop Cusp guns 34 GHz 2 nd -harmonic peniotron design - 125 kW with 47% device efficiency - Employs Northrop Cusp gun 34 GHz slotted cavity and coupler was designed with HFSS for high efficiency Axis-encircling electrons generate m th -order azimuthal mode in s th -harmonic peniotron if m=s+1 Slotted circuit enhances interaction and allows stable, lowest-order mode to have desired m th -order symmetry Cusp gun produces needed axis-encircling electron beam Improve device efficiency of Tohoku’s recent third-harmonic  =35% peniotron Achieve device efficiency of 50% in harmonic gyro-device Foundation for peniotron-amplifiers TE 11 -Like Mode in Slotted Cavity 3 of 221c

4. 4 of 22 Description of Peniotron –Fast-Wave Device –Similar to Gyrotron –Driven by Electrons’ Transverse Velocity –Optimized for Axis-Encircling Electron Beam –Resonance Condition with TE m1 Wave:  = s  c + k z v z s  Cyclotron Harmonic s = m for Gyrotron(Synchronism) s = m-1 for Peniotron(Asynchronism) 2a

5. 5 of 22 Motivation for Peniotron Proven High Efficiency –75% Electronic Efficiency Predicted Higher Efficiency –Efficiency >80% is Predicted Gyrotron Replacement –Higher Efficiency than Gyrotron –High Frequency Source well suited for Cyclotron Harmonic Emission 2b

6. State of the Art Tohoku University Team Recently Demonstrated Extremely High Efficiency –[ T. Ishihara, et al., IEEE-ED 46, p. 798, 1999 ] –30 GHz, 3rd-Harmonic Peniotron –Slotted (Magnetron Type) Waveguide, 2  Mode –Significant Achievement: Electronic Efficiency of 75% –35% Device Efficiency due to Critically Coupled Cavity 6 of 22 2c

7. Peniotron Resonance with TE m1 Wave:  = (m-1)  c + k z v z Electrons Move Forward by 360 o each Orbit Wave Appears as “DC” Electric Field Electrons E x B Drift to Deceleration Phase Peniotron Interaction 7 of 22 3a

8. E-Field Slotted Circuit TE 11 -Like Mode with TE 31 Content UCD Peniotron Features Second-Harmonic Operation - 34 GHz Operation in  /2 Cavity Mode –4-Vane Slotted Waveguide –Lowest Order Mode –Contains Needed m=3 Component –Suppresses Gyrotron Modes –Easily Couples to Circular Output Waveguide New Northrop Grumman Cusp Gun –High Quality Axis-Encircling Beam –High Efficiency Interaction –High Power (125 kW) 8 of 22 3b

9. Mode Selection for Axis-Encircling Electrons: s = Cyclotron Harmonic m = s for Gyrotron m = s+1 for Peniotron 4-Vane Slotted Circuit Yields m=3 for Lowest Order Mode Dispersion Diagram/Mode Selection Lowest Order Mode Ensures Stability 9 of 22 Peniotron  r w /c 2  (m=0,4)  (m=1,3)  (m=2) Strongest Competing Mode is 4th-Harmonic Gyrotron kzrwkzrw 4a

10. Nearest Competing Mode: 4th-Harmonic Gyrotron Start oscillation current for competing 4th-Harmonic Gyrotron is four times higher than Peniotron start current. Excellent Stability Predicted Magnetic Tuning Curve 10 of 22 Gyrotron Starting Current is Above Peniotron’s 4b

11.  50% Efficiency Predicted Efficiency Predictions: Electron Efficiency58% Device Efficiency47% Device Efficiency with Depressed Collector57% Collector Potential 12.8 kV Power and Efficiency Peniotron has been Simulated with Nonlinear Code 11 of 22  dev  dep  elec 4c

12. Beam Voltage70 kV Beam Current3.5 A Velocity Ratio, v  /v z 1.5 Magnetic Field6.5 kG Velocity Spread,  v z /v z 5% Guiding Center Spread,  r c /r L 10% Mode  /2 Axial Mode Number1 Vane Depth, b/a1.45 Electron-Vane Ratio, r L /a0.65 Inner Vane Radius, a1.82 mm Cavity Length31 mm Slot Angle,  o 22.5 Unloaded Q, Q 0 1900 Loaded Q, Q L 357 Design Parameters 12 of 22 5a

13. Slotted Cavity Cutoff Drift Tube Circular Iris (Removable) Cavity Design 13 of 22 Output Circular Waveguide r=4.5 mm Iris Radius for Critical Coupling: 2.35mm, Q L =990 Iris Radius for Over Coupling: 2.55mm, Q L =357 Q 0 =190 0 Diagnostic Coupling Ports 5b

14. Couples to TE 11 Circular Waveguide Mode Diffraction Coupling – Overcoupled for High Device Efficiency - 47% Predicted – Efficiency Increased by Depressed Collector - 57% Expected – Suppresses Higher Order Axial Modes Cavity Design–Diffraction Coupling 14 of 22 1st Axial Mode, 34 GHz 5c

15. Diagnostic Coupling – Couples to standard WR-28 rectangular waveguide – Coupling to adjacent slots will load both components of circularly polarized wave Diagnostic Sidewall Couplers 15 of 22 Diagnostic Coupling Couples to TE 10 Mode EE E 6a

16. Conversion to TM 11 Mode Only Above 40.0 GHz Mode Conversion Occurs Only at Higher Frequencies - TM 11 Mode is Excited Output Mode Conversion 16 of 22 6b

17. UCD Peniotron will use state-of-the-art Cusp gun developed by Northrop Grumman Cusp Gun 17 of 22 7a

18. Axis Encircling Beam Parameters: Beam Voltage70 kV Beam Current3.5 A Velocity Ratio, v  /v z 1.5 Velocity Spread,  v z /v z 5% Guiding Center Spread,  r c /r L 10% Cusp Gun 18 of 22 7b

19. Four Independently Controlled Coils Superconducting Magnet 19 of 22 Gun Coil Components Cusp Gradient from Internal Gun Coil and Two Supplemental Gun Coils 7c

20.  50% Device Efficiency Predicted Summary Peniotron Demonstrated Very High Efficiency (Tohoku) UCD Peniotron Designed For High Device Efficiency  /2 Slotted Circuit Mode Provides Stability and m=3 Component for s=2 Peniotron Overcoupled Cavity Provides High Device Efficiency Northrop Grumman Cusp Gun Provides Required Axis-Encircling Beam 20 of 22 8a

21. Future Work Circuit Fabrication Cold Test Electron Beam Test Hot Test the Peniotron 21 of 22 8b

22. Sign Up Sheet 22 of 22 8c