Second-Harmonic Fundamental Mode Slotted Peniotron

Transcription

1 Second-Harmonic Fundamental Mode Slotted Peniotron 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. Pulsed Power Plasma Science Conference, PPPS-2001 Las Vegas, Nevada June 7-22, 2001 This work has been supported by AFOSR under Grant F (MURI MVE). Distribution Statement A: Approved for Public Release; Distribution is Unlimited 1 of 22 1a

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 π/2 mode. The TE 11 -like π/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, α=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). 2 of 22 1b

3 S e c ond-harmonic F undamental Mode Slotted Peniotron Objectives Improve device efficiency of Tohoku s recent third-harmonic η=35 % peniotron A c hiev e d e vic e efficien c y of 50 % in harmonic gyro-device Foundation for peniotron-amplifiers Approach TE 11 -Like Mode in Slotted Cavity Accomplishments Axis-encircling electrons generate Received two Northrop C usp guns m th -order azimuthal mode in s th -harmonic peniotron if m= s+1 34 G H z 2 nd -harm onic peniotron de sig Slotted circuit enhances interaction k W with 47 % d e vice efficien c y and allow s s table, low e st-ord er m o d e - E m ploy s N orthrop C u s p gun to have desired m th -order sym metry 3 4 G H z sl otte d c a vity a n d c o u pl er w a Cusp gun produces needed designed with HF S S for high efficienc axis-encircling electron beam 3 of 22 1c

4 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: ω = sω c + k z v z s Cyclotron Harmonic s = m for Gyrotron s = m-1 for Peniotron (Synchronism) (Asynchronism) 4 of 22 2a

5 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 5 of 22 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, 2π Mode Significant Achievement: Electronic Efficiency of 75% 35% Device Efficiency due to Critically Coupled Cavity 6 of 22 2c

7 Peniotron Interaction Peniotron Resonance with TE m1 Wave: ω = (m-1)ω 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 7 of 22 3a

8 UCD Peniotron Features Second-Harmonic Operation - 34 GHz Operation in π/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) Slotted Circuit TE 11 -Like Mode with TE 31 Content E-Field 8 of 22 3b

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

10 Magnetic Tuning Curve Gyrotron Starting Current is Above Peniotron s Nearest Competing Mode: 4th-Harmonic Gyrotron 6 I s (A) Gyrotron Operating Current Peniotron Start oscillation current for competing 4th-Harmonic Gyrotron is four times higher than Peniotron start current B 0 (kg) Excellent Stability Predicted 10 of 22 4b

11 Power and Efficiency Peniotron has been Simulated with Nonlinear Code Efficiency Predictions: Electron Efficiency 58% Device Efficiency 47% Device Efficiency with Depressed Collector 57% Collector Potential 12.8 kv Efficiency (%) η dep η dev η elec P out I b (A) P out (kw) 50% Efficiency Predicted 11 of 22 4c

12 Design Parameters Beam Voltage 70 kv Beam Current 3.5 A Velocity Ratio, v /v z 1.5 Magnetic Field 6.5 kg Velocity Spread, v z /v z 5% Guiding Center Spread, r c /r L 10% Mode π/2 Axial Mode Number 1 Vane Depth, b/a 1.45 Electron-Vane Ratio, r L /a 0.65 Inner Vane Radius, a 1.82 mm Cavity Length 31 mm Slot Angle, θ o 22.5 Unloaded Q, Q Loaded Q, Q L θ ο a r L b 12 of 22 5a

13 Cavity Design Diagnostic Coupling Ports Output Circular Waveguide r=4.5 mm Slotted Cavity Q 0 =1900 Circular Iris (Removable) Cutoff Drift Tube Iris Radius for Critical Coupling: 2.35mm, Q L = of 22 Iris Radius for Over Coupling: 2.55mm, Q L =357 5b

14 Cavity Design Diffraction Coupling 1st Axial Mode, 34 GHz 0 Couples to TE 11 Circular Waveguide Mode -1 S 11 (db) rd Axial Mode, Q L =59 2 nd Axial Mode, Q L =102 Operating Mode, Q L = of 22 Diffraction Coupling Frequency (GHz) Overcoupled for High Device Efficiency - 47% Predicted Efficiency Increased by Depressed Collector - 57% Expected Suppresses Higher Order Axial Modes 5c

15 Diagnostic Sidewall Couplers Couples to TE 10 Mode E E -30 Diagnostic Coupling Coupling to Operating Mode -40 Coupling to 2 nd Axial Mode E S 21 (db) of 22 Diagnostic Coupling Frequency (GHz) Couples to standard WR-28 rectangular waveguide Coupling to adjacent slots will load both components of circularly polarized wave 6a

16 Output Mode Conversion Mode Conversion Occurs Only at Higher Frequencies -TM 11 Mode is Excited TM 11 Output S TE 11 Output of 22 Conversion to TM 11 Mode Only Above 40.0 GHz Frequency (GHz) 6b

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

18 Cusp Gun Axis Encircling Beam Parameters: Beam Voltage 70 kv Beam Current 3.5 A Velocity Ratio, v /v z 1.5 Velocity Spread, v z /v z 5% Guiding Center Spread, r c /r L 10% 18 of 22 7b

19 Superconducting Magnet Four Independently Controlled Coils Cusp Gradient from Internal Gun Coil and Two Supplemental Gun Coils Total Gun Coil Components z (cm) 19 of 22 7c

20 Summary Peniotron Demonstrated Very High Efficiency (Tohoku) UCD Peniotron Designed For High Device Efficiency π/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 50% Device Efficiency Predicted 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