TE 21 Second-Harmonic Gyro-TWT Amplifier with an Axis-Encircling Beam S.B. Harriet* , D.B. McDermott, and N.C. Luhman

1. TE21 Second-Harmonic Gyro-TWT Amplifier with an Axis-Encircling Beam S.B. Harriet*, D.B. McDermott, and N.C. Luhmann, Jr. Department of Applied Science, University of California, Davis *Also with NSWC Crane 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 1 of 17

2. Overview • Applications include mm-Wave Radar and Broadband Communications • Harmonic Operation Provides Higher Power and Greater Stability • Significant Reduction of Magnetic Field from Harmonic Operation • Axis-Encircling Electrons Provide Strongest Harmonic Interaction • Second-Harmonic TE21 Gyro-TWT Designed for • 50 kW at 30 GHz with 20% Efficiency • 30 dB Saturated Gain • 3% Saturated Bandwidth • Axis-Encircling Electron Beam Produced by Northrop Grumman Cusp Gun 2 of 17

3. Previous UCD 200 kW TE21(2) Gyro-TWT With MIG MIG TE21(2) Gyro-TWT Produced 200 kW of output power Efficiency reduced because electrons were constrained to a fixed azimuth so some electrons experienced a weak RF field while others oversaturated in a strong RF field. Cusp-Gun TE21(2) Gyro-TWT Axis-encircling beam will yield full (20%) efficiency because electrons rotate through the peaks and nulls. Efficiency will nearly double and gain will increase Cusp MIG 3 of 17 Measured Bandwidth of 200 kW MIG TE21(2) Gyro-TWT

4. Higher Power at Higher Harmonics Threshold Beam Current for Absolute Instability 70 kV, rgc= 0 Oscillation at Cutoff of Operating Mode Limits Gyro-TWTs Start-Oscillation Current is Higher for Harmonic Operation Higher Current Yields Higher Power Is = 5 A Gives 30% Safety Margin for Planned 3.5 A Beam Is (A) v / v|| 4 of 17

5. Mode-Selective Interaction Circuit Previous Experiment Measured Insertion Loss TE21 Operating Mode < 1.5 dB TE11 Unwanted Mode > 24 dB Slices Suppress Odd-Order Azimuthal Modes by Interrupting their Wall Currents Circular Waveguide with Two Orthogonal Slices through the Axis Lossy MgO-SiC Vacuum Jacket Circular Waveguide 5 of 17

6. Circuit Dispersion Diagram Competing Modes must be Suppressed Second Cyclotron Harmonic Line Grazes the TE21 Mode s=4 TE01 TE31 TE41 Odd-Order Azimuthal Modes Suppressed by Sliced Circuit s=3 s=2 TE41(4) is Strongest Gyro-BWO Threat rw / c TE21 TE11 s=1 Oscillation can Occur at Cutoff (Absolute Instability)  = sc + kzvz No Interaction at First Cyclotron Harmonic kzrw For Axis-Encircling Beam only s=m Modes are Excited 6 of 17

7. Gyro-BWO Oscillation Suppress Gyro-BWO Instability with Distributed Loss • TE41(4) is Strongest Competing Mode in Mode Selective Circuit • Loss Added to Circuit Wall to Suppress TE41(4) Gyro-BWO • Figure shows Dependence on Wall Resistivity of Critical Length for TE41(4) Gyro-BWO 7 of 17

8. Spatial Power Growth Loss added to first 30.5 cm of circuit to suppress Gyro-BWO Last 11.5 cm of circuit not loaded to avoid attenuating the high power wave 8 of 17

9. Constant Drive Bandwidth Predicted Bandwidth is Greater than 3% Device Parameters Voltage 70 KV Current 3.5 A  = v /v|| 1.2 vz / vz 7% Magnetic Field, Bo 5.48 kG B/Bg 0.99 Cutoff Frequency 28.6 GHz Guiding Center Radius 0.0 rw Circuit Radius, rw 0.509 cm Lossy Circuit Length 30.5 cm Wall Resistivity 2300 x copper Total Circuit Length 42.0 cm Saturated Drive Bandwidth (Dvz/vz=0%, 7%, 10%) 9 of 17

10. Magnetic Circuit Water Cooled Copper Coil Interaction Circuit Region Pole Piece NG Cusp Gun Magnetics Cathode Vacuum Jacket of Gyro-TWT • - Magnetic Circuit Configuration: • Incorporates gun coil and pole pieces provided by Northrop Grumman • Water cooled copper magnet coils and steel pole pieces for interaction region • Good match to Northrop Grumman recommended magnetic fields in field reversal region 10 of 17

11. Magnetic Field Profile Magnetic Field Profile Designed and Simulated in Maxwell 2D® • Provides field reversal and fast rise required by Cusp gun • Provides flat region for interaction circuit • Designed for stability from gyrotron oscillations in each section of the device. 11 of 17

12. Northrop Grumman Cusp Electron Gun UCD TE21(2) Gyro-TWT Cusp Gun developed by Northrop Grumman • NG Cusp Gun Produces Axis-Encircling Electron Beam • Axial Electron Beam Starts Spinning Due to vz x Br Force in Magnetic Reversal Region EGUN Simulation of Cusp Gun with new Magnetics 12 of 17

13. 0 dB Input Coupler for TE21(2) Gyro-TWT • Multi-Hole Directional Coupler was Designed with HFSS • All Modes are Matched due to Upstream Termination • Excellent Selectivity to TE21 Mode • -1 dB Coupling in Previous Gyro-TWT Experiment Schematic Excited TE21 Wave Input TE10 Mode Input Waveguide Interaction Waveguide Excited TE21 Mode HFSS Intensity Plot of the Input and Generated Waves 13 of 17

14. HFSS Simulation of 0 dB Input Coupler < 0.5 dB Coupling Loss Simulated over the 28.5 – 31.5 GHz Bandwidth of the Gyro-TWT Amplifier > 45 dB Round Trip Return Loss for the Operating TE21 Circular Mode from 28.5 – 31.5 GHz 14 of 17

15. 0 dB Input Coupler for TE21(2) Gyro-TWT Three dimensional view of coupler being fabricated for the gyro-TWT 15 of 17

16. RF Driver for Gyro-TWT CPI Ka-Band TWT Driver ETM Modulator Supply for Driver TWT, 16 of 17

17. Summary • A novel TE21 Second-Harmonic Gyro-TWT Amplifier with an axis-encircling beam from a Northrop Grumman Cusp Gun is being constructed at UC Davis to demonstrate the advantages of a large orbit axis-encircling beam in a harmonic Gyro-TWT. Cusp-Gun Second-Harmonic Gyro-TWT Amplifier Frequency 30 GHz Output Power 50 kW Efficiency 20 % Bandwidth 3 % Gain 30 dB 17 of 17