TE 21 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 F (MURI MVE). Distribution Statement A: Approved for Public Release; Distribution is Unlimited 1 of 17

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 TE 21 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 Overview 2 of 17

Measured Bandwidth of 200 kW MIG TE 21 (2) Gyro-TWT Previous UCD 200 kW TE 21 (2) Gyro-TWT With MIG MIG TE 21 (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 TE 21 (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 3 of 17 MIG Cusp

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

Circular Waveguide with Two Orthogonal Slices through the Axis Suppress Odd-Order Azimuthal Modes by Interrupting their Wall Currents Mode-Selective Interaction Circuit Slices Circular Waveguide Lossy MgO-SiC Vacuum Jacket Previous Experiment Measured Insertion Loss TE 21 Operating Mode < 1.5 dB TE 11 Unwanted Mode > 24 dB 5 of 17

Oscillation can Occur at Cutoff (Absolute Instability) Second Cyclotron Harmonic Line Grazes the TE 21 Mode No Interaction at First Cyclotron Harmonic Circuit Dispersion Diagram TE 41 (4) is Strongest Gyro-BWO Threat Odd-Order Azimuthal Modes Suppressed by Sliced Circuit Competing Modes must be Suppressed 6 of 17  r w / c s=1 s=3 s=2 s=4 TE 11 TE 01 TE 21 TE 41 TE 31  = s  c + k z v z kzrwkzrw For Axis-Encircling Beam only s=m Modes are Excited

TE 41 (4) is Strongest Competing Mode in Mode Selective Circuit Loss Added to Circuit Wall to Suppress TE 41 (4) Gyro-BWO Figure shows Dependence on Wall Resistivity of Critical Length for TE 41 (4) Gyro-BWO Suppress Gyro-BWO Instability with Distributed Loss Gyro-BWO Oscillation 7 of 17

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

Saturated Drive Bandwidth  v z /v z =0%, 7%, 10%) Predicted Bandwidth is Greater than 3% Constant Drive Bandwidth Device Parameters Voltage70 KV Current3.5 A  = v  /v || 1.2  v z / v z 7% Magnetic Field, B o 5.48 kG B/B g 0.99 Cutoff Frequency28.6 GHz Guiding Center Radius0.0 r w Circuit Radius, r w cm Lossy Circuit Length30.5 cm Wall Resistivity2300 x copper Total Circuit Length42.0 cm 9 of 17

Magnetic Circuit 10 of 17 - 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 NG Cusp Gun Magnetics Vacuum Jacket of Gyro-TWT Pole Piece Water Cooled Copper Coil Cathode Interaction Circuit Region

Magnetic Field Profile 11 of 17 -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. Magnetic Field Profile Designed and Simulated in Maxwell 2D ®

UCD TE 21 (2) Gyro-TWT Cusp Gun developed by Northrop Grumman Northrop Grumman Cusp Electron Gun NG Cusp Gun Produces Axis-Encircling Electron Beam Axial Electron Beam Starts Spinning Due to v z x B r Force in Magnetic Reversal Region 12 of 17 EGUN Simulation of Cusp Gun with new Magnetics

Multi-Hole Directional Coupler was Designed with HFSS All Modes are Matched due to Upstream Termination Excellent Selectivity to TE 21 Mode -1 dB Coupling in Previous Gyro-TWT Experiment HFSS Intensity Plot of the Input and Generated Waves Schematic Excited TE 21 Wave Interaction Waveguide Input Waveguide Input TE 10 Mode Excited TE 21 Mode 0 dB Input Coupler for TE 21 (2) Gyro-TWT 13 of 17

< 0.5 dB Coupling Loss Simulated over the 28.5 – 31.5 GHz Bandwidth of the Gyro-TWT Amplifier HFSS Simulation of 0 dB Input Coupler 14 of 17 > 45 dB Round Trip Return Loss for the Operating TE 21 Circular Mode from 28.5 – 31.5 GHz

0 dB Input Coupler for TE 21 (2) Gyro-TWT 15 of 17 Three dimensional view of coupler being fabricated for the gyro-TWT

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

Frequency 30 GHz Output Power 50 kW Efficiency 20 % Bandwidth 3 % Gain 30 dB -A novel TE 21 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 Summary 17 of 17