140kW, 94GHz Heavily Loaded TE01 Gyro-TWT D.B. McDermott, H.H. Song, Y. Hirata, A.T. Lin1, T.H. Chang2, K.R. Chu2 and N.C. Luhmann, Jr. Department of Applied Science, UC Davis 1 Department of Physics, UCLA 2 Department of Physics, NTHU This work has been supported by AFOSR under Grants F49620-99-1-0297 (MURI-MVE) and F49620-00-1-0339.

Outline Small-Signal Design for Stability Wall Loss Large-Signal Characteristics Circuit Components

Motivation Why Gyro-TWT? Why TE01 Mode? Wider Bandwidth than Gyro-Klystron Higher Circuit Efficiency  Higher Power Capability Why TE01 Mode? Low Loss Well Centered for MIG Electron Beam (Peaks for r/rw=0.5) Azimuthal Symmetry is Favorable for MIG Beam Field Pattern is Unique (Jz=0 and Er=0) - Useful for Mode Selective Circuit

Dispersion Diagram - TE01 Gyro-TWT 100 kV, v^/vz=1.0 Must Suppress TE11(1) , TE21(1) and TE02(2) Gyro-BWO Interactions

Stable Beam Current (Absolute Instability at Cutoff) Beam Current can be Higher for Lower v^/vz and Lower Bo/Bg 100 kV, v^/vz=1.0 Unloaded TE01(1) Circuit is Stable for 5 A, v^/vz=1.0, and Bo/Bg=1.0

Gyro-BWO Stability in Lossy TE01(1) Circuit Wall is Coated with Lossy Graphite to Suppress Gyro-BWO [ NTHU's Technique, PRL 81, 4760 (1998)] r/rcopper = 7.104 yields Stability and 100 dB Loss for 14.5 cm Circuit

CW Wall Loading < 50 W/cm2 Power Growth in Lossy Single-Stage Device Self-Consistent Large-Signal Simulation Code Large-Signal Gain = 50 dB Efficiency = 28% Peak Power = 140 kW 100 kV, 5 A, v^/vz =1 Dvz/vz = 5% Electron efficiency is nearly independent of loss Final 2.5 cm is unloaded to avoid damping high power wave CW Wall Loading < 50 W/cm2 92.25 GHz

Predicted Saturated Bandwidth 5% Bandwidth is Predicted Dw/w = 5% Pout = 140 kW h = 28% Gain = 50 dB rw = 2.01 mm rc/rw = 0.45 r/rcopper = 70,000 Llossy = 11.0 cm Lcopper = 2.5 cm Lloss-taper = 1.0 cm Lcircuit = 14.5 cm 7

Gyro-TWT Circuit has been Fabricated MIG Connection Input Coupler Interaction Region Output Coupler Collector 30 cm ruler Axial View

Gyro-TWT Circuit has been Fabricated Rectangular Input Waveguide Cross-Section of Coaxial Coupler Rectangular Input Waveguide Coaxial Cavity Interaction Circuit

0 dB TE01 Input Coupler HFSS Design Similar to UCLA’s TE81 Azimuthal Phase-Velocity Coupler HFSS Design Similar to UCLA’s TE81 Gyro-TWT Coupler NRL’s Gyroklystron Coax Coupler All Modes are Matched

TE51/TE01 Coax-Cavity Input Coupler TE10 Rectangular Waveguide into TE51 Coax-Cavity into TE01 Circular Waveguide

RF Measurement Set-up for Coupler and Circuit Loss MPI Flower-Petal TE10 / TE01 Transducers Give <1.3 VSWR over 5% Bandwidth DURIP W-Band Vector Network Analyzer at SLAC will Measure Optimized Components Set-up for Coupler Measurement W-Band Scalar Network Analyzer 12

Bandwidth of Coaxial Input Coupler Coupler exhibits > 2 dB coupling for 3% bandwidth Performance is limited by cutoff of short Cutoff of short Predicted for 93.0 - 96.5 GHz Coupling > 1 dB Selectivity > 40 dB Return Loss (TE01) > 7 dB Return Loss (TE21) > 14 dB Return Loss (TE11) > 28 dB Feature: No tapering is needed between coupler and gain region

Future Coaxial Input Coupler Although the initial Gyro-TWT experiment will employ the previous coaxial couplers, plans have been initiated to develop an improved coupler for future experiments. These three modifications of the original display a 7% bandwidth. 15

Measured Loss in Circuit Interaction Circuit has been Coated with Aquadag Aquadag is a Carbon Colloid with r/rCu=70,000 and dskin=0.06 mm Measurements versus HFSS Modeling 90 dB Loss Measured at 93 GHz

Single-Anode MIG (100 kV, 5 A, v^/vz = 1 Designed with FINELGUN Fabricated by NTHU Mo Coating - Edge Emission Cathode Angle 74o Magnetic Compression 32 Guiding Center Radius 0.9 mm Cathode Radius 5.1 mm Emitting Strip Length 1.9 mm Guiding Center Spread 10% Axial Velocity Spread 5% Electric Field 70 kV Cathode Loading 9 A/cm2 Jemis/JL 0.3

MIG Has Been Activated Cathode Stalk Very Steep Cathode (74) Emitting Ring

I-V Characteristic of MIG

Superconducting Magnet Profile 50 kG ± 0.1% over 50 cm Large 6" ID Bore Refrigerated Field Profile of the Four Independent Coils

Hughes 987 Coupled-Cavity TWT CPI 1kW EIO is Also Available 100W 94GHz TWT Input Driver Hughes 987 Coupled-Cavity TWT CPI 1kW EIO is Also Available

Summary UCD 94GHz Gyro-TWT has been Constructed - Capable of 140kW with Dw/w=5% and h=28% Circuit is Heavily Loaded to Suppress Gyro-BWO - Final 2.5 cm is Unloaded to Avoid Damping Saturated Wave - Loss has Negligible Effect on Efficiency - 90 dB Loss Measured at 93 GHz MIG was Designed with Dvz/vz = 5% and v^/vz = 1.0 - MIG has been Activated Coax Couplers were Designed with HFSS - Good Match for All Modes - Very Short Length (5 mm) - Input and Output Couplers have been Measured