Development of a W-Band TE 01 Gyrotron Traveling-Wave Amplifier (Gyro-TWT) for Advanced Radar Applications 1 Department of Applied Science, Univ. of California, Davis * Department of Physics, National Tsing-Hua Univ., Taiwan H. H. Song, D. B. McDermott, Y. Hirata, L. R. Barnett *, C. W. Domier, H. L. Hsu, T. H. Chang *, W.C. Tsai *, K. R. Chu *, and N. C. Luhmann, Jr.

Motivation US Navy 94 GHz High Power WORLOC Radar Increasing needs for broadband, high power millimeter wave sources for: High resolution imaging radar Radar tracking for space debris Atmospheric sensing (ozone mapping etc.) Communication systems Gyro-TWT has a higher power capability ( > 100 kW) than conventional linear TWT Gyro-TWT has wider bandwidth than other Gyro-devices (Gyroklystron, Gyrotwystron) Univ. of Miami 94GHz Cloud Radar Why Gyro-TWT (Gyrotron Traveling Wave Tube) ? 2

UCD W-band TE 01 Gyro-TWT Amplifier Overall system setup for hot test of the W-band TE 01 gyro-TWT Extend the state-of-the-art wide bandwidth, high power millimeter wave amplifier technology by developing a stable W-band gyro-TWT (Goal performance: P out =110 kW, Gain=45 dB,  22%, BW 3dB =5%) Gyro-TWT’s offer wide bandwidth TE 01 mode transmits high power Distributed wall loss configuration stabilizes amplifier Objectives Approach Accomplishments Recent gyro-TWT under hot test with 61.2 kW saturated output power, 40 dB gain, 17.9 % efficiency, 1.5 GHz (1.6%) bandwidth in zero drive stable condition (unoptimized) 3

Dispersion Diagram of TE 01 Gyro-TWT Beam mode dispersion:  s  c + k z v z Wave mode dispersion:    c  +c 2 k z 2 Absolute instabilities must be stabilized : TE 11 (1), TE 21 (1), TE 02 (2),TE 01 (1)  s  c + k z v z s = 1 s = 2 k z (/m)  GHz) TE 11 (1) TE 21 (1) TE 01 (1) TE 02 (2) operating point (grazing intersection) Potential Gyro-BWO interaction s=1 s= kV,  =1.0

Design Approach Beam voltage, velocity ratio, Mode, guiding center radius etc. Choose Device parameters Simulation using ‘Absolute Instability’ code [1] Determine stable beam current Simulation using ‘Gyro-BWO’ code [2] Determine Circuit Length and Loss Value Simulation using nonlinear code [3] Check Large Signal Characteristics Iterate the loop to optimize the gain, power, efficiency, and bandwidth [1] ‘Absolute Instability’ code is based on K.R.Chu et. al, “Gain and Bandwidth of the Gyro-TWT and CARM Amplifiers”, IEEE Trans. Plasma Sci., vol.16, pp , 1988) [2] ‘Gyro-BWO’ code is based on C.S.Kou et. al, “High Power Harmonic Gyro-TWT-Linear Theory and Oscillation Study”, IEEE Trans. Plasma Sci., vol.20, pp , 1992) [3] Nonlinear code is based on (K.R.Chu et. al, “Theory and Experiment of Ultrahigh-Gain Gyrotron Traveling Wave Amplifier”, IEEE Trans. Plasma Sci., vol.27, pp , 1999) 5

Device Parameters Voltage100 kV Current5 A  = v  /v z 1.0  v z / v z 5 % Magnetic Field(B o )35.6 kG B o /B g Cutoff Frequency90.97 GHz Wall Resistivity70,000  Cu Circuit Radius, r w cm Guiding Center Radius, r c 0.45 r w Circuit Length13.6 cm 6

Stable Beam Current Gyro-TWT exhibits absolute instability near cutoff at sufficiently high beam current Beam current can be higher for lower  v  /v z ) and lower B o /B g Unloaded TE 01 circuit is stable for beam current = 5 A for design value  1.0 and B o /B g =  = B o /B g I s (A) Design value Stability from TE 01 Cutoff Oscillation Keep I < I s Simulation results using ‘Absolute Instability’ code 7

Predicted Gyro-TWT Performance For predicted velocity spread  v z /v z = 5% -Bandwidth  /  = 5% - P out = 110 kW -  = 22% - Large signal gain = 45 dB Nonlinear large signal code predicts output power, efficiency and gain 8

Application of Loss Loss has been added to circuit to suppress Gyro-BWO Theory   Cu = 70,000 is needed ‘Aquadag’ (a Carbon colloid) has the desired loss of  Cu  70,000 input output loss 12cm 1.6cm Axial view of TE 01 Gyro-TWT circuit Initial 12 cm is coated. Final 1.6 cm is uncoated to prevent wave damping Measurement versus HFSS simulation 90 dB loss is measured at 93 GHz Loss lowers the gain but this can be compensated by increasing the circuit length to just below the critical length 9

Experimental Design and Setup Single Anode MIG High Voltage Modulator RF Couplers Interaction Circuit Vacuum System Superconducting Magnet System RF Drive Sources RF Diagnostics 10

Single Anode MIG Glowing Cathode Emission Ring Activated MIG Assembled MIG Cathode Stalk Cathode Emission Ring Designed MIG beam parameters Beam voltage 100 kV Beam current 5 A Velocity ratio (v  / v z ) 1.0 Velocity spread 2% Cathode radius 5.1 mm Guiding center radius 0.9 mm EGUN simulation of electron trajectory and magnetic field profile 11

RF Couplers Cross section of the Fabricated Coax Coupler TE 10  TE 51  TE 01 Coax Coupler Designed with HFSS All Modes are Matched 0 dB input coupler and 10 dB output coupler are employed Rectangular Input waveguide (TE 10 ) Coaxial Cavity (TE 51 ) Interaction Circuit (TE 01 ) HFSS cross sectional view of electromagnetic field intensity 12

RF Coupler Characterization RF couplers are characterized using both scalar and vector network analyzers Input coupler Scalar measurement Vector measurement Output coupler Scalar measurement Vector measurement 13

Interaction Circuit Interaction region is heavily loaded with ‘Aquadag’, a carbon colloid with  /  cu = 70,000 Final 1.6 cm of interaction region is unloaded to avoid damping of high power wave Axial View of Fabricated TE 01 interaction circuit Beam Tunnel Interaction Region (13.6cm) Output Coupler Input Coupler Load Collector Coated with Aquadag Uncoated 30cm ruler 14

RF Input Driver W-Band input driver is capable of driving either Hughes Folded Waveguide TWT (94 GHz, 100W, BW=5%) or CPI EIO (93 GHz, 1 kW, BW=5%) Hughes 94 GHz, 100 W Folded Waveguide TWT SLAC-UC Davis W-Band Modulator 15

RF Diagnostics RF diagnostics are setup to monitor the output power w/ and w/o input drive Various modes are measured simultaniously using waveguide switch, cavity filter, waveguide cutoff sections, and Fabry-Perot interferometer High power load Circulator Input driver Gyro-TWT IN Frequency meter Directional coupler OUT Variable attenuator scope Cross guide coupler Crystal detector Ka-Band overmoded waveguide 3 2 Fabry-Perot interferometer 16

Magnet System Refrigerated Superconducting Magnet Superconducting magnet Coil power supply Axial position (cm) Magnetic Field (kG) Magnetic field profile for 4 coils - 50 kG ± 0.1% over 50 cm - 4 compensated independent coils - 6” large bore 17

Integrated Gyro-TWT System Superconducting Magnet MIG Main Vacuum Pump RF Input RF Output Gun Vacuum Pump Collector Beam Tunnel Axial Position of Superconducing Magnet (cm) Magnetic Field (kG) 18

Experimental Progress Flowchart 1 st version Gyro-TWT - Employed MIG  v z /v z =5% (predicted) - Small signal gain=34dB, BW=2% - Performance hampered by misaligned MIG (  v z /v z =10% inferenced by nonlinear code) 2 nd version Gyro-TWT - Employed realigned MIG  v z /v z =2% (predicted) - 59kW output power, 42 dB gain, 26.6% efficiency, and BW=1.3 GHz - Performance limited by spurious oscillations (TE 02 and TE 01 mode oscillations) 3 rd version Gyro-TWT 4 th version Gyro-TWT - Employed shortened interaction circuit - 61kW output power, 40 dB gain, 17.9% efficiency, and BW=1.5 GHz - Performance limited by reflections at the output end and gun misalignment - Employed well matched output section and well aligned MIG - Currently under hot test 19

Measured Transfer Characteristics - Gyro-TWT shows good linearity at lower voltages (< 70 kV) V b =56 kV, I b =3.7 A and B o =34.1 kG 2 nd version Gyro-TWT 20

Measured Bandwidth GHz 3 dB bandwidth has been measured V b =60 kV, I b =3.7 A and B o =34.0 kG 2 nd version Gyro-TWT 21

Frequency Identification using Fabry-Perot Interferometer Fabry-Perot interferometer using two horn antennas, metal mesh, and translational stage employed to identify competing modes crystal detector horn antenna micrometer metal mesh 22

Mode Competition Identification 23 2 nd version Gyro-TWT3 rd version Gyro-TWT TE 02 mode oscillation (170 GHz) Eliminated Shorten circuit length TE 01 mode drift tube oscillation (85 GHz) Eliminated Reduced drift tube radius TE 01 mode cutoff oscillation (91 GHz) Higher start oscillation current Shorten circuit length

Measured Start Oscillation Current Start oscillation current for TE 01 cutoff oscillation were measured Oscillation threshold decreases for increasing magnetic field By shortening circuit length, start oscillation current has been increased 24 2 nd version 3 rd version 60 kV 85 kV 60 kV

Drift Tube Oscillation - In 2 nd version, oscillation has been measured at 85 GHz at the drift tube using Fabry-Perot interferometer - TE 01 mode at the drift tube has been identified to be the source of oscillation  drift tube radius reduced in 3 rd version and oscillation eliminated Cyclotron and cutoff frequency vs. axial position of beam tunnel region TE 11 cutoff TM 01 cutoff TE 21 cutoff TE 01 cutoff cyclotron Frequency (100 kV) cyclotron frequency (61 kV) 2 nd version Gyro-TWT 25

Mode Competition - 2 nd version Gyro-TWT performance limited to lower voltage due to mode competition - Competing mode are identified to be TE 02 mode measured at 170 GHz using Fabry-Perot interferometer- V b =70 kV, I b =5.3 A, Bo=34.3 kG I b =5.4 A, B o =34.3 kG 2 nd version Gyro-TWT 26

Measured Absolute Instability - In 2 nd version, oscillations near cutoff frequency (~91 GHz) have been observed at higher voltages than > 70 kV - The cutoff oscillation degrades the amplified signal V b =72 kV, I b =5.3 A, B o =34.1 kG V b =80 kV, I b =5.1 A, B o =34.8 kG 2 nd version Gyro-TWT 27

Measured Bandwidth - 3 rd version gyro-TWT performance limited due to the excessive return loss at the output end (verified by simulation) 3 rd version Gyro-TWT 28 Different return loss assumed in simulation Effect of return loss on bandwidth and comparison with measurement

Improved Output Reflection - Output section reflection has been improved using heavily loaded output load - 10-layer coated output load currently employed in the hot test (4 th version gyro-TWT) 29

Summary UCD 94 GHz TE 01 Gyro-TWT has been constructed with predicted capability of 110 kW with  =5% and  =22%. Circuit has been heavily loaded to suppress Gyro-BWO with 90 dB loss measured at 93 GHz. 1 st and 2 nd version gyro-TWT performance limited by velocity spread and competing modes. Recent 3 rd version gyro-TWT hot tested with 61.2 kW saturated output power, 40 dB gain, 17.9% efficiency, and 1.5 GHz bandwidth (1.6 % BW). To enhance the bandwidth and the output power, improved output section with reduced reflection and well aligned MIG are employed in the 4 th version of gyro-TWT (currently under hot test). 30