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Development of a W-Band TE01 Gyrotron Traveling-Wave Amplifier (Gyro-TWT) for Advanced Radar Applications

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.

Department of Applied Science, Univ. of California, Davis *Department of Physics, National Tsing-Hua Univ., Taiwan


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


US Navy 94 GHz High Power WORLOC Radar

Why Gyro-TWT (Gyrotron Traveling Wave Tube) ?? Why Gyro-TWT (Gyrotron Traveling Wave Tube) · 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

3 UCD W-band TE01 Gyro-TWT Amplifier


· Extend the state-of-the-art wide bandwidth,

high power millimeter wave amplifier technology by developing a stable W-band gyro-TWT (Goal performance: Pout=110 kW, Gain=45 dB, =22%, BW3dB=5%) Overall system setup for hot test of the W-band TE01 gyro-TWT


· Gyro-TWT's offer wide bandwidth

· TE01 mode transmits high power · Distributed wall loss configuration stabilizes amplifier


· 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)


Dispersion Diagram of TE01 Gyro-TWT

100 kV, =1.0 · Beam mode dispersion: = sc + kzvz Wave mode dispersion: 2 = c2 +c2kz2 · Absolute instabilities must be stabilized : TE11(1), TE21(1), TE02(2) ,TE01(1)



/2 (GHz)




s=2 2


operating point (grazing intersection)


vz + kz c

TE01(1) TE21(1)


s=11 s=

vz TE11(1) + kz Potential Gyro-BWO c =s interaction


0 kz(/m)


Design Approach

· Iterate the loop to optimize the gain, power, efficiency, and bandwidth Choose Device parameters

Beam voltage, velocity ratio, Mode, guiding center radius etc.


Determine stable beam current

Simulation using `Absolute Instability' code [1]

Check Large Signal Characteristics

Simulation using nonlinear code [3]

Determine Circuit Length and Loss Value

Simulation using `Gyro-BWO' code [2]

[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.90-104, 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.155-162, 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.391-402, 1999)

Device Parameters

Voltage Current = v/vz vz/ vz Magnetic Field(Bo) Bo/Bg Cutoff Frequency Wall Resistivity Circuit Radius, rw Guiding Center Radius, rc Circuit Length 100 kV 5A 1.0 5% 35.6 kG 0.995 90.97 GHz 70,000 Cu 0.201 cm 0.45 rw 13.6 cm


Stable Beam Current

· Gyro-TWT exhibits absolute instability near cutoff at sufficiently high beam current · Unloaded TE01 circuit is stable for beam current = 5 A for design value =1.0 and Bo/Bg= 0.995 · Beam current can be higher for lower (=v/vz) and lower Bo/Bg Stability from TE01 Cutoff Oscillation Keep I < Is

= 0.9 1.0



1.1 1.2 1.3

Design value

Bo/Bg Simulation results using `Absolute Instability' code

Predicted Gyro-TWT Performance

· Nonlinear large signal code predicts output power, efficiency and gain


For predicted velocity spread vz/vz = 5% -Bandwidth / = 5% - Pout= 110 kW - = 22%

- Large signal gain = 45 dB

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 Axial view of TE01 Gyro-TWT circuit input loss 1.6cm output Insertion Loss (dB / 12 cm)



Measurement versus HFSS simulation

HFSS-Copper Guide




HFSS-Resistive Guide / =70,000)


· Initial 12 cm is coated. Final 1.6 cm is uncoated to prevent wave damping · 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



HFSS-Copper Guide with Inner Semiconductor Tube ( r=0.05 mm, / =70,000)


-200 90 92 94 96

rw=2.01 mm

98 100

Frequency (GHz)

Experimental Design and Setup

· · · · · · · · Single Anode MIG High Voltage Modulator RF Couplers Interaction Circuit Vacuum System Superconducting Magnet System RF Drive Sources RF Diagnostics


Single Anode MIG

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


Activated MIG

Glowing Cathode Emission Ring

RF Couplers

· 0 dB input coupler and 10 dB output coupler are employed

TE10 TE51 TE01 Coax Coupler Designed with HFSS All Modes are Matched


Cross section of the Fabricated Coax Coupler

Rectangular Input waveguide (TE10 ) Coaxial Cavity (TE51 )

HFSS cross sectional view of electromagnetic field intensity

Interaction Circuit (TE01 )

RF Coupler Characterization

Input coupler Output coupler


· RF couplers are characterized using both scalar and vector network analyzers

Scalar measurement Scalar measurement Vector measurement Vector measurement

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

Input Coupler Beam Tunnel Interaction Region (13.6cm) Output Coupler Load Collector


30cm ruler

Coated with Aquadag


Axial View of Fabricated TE01 interaction circuit

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%)

SLAC-UC Davis W-Band Modulator Hughes 94 GHz, 100 W Folded Waveguide TWT


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

Directional coupler 3 2 Cross guide coupler Variable Circulator attenuator 2 Ka-Band overmoded waveguide Frequency meter 3 1 Fabry-Perot interferometer




Input driver

Crystal detector scope

High power load

Magnet System

· Refrigerated Superconducting Magnet


· Magnetic field profile for 4 coils

Magnetic Field (kG)

Superconducting magnet

Coil power supply

Axial position (cm)

- 50 kG ± 0.1% over 50 cm - 4 compensated independent coils - 6" large bore

Integrated Gyro-TWT System

Magnetic Field (kG)


Axial Position of Superconducing Magnet (cm) Gun Vacuum Pump Beam Tunnel

Superconducting Magnet

MIG RF Input


RF Output

Main Vacuum Pump

Experimental Progress Flowchart

1st version Gyro-TWT - Employed MIG vz/vz=5% (predicted) - Small signal gain=34dB, BW=2% - Performance hampered by misaligned MIG (vz/vz=10% inferenced by nonlinear code)


2nd version Gyro-TWT

- Employed realigned MIG vz/vz=2% (predicted) - 59kW output power, 42 dB gain, 26.6% efficiency, and BW=1.3 GHz - Performance limited by spurious oscillations (TE02 and TE01 mode oscillations) - 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

3rd version Gyro-TWT


version Gyro-TWT

- Employed well matched output section and well aligned MIG - Currently under hot test

Measured Transfer Characteristics

- Gyro-TWT shows good linearity at lower voltages (< 70 kV)

· Vb=56 kV, Ib=3.7 A and Bo=34.1 kG


2nd version Gyro-TWT

Measured Bandwidth

- 1.2 GHz 3 dB bandwidth has been measured

· Vb=60 kV, Ib=3.7 A and Bo=34.0 kG


2nd version Gyro-TWT

Frequency Identification using Fabry-Perot Interferometer

· Fabry-Perot interferometer using two horn antennas, metal mesh, and translational stage employed to identify competing modes horn antenna


crystal detector metal mesh micrometer

Mode Competition Identification

2nd version Gyro-TWT

Shorten circuit length TE02 mode oscillation (170 GHz) Eliminated


3rd version Gyro-TWT

TE01 mode drift tube oscillation (85 GHz)

Reduced drift tube radius


TE01 mode cutoff oscillation (91 GHz)

Shorten circuit length

Higher start oscillation current

Measured Start Oscillation Current

· Start oscillation current for TE01 cutoff oscillation were measured · Oscillation threshold decreases for increasing magnetic field · By shortening circuit length, start oscillation current has been increased

2nd version 3rd version


85 kV 85 kV 60 kV 60 kV

Drift Tube Oscillation

- In 2nd version, oscillation has been measured at 85 GHz at the drift tube using Fabry-Perot interferometer - TE01 mode at the drift tube has been identified to be the source of oscillation drift tube radius reduced in 3rd version and oscillation eliminated

· Cyclotron and cutoff frequency vs. axial position of beam tunnel region


cyclotron TE01 cutoff frequency (61 kV) cyclotron TE21 cutoff Frequency (100 kV) TM01 cutoff TE11 cutoff

2nd version Gyro-TWT

Mode Competition

- 2nd version Gyro-TWT performance limited to lower voltage due to mode competition - Competing mode are identified to be TE02 mode measured at 170 GHz using Fabry-Perot interferometer· Vb=70 kV, Ib=5.3 A, Bo=34.3 kG · Ib=5.4 A, Bo=34.3 kG


2nd version Gyro-TWT

Measured Absolute Instability

- In 2nd version, oscillations near cutoff frequency (~91 GHz) have been observed at higher voltages than > 70 kV - The cutoff oscillation degrades the amplified signal

·Vb=80 kV, Ib=5.1 A, Bo=34.8 kG · Vb=72 kV, Ib=5.3 A, Bo=34.1 kG


2nd version Gyro-TWT

Measured Bandwidth

- 3rd version gyro-TWT performance limited due to the excessive return loss at the output end (verified by simulation)

· Effect of return loss on bandwidth and comparison with measurement


· Different return loss assumed in simulation

3rd version Gyro-TWT

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 (4th version gyro-TWT)



· · · · · UCD 94 GHz TE01 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. 1st and 2nd version gyro-TWT performance limited by velocity spread and competing modes. Recent 3rd 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 4th version of gyro-TWT (currently under hot test).



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