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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

Motivation

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

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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

Objectives

· 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

Approach

· Gyro-TWT's offer wide bandwidth

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

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)

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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)

200

TE02(2)

/2 (GHz)

150

s=

=

s=2 2

s

operating point (grazing intersection)

100

vz + kz c

TE01(1) TE21(1)

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s=11 s=

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

-4000

0 kz(/m)

4000

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.

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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

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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

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Is(A)

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

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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)

0

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Measurement versus HFSS simulation

HFSS-Copper Guide

Measurement

12cm

-50

HFSS-Resistive Guide / =70,000)

Cu

· 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

-100

-150

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

Cu

-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

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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

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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

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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

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· 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

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30cm ruler

Coated with Aquadag

Uncoated

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

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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

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Gyro-TWT OUT

IN

Input driver

Crystal detector scope

High power load

Magnet System

· Refrigerated Superconducting Magnet

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· 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)

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

Superconducting Magnet

MIG RF Input

Collector

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)

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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

4th

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

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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

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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

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crystal detector metal mesh micrometer

Mode Competition Identification

2nd version Gyro-TWT

Shorten circuit length TE02 mode oscillation (170 GHz) Eliminated

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3rd version Gyro-TWT

TE01 mode drift tube oscillation (85 GHz)

Reduced drift tube radius

Eliminated

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

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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

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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

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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

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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

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· 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)

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Summary

· · · · · 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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Information

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