Development of a W-Band TE01 Gyrotron

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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
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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

US Navy 94 GHz High Power WORLOC Radar
Why Gyro-TWT (Gyrotron Traveling Wave Tube) ?

• Gyro-TWT has a higher power capability ( gt 100
  kW)
• than conventional linear TWT
• Gyro-TWT has wider bandwidth than other
  Gyro-devices
• (Gyroklystron, Gyrotwystron)

Univ. of Miami 94GHz Cloud Radar
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UCD W-band TE01 Gyro-TWT Amplifier
Objectives
Overall system setup for hot test of the W-band
TE01 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 Pout110 kW,
• Gain45 dB, h22, BW3dB5)

Accomplishments
Approach

• Gyro-TWTs 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)

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Dispersion Diagram of TE01 Gyro-TWT
100 kV, a1.0

• Beam mode dispersion w sWc kzvz
• Wave mode dispersion w2 wc2 c2kz2
• Absolute instabilities must be stabilized
• TE11(1), TE21(1), TE02(2) ,TE01(1)

operating point (grazing intersection)
s2
s 2
w sWc kzvz
s1
s 1
w sWc kzvz
Potential Gyro-BWO interaction
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Design Approach
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• Iterate the loop to optimize the gain, power,
  efficiency, and bandwidth

Choose Device parameters
Beam voltage, velocity ratio, Mode, guiding
center radius etc.
Check Large Signal Characteristics
Determine stable beam current
Simulation using Absolute Instability
code 1
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)
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Device Parameters

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

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Stable Beam Current

• Gyro-TWT exhibits absolute instability near
  cutoff at sufficiently high beam
• current
• Beam current can be higher for lower a (v/vz)
  and lower Bo/Bg

• Unloaded TE01 circuit is stable for beam current
  5 A for design
• value a 1.0 and Bo/Bg 0.995

Stability from TE01 Cutoff Oscillation
Keep I lt Is
Simulation results using Absolute Instability
code
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Predicted Gyro-TWT Performance

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

• For predicted velocity spread Dvz/vz 5
• -Bandwidth Dw/w 5
• - Pout 110 kW
• h 22

- Large signal gain 45 dB
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Application of Loss

• Loss has been added to circuit to suppress
  Gyro-BWO
• Theory ? r/rCu 70,000 is needed
• Aquadag (a Carbon colloid) has the desired
  loss of r/rCu ? 70,000

Measurement versus HFSS simulation

• 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

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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

• Designed MIG beam parameters
• Beam voltage 100 kV
• Beam current 5 A
• Velocity ratio (v/ vz) 1.0
• Velocity spread 2
• Cathode radius 5.1 mm
• Guiding center radius 0.9 mm

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RF Couplers

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

Cross section of the Fabricated Coax
Coupler
HFSS cross sectional view of electromagnetic
field intensity
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RF Coupler Characterization

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

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Interaction Circuit

• Interaction region is heavily loaded with
  Aquadag, a carbon colloid
• with r/rcu 70,000
• Final 1.6 cm of interaction region is unloaded
  to avoid damping of high
• power wave

Axial View of Fabricated TE01 interaction circuit
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RF Input Driver

• W-Band input driver is capable of driving either
  Hughes Folded Waveguide
• TWT (94 GHz, 100W, BW5) or CPI EIO (93 GHz, 1
  kW, BW5)

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

Fabry-Perot interferometer
Directional coupler
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Cross guide coupler
Gyro-TWT
Input driver
Variable attenuator
Circulator
OUT
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IN
Ka-Band overmoded waveguide
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Frequency meter
High power load
Crystal detector
scope
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Magnet System

• Refrigerated Superconducting Magnet

• Magnetic field profile for 4 coils

- 50 kG 0.1 over 50 cm - 4 compensated
independent coils - 6 large bore
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Integrated Gyro-TWT System
Gun Vacuum Pump
Beam Tunnel
Superconducting Magnet
Collector
MIG
RF Input
Main Vacuum Pump
RF Output
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Experimental Progress Flowchart

• Employed MIG Dvz/vz5 (predicted)
• Small signal gain34dB, BW2
• Performance hampered by misaligned MIG
• (Dvz/vz10 inferenced by nonlinear code)

1st version Gyro-TWT

• Employed realigned MIG Dvz/vz2 (predicted)
• 59kW output power, 42 dB gain, 26.6 efficiency,
  
• and BW1.3 GHz
• Performance limited by spurious oscillations
• (TE02 and TE01 mode oscillations)

2nd version Gyro-TWT

• Employed shortened interaction circuit
• 61kW output power, 40 dB gain, 17.9 efficiency,
• and BW1.5 GHz
• Performance limited by reflections at the
• output end and gun misalignment

3rd version Gyro-TWT

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

4th version Gyro-TWT
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Measured Transfer Characteristics
- Gyro-TWT shows good linearity at lower voltages
(lt 70 kV)

• Vb56 kV, Ib3.7 A and Bo34.1 kG

2nd version Gyro-TWT
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Measured Bandwidth
- 1.2 GHz 3 dB bandwidth has been measured

• Vb60 kV, Ib3.7 A and Bo34.0 kG

2nd version Gyro-TWT
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Frequency Identification using Fabry-Perot
Interferometer

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

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Mode Competition Identification
2nd version Gyro-TWT
3rd version Gyro-TWT
Shorten circuit length
TE02 mode oscillation (170 GHz)
Eliminated
Reduced drift tube radius
TE01 mode drift tube oscillation (85 GHz)
Eliminated
Shorten circuit length
Higher start oscillation current
TE01 mode cutoff oscillation (91 GHz)
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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

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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

2nd version Gyro-TWT
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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-

• Ib5.4 A, Bo34.3 kG

• Vb70 kV, Ib5.3 A, Bo34.3 kG

2nd version Gyro-TWT
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Measured Absolute Instability

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

• Vb80 kV, Ib5.1 A, Bo34.8 kG

• Vb72 kV, Ib5.3 A, Bo34.1 kG

2nd version Gyro-TWT
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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
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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 Dw/w5 and h22.
• 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).