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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
US Navy 94 GHz High Power
WORLOC Radar
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
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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: Pout=110 kW,
Gain=45 dB, h=22%, BW3dB=5%)
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, a=1.0
w/2p (GHz)
200
• Beam mode dispersion: w = sWc + kzvz
Wave mode dispersion: w2 = wc2 +c2kz2
TE02(2)
• Absolute instabilities must be stabilized
: TE11(1), TE21(1), TE02(2) ,TE01(1)
150
s=2
operating point
(grazing intersection)
TE01(1)
100
TE21(1)
50
s=1
Potential Gyro-BWO
interaction
-4000
TE11(1)
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.
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
Current
a= v^/vz
Dvz/ 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 rCu
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 a =1.0 and Bo/Bg= 0.995
• Beam current can be higher for lower a (=v^/vz) and lower Bo/Bg
Stability from TE01 Cutoff Oscillation
Keep I < Is
a = 0.9
Is(A)
1.0
1.1
Design value
1.2
1.3
Bo/Bg
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
Axial view of TE01 Gyro-TWT circuit
output
loss
12cm
1.6cm
• 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
Insertion Loss (dB / 12 cm)
input
Measurement
0
HFSS-Resistive Guide
r/r =70,000)
Cu
-50
-100
HFSS-Copper Guide with Inner
Semiconductor Tube
( Dr=0.05 mm, r/r =70,000)
-150
Cu
rw=2.01 mm
-200
90
92
94
96
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Frequency (GHz)
100
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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
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
Velocity spread
2%
Ring
Cathode radius
5.1 mm
Guiding center radius 0.9 mm
Activated MIG
Glowing Cathode Emission Ring
EGUN simulation of electron trajectory
and magnetic field profile
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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 )
Interaction Circuit
(TE01 )
HFSS cross sectional view of
electromagnetic field intensity
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RF Coupler Characterization
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• RF couplers are characterized using both scalar and vector network analyzers
Input coupler
Output coupler
Scalar measurement
Scalar measurement
Vector measurement
Vector measurement
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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
Input
Coupler
Beam Tunnel
30cm ruler
Output
Coupler
Interaction Region
(13.6cm)
Coated with
Aquadag
Load Collector
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
Fabry-Perot
interferometer
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Cross guide
coupler
Variable
Circulator
attenuator
Gyro-TWT OUT
IN
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Ka-Band overmoded waveguide
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3
Frequency
meter
High
power
load
Crystal
detector
scope
Input
driver
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Magnet System
• Refrigerated Superconducting Magnet
Magnetic Field (kG)
• Magnetic field profile for 4 coils
Superconducting
magnet
Coil power supply
- 50 kG ± 0.1% over 50 cm
- 4 compensated independent coils
- 6” large bore
Axial position (cm)
Magnetic Field (kG)
Integrated Gyro-TWT System
Axial Position of Superconducing Magnet (cm)
Gun Vacuum Pump
Superconducting
Magnet
Beam Tunnel
MIG RF Input
Collector
RF Output
Main
Vacuum Pump
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Experimental Progress Flowchart
1st
version Gyro-TWT
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- Employed MIG Dvz/vz=5% (predicted)
- Small signal gain=34dB, BW=2%
- Performance hampered by misaligned MIG
(Dvz/vz=10% inferenced by nonlinear code)
2nd version Gyro-TWT
- Employed realigned MIG Dvz/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)
3rd 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
4th
- Employed well matched output section and well
aligned MIG
- Currently under hot test
version Gyro-TWT
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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
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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
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Mode Competition Identification
2nd version Gyro-TWT
3rd version Gyro-TWT
Shorten circuit length
TE02 mode oscillation
(170 GHz)
TE01 mode drift tube
oscillation (85 GHz)
TE01 mode cutoff
oscillation (91 GHz)
Eliminated
Reduced drift tube
radius
Eliminated
Shorten circuit length
Higher start
oscillation current
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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
2nd version
3rd version
85 kV
85 kV
60 kV
60 kV
Drift Tube Oscillation
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- 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
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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• Vb=70 kV, Ib=5.3 A, Bo=34.3 kG
• Ib=5.4 A, Bo=34.3 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 > 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
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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)
• Different return loss assumed in simulation
• Effect of return loss on bandwidth and
comparison with measurement
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)
Summary
• UCD 94 GHz TE01 Gyro-TWT has been constructed with predicted capability
of 110 kW with Dw/w=5% and h=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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