140kW, 94GHz Heavily Loaded TE01 Gyro-TWT

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140kW, 94GHz Heavily Loaded TE01 Gyro-TWT
D.B. McDermott, H.H. Song, Y. Hirata, A.T. Lin1,
T.H. Chang2, K.R. Chu2 and N.C. Luhmann, Jr.
Department of Applied Science, UC Davis 1
Department of Physics, UCLA 2 Department of
Physics, NTHU This work has been supported by
AFOSR under Grants F49620-99-1-0297 (MURI-MVE)
and F49620-00-1-0339.
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Outline

• Small-Signal Design for Stability
• Wall Loss
• Large-Signal Characteristics
• Circuit Components

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Motivation

• Why Gyro-TWT?
• Wider Bandwidth than Gyro-Klystron
• Higher Circuit Efficiency ? Higher Power
  Capability
• Why TE01 Mode?
• Low Loss
• Well Centered for MIG Electron Beam (Peaks for
  r/rw0.5)
• Azimuthal Symmetry is Favorable for MIG Beam
• Field Pattern is Unique (Jz0 and Er0)
• - Useful for Mode Selective Circuit

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Dispersion Diagram - TE01 Gyro-TWT
100 kV, v/vz1.0
Must Suppress TE11(1) , TE21(1) and TE02(2)
Gyro-BWO Interactions
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Stable Beam Current (Absolute Instability at
Cutoff)
Beam Current can be Higher for Lower v/vz and
Lower Bo/Bg
100 kV, v/vz1.0
Unloaded TE01(1) Circuit is Stable for 5 A,
v/vz1.0, and Bo/Bg1.0
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Gyro-BWO Stability in Lossy TE01(1) Circuit

• Wall is Coated with Lossy Graphite to Suppress
  Gyro-BWO
• NTHU's Technique,
• PRL 81, 4760 (1998)
• r/rcopper 7.104 yields Stability and 100 dB
  Loss for 14.5 cm Circuit

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Power Growth in Lossy Single-Stage
DeviceSelf-Consistent Large-Signal Simulation
Code
CW Wall Loading lt 50 W/cm2

• Large-Signal Gain 50 dB
• Efficiency 28
• Peak Power 140 kW
• 100 kV, 5 A, v/vz 1
• Dvz/vz 5
• Electron efficiency is nearly independent of loss
• Final 2.5 cm is unloaded to avoid damping high
  power wave

92.25 GHz
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Predicted Saturated Bandwidth
5 Bandwidth is Predicted

• Dw/w 5
• Pout 140 kW
• h 28
• Gain 50 dB
• rw 2.01 mm
• rc/rw 0.45
• r/rcopper 70,000
• Llossy 11.0 cm
• Lcopper 2.5 cm
• Lloss-taper 1.0 cm
• Lcircuit 14.5 cm

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Gyro-TWT Circuit has been Fabricated
MIG Connection Input Coupler Interaction
Region Output Coupler Collector
30 cm ruler
Axial View
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Gyro-TWT Circuit has been Fabricated
Cross-Section of Coaxial Coupler
Rectangular Input Waveguide
Coaxial Cavity
Interaction Circuit
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0 dB TE01 Input Coupler
Azimuthal Phase-Velocity Coupler

• HFSS Design
• Similar to
• UCLAs TE81
• Gyro-TWT Coupler
• NRLs Gyroklystron Coax Coupler
• All Modes are Matched

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TE51/TE01 Coax-Cavity Input Coupler
TE10 Rectangular Waveguide into TE51
Coax-Cavity into TE01 Circular Waveguide
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RF Measurement Set-up for Coupler and Circuit Loss

• MPI Flower-Petal TE10? / TE01? Transducers Give
  lt1.3 VSWR over 5 Bandwidth
• DURIP W-Band Vector Network Analyzer at SLAC
  will Measure Optimized Components

Set-up for Coupler Measurement
W-Band Scalar Network Analyzer
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Bandwidth of Coaxial Input Coupler

• Coupler exhibits gt 2 dB coupling for 3
  bandwidth
• Performance is limited by cutoff of short

Cutoff of short

• Predicted for 93.0 - 96.5 GHz
• Coupling gt 1 dB
• Selectivity gt 40 dB
• Return Loss (TE01) gt 7 dB
• Return Loss (TE21) gt 14 dB
• Return Loss (TE11) gt 28 dB
• Feature No tapering is needed between coupler
  and gain region

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Future Coaxial Input Coupler
Although the initial Gyro-TWT experiment will
employ the previous coaxial couplers, plans
have been initiated to develop an improved
coupler for future experiments.
These three modifications of the original display
a 7 bandwidth.
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Measured Loss in Circuit
Interaction Circuit has been Coated with Aquadag
Aquadag is a Carbon Colloid with r/rCu70,000 and
dskin0.06 mm
Measurements versus HFSS Modeling
90 dB Loss Measured at 93 GHz
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Single-Anode MIG (100 kV, 5 A, v/vz 1

• Designed with FINELGUN
• Fabricated by NTHU
• Mo Coating - Edge Emission
• Cathode Angle 74o
• Magnetic Compression 32
• Guiding Center Radius 0.9 mm
• Cathode Radius 5.1 mm
• Emitting Strip Length 1.9 mm
• Guiding Center Spread 10
• Axial Velocity Spread 5
• Electric Field 70 kV
• Cathode Loading 9 A/cm2
• Jemis/JL 0.3

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MIG Has Been Activated
Cathode Stalk
Very Steep Cathode (74?)
Emitting Ring
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I-V Characteristic of MIG
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Superconducting Magnet Profile
Field Profile of the Four Independent Coils

• 50 kG 0.1 over 50 cm
• Large 6" ID Bore
• Refrigerated

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100W 94GHz TWT Input Driver
Hughes 987 Coupled-Cavity TWT
CPI 1kW EIO is Also Available
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Summary

• UCD 94GHz Gyro-TWT has been Constructed
• - Capable of 140kW with Dw/w5 and h28
• Circuit is Heavily Loaded to Suppress Gyro-BWO
• - Final 2.5 cm is Unloaded to Avoid Damping
  Saturated Wave
• - Loss has Negligible Effect on Efficiency
• - 90 dB Loss Measured at 93 GHz
• MIG was Designed with Dvz/vz 5 and v/vz 1.0
• - MIG has been Activated
• Coax Couplers were Designed with HFSS
• - Good Match for All Modes
• - Very Short Length (5 mm)
• - Input and Output Couplers have been Measured