1. 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
F (MURI-MVE) and F

2. Outline Small-Signal Design for Stability Wall Loss
Large-Signal Characteristics
Circuit Components

3. Motivation Why Gyro-TWT? Why TE01 Mode?
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/rw=0.5)
Azimuthal Symmetry is Favorable for MIG Beam
Field Pattern is Unique (Jz=0 and Er=0)
- Useful for Mode Selective Circuit

4. Dispersion Diagram - TE01 Gyro-TWT
100 kV, v^/vz=1.0
Must Suppress TE11(1) , TE21(1) and TE02(2) Gyro-BWO Interactions

5. Stable Beam Current (Absolute Instability at Cutoff)
Beam Current can be Higher for Lower v^/vz and Lower Bo/Bg
100 kV, v^/vz=1.0
Unloaded TE01(1) Circuit is Stable for 5 A, v^/vz=1.0, and Bo/Bg=1.0

6. 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 = yields Stability and 100 dB Loss for 14.5 cm Circuit

7. CW Wall Loading < 50 W/cm2
Power Growth in Lossy Single-Stage Device Self-Consistent Large-Signal Simulation Code
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
CW Wall Loading < 50 W/cm2
92.25 GHz

8. 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 = cm
Lcopper = 2.5 cm
Lloss-taper = 1.0 cm
Lcircuit = 14.5 cm 7

9. Gyro-TWT Circuit has been Fabricated
MIG Connection Input Coupler Interaction Region Output Coupler Collector
30 cm ruler
Axial View

10. Gyro-TWT Circuit has been Fabricated Rectangular Input Waveguide
Cross-Section of Coaxial Coupler
Rectangular Input Waveguide
Coaxial Cavity
Interaction Circuit

11. 0 dB TE01 Input Coupler HFSS Design Similar to UCLA’s TE81
Azimuthal Phase-Velocity Coupler
HFSS Design
Similar to
UCLA’s TE81
Gyro-TWT Coupler
NRL’s Gyroklystron Coax Coupler
All Modes are Matched

12. TE51/TE01 Coax-Cavity Input Coupler
TE10 Rectangular Waveguide
into TE51 Coax-Cavity
into TE01 Circular Waveguide

13. RF Measurement Set-up for Coupler and Circuit Loss
MPI Flower-Petal TE10 / TE01 Transducers Give <1.3 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 12

14. Bandwidth of Coaxial Input Coupler
Coupler exhibits > 2 dB coupling for 3% bandwidth
Performance is limited by cutoff of short
Cutoff of short
Predicted for GHz
Coupling > 1 dB
Selectivity > 40 dB
Return Loss (TE01) > 7 dB
Return Loss (TE21) > 14 dB
Return Loss (TE11) > 28 dB
Feature: No tapering is needed between coupler and gain region

15. 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. 15

16. Measured Loss in Circuit
Interaction Circuit has been Coated with Aquadag
Aquadag is a Carbon Colloid with r/rCu=70,000 and dskin=0.06 mm
Measurements versus HFSS Modeling
90 dB Loss Measured at 93 GHz

17. 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

18. MIG Has Been Activated Cathode Stalk Very Steep Cathode (74)
Emitting Ring

19. I-V Characteristic of MIG

20. Superconducting Magnet Profile
50 kG ± 0.1% over 50 cm
Large 6" ID Bore
Refrigerated
Field Profile of the Four Independent Coils

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

22. Summary UCD 94GHz Gyro-TWT has been Constructed
- Capable of 140kW with Dw/w=5% and h=28%
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