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.

Outline

• Small-Signal Design for Stability

– Wall Loss

• Large-Signal Characteristics

• Circuit Components

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/rw=0.5)

• Azimuthal Symmetry is Favorable for MIG Beam

• Field Pattern is Unique (Jz=0 and Er=0)

- Useful for Mode Selective Circuit

Motivation

Dispersion Diagram - TE01 Gyro-TWT

100 kV, v/vz=1.0

Must Suppress TE11(1) , TE21

(1) and TE02(2) Gyro-BWO Interactions

0

2

4

6

8

10

-3 -2 -1 0 1 2 3

kzrw

TE02

TE21

TE11

TE01

w = sWc + k

zv

zw

r w/c

s = 1

s = 2

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

0.1

1

10

100

1000

0 0.5 1 1.5 2 2.5

I s(A)

v/v

z

Bo/B

g = 1.00

0.98

0.96

Design Values

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)]

• copper = 7.104 yields Stability and 100 dB Loss for 14.5 cm Circuit 0

20

40

60

80

102 103 104 105 106L

c/rw

copper

TE02

(2) TE21

(1)

TE11

(1)

100 kV, 5 A, v/v

z = 1.0

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

vz/vz = 5%

• Electron efficiency is nearly independent of loss

• Final 2.5 cm is unloaded to avoid damping high power wave

92.25 GHz

10-2

10-1

100

101

102

103

104

105

106

0 5 10 15

Pow

er (

W)

z (cm)

lossy wall Cu wall

loss taperCu

= 70,000

Pin = 5.0 W

1.25 W

0.3 W

CW Wall Loading < 50 W/cm2

Predicted Saturated Bandwidth

• ww = 5%

• Pout = 140 kW

• = 28%

• Gain = 50 dB

• rw = 2.01 mm

• rc/rw = 0.45

• copper = 70,000

• Llossy = 11.0 cm

• Lcopper = 2.5 cm

• Lloss-taper = 1.0 cm

• Lcircuit = 14.5 cm

5% Bandwidth is Predicted

7

0

50

100

150

200

0

10

20

30

40

90 92 94 96 98 100

Pou

t (kW

)

Efficiency (%

)

Frequency (GHz)

vz/v

z = 0%

5%

Gyro-TWT Circuit has been Fabricated

Axial View

MIG Connection Input Coupler Interaction Region Output Coupler Collector

30 cm ruler

Cross-Section of Coaxial Coupler

Gyro-TWT Circuit has been Fabricated

Rectangular Input Waveguide

Coaxial Cavity

Interaction Circuit

0 dB TE01 Input Coupler

• HFSS Design

• Similar to

– UCLA’s TE81 Gyro-TWT Coupler

– NRL’s Gyroklystron Coax Coupler

• All Modes are Matched

Azimuthal Phase-Velocity Coupler

TE51/TE01 Coax-Cavity Input Coupler

TE10 Rectangular Waveguide into TE51 Coax-Cavity

into TE01 Circular Waveguide

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

W-Band Scalar Network AnalyzerSet-up for

Coupler Measurement

12

Bandwidth of Coaxial Input Coupler

Predicted for 93.0 - 96.5 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

Cutoff of short

-25

-20

-15

-10

-5

0

90 92 94 96 98 100

Cou

plin

g (d

B)

Frequency (GHz)

Return Loss (HFSS)

Coupling (HFSS)

Coupling (Measurement)

• Coupler exhibits > 2 dB coupling for 3% bandwidth

• Performance is limited by cutoff of short

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.

9 0 9 2 9 4 9 6 9 8 1 0 0 1 0 2

- 1 5

- 1 2

- 9

- 6

- 3

0

Cou

plin

g (d

B)

f (G H z )

o r ig ina l

o ptim iz a tio n 1

o ptim iz a tio n 2

o ptim iz a tio n 3

These three modifications of the original display a 7% bandwidth.

15

Measured Loss in Circuit

-200

-150

-100

-50

0

90 92 94 96 98 100Inse

rtio

n L

oss

(dB

/ 12

cm

)

Frequency (GHz)

HFSS-Copper Guide

HFSS-Copper Guide with Inner Semiconductor Tube (r=0.05 mm,

Cu=70,000)

HFSS-Resistive Guide (

Cu=70,000)

rw

=2.01 mm

Interaction Circuit has been Coated with AquadagAquadag is a Carbon Colloid with Cu=70,000 and skin=0.06 mm

Measurements versus HFSS Modeling

90 dB Loss Measured at 93 GHz

• 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

Single-Anode MIG (100 kV, 5 A, v/vz = 1

MIG Has Been Activated

Emitting Ring

Cathode Stalk

Very Steep Cathode (74)

I-V Characteristic of MIG

0

5

10

15

20

25

0 100 200 300 400 500 600 700 800

780degree

820degree

845degree

898degree

1010degree

I dc(m

A)

Vdc

(V)

Superconducting Magnet Profile

0

20

40

60

-50 0 50 100 150 200

z (cm)

Interaction

Gun

Field Profile of the Four Independent Coils

• 50 kG ± 0.1% over 50 cm

• Large 6" ID Bore

• Refrigerated

100W 94GHz TWT Input Driver

Hughes 987 Coupled-Cavity TWT

CPI 1kW EIO is Also Available

Summary

• UCD 94GHz Gyro-TWT has been Constructed - Capable of 140kW with ww=5% and =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 vz/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