Development of a W-Band TE01 GyrotronTraveling-Wave Amplifier (Gyro-TWT)

for Advanced Radar Applications

1

Department of Applied Science, Univ. of California, Davis*Department of Physics, National Tsing-Hua Univ., Taiwan

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.

Motivation

US Navy 94 GHz High Power WORLOC Radar

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

• 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

Why Gyro-TWT (Gyrotron Traveling Wave Tube) ?Why Gyro-TWT (Gyrotron Traveling Wave Tube) ?

2

UCD W-band TE01 Gyro-TWT Amplifier

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 amplifiertechnology by developing a stable W-bandgyro-TWT(Goal performance: Pout=110 kW,Gain=45 dB, η=22%, BW3dB=5%)

• Gyro-TWT’s offer wide bandwidth

• TE01 mode transmits high power

• Distributed wall loss configurationstabilizes amplifier

Objectives

Approach Accomplishments

• Recent gyro-TWT under hot testwith 61.2 kW saturated output power,40 dB gain, 17.9 % efficiency, 1.5GHz (1.6%) bandwidth in zero drivestable condition (unoptimized)

3

Dispersion Diagram of TE01 Gyro-TWT

• Beam mode dispersion: ω = sΩc + kzvzWave mode dispersion: ω2 = ωc

2 +c2kz2

• Absolute instabilities must be stabilized: TE11

(1), TE21(1), TE02

(2) ,TE01(1)

ω = sΩ c + k zv z

ω = sΩ c + k zv z

s = 1

s = 2

kz(/m)

50

100

150

200

0-4000 4000

ω/2

π (G

Hz)

TETE1111(1)(1)

TE21(1)

TE01(1)

TE02(2)

operating point(grazing intersection)

Potential Gyro-BWOinteraction

s=1

s=2

4

100 kV, α=1.0

Design Approach

Beam voltage, velocity ratio,Mode, guiding center radius etc.

Choose Device parameters

Simulation using ‘Absolute Instability’ code [1]

Determine stable beam current

Simulation using ‘Gyro-BWO’ code [2]

Determine Circuit Lengthand Loss Value

Simulation using nonlinear code [3]

Check Large Signal Characteristics

• Iterate the loop to optimize the gain, power, efficiency, and bandwidth

[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”, IEEETrans. Plasma Sci., vol.27, pp.391-402, 1999)

5

Device Parameters

Voltage 100 kV Current 5 Aα= v⊥/vz 1.0∆vz/ vz 5 %Magnetic Field(Bo) 35.6 kGBo/Bg 0.995Cutoff Frequency 90.97 GHzWall Resistivity 70,000 ρCuCircuit Radius, rw 0.201 cmGuiding Center Radius, rc 0.45 rw

Circuit Length 13.6 cm

6

Stable Beam Current• Gyro-TWT exhibits absolute instability near cutoff at sufficiently high beam

current

• Beam current can be higher for lower α (=v⊥/vz) and lower Bo/Bg

• Unloaded TE01 circuit is stable for beam current = 5 A for designvalue α =1.0 and Bo/Bg= 0.995

α = 0.9

1.0

1.11.2

1.3

Bo/Bg

I s(A) Design value

Stability from TE01 Cutoff OscillationKeep I < Is

Simulation results using ‘Absolute Instability’ code

7

Predicted Gyro-TWT Performance

For predicted velocity spread ∆vz/vz = 5%-Bandwidth ∆ω/ω = 5%- Pout= 110 kW- η = 22%

- Large signal gain = 45 dB

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

8

Application of Loss• Loss has been added to circuit to suppress Gyro-BWO

Theory ⇒ ρ/ρCu = 70,000 is needed• ‘Aquadag’ (a Carbon colloid) has the desired loss of ρ/ρCu ≅ 70,000

input outputloss

12cm 1.6cm

Axial view of TE01 Gyro-TWT circuit

• Initial 12 cm is coated. Final 1.6 cmis uncoated to prevent wave damping

Inse

rtion

Los

s (dB

/ 12

cm

)

Frequency (GHz)

-200

-150

-100

-50

0

90 92 94 96 98 100

HFSS-Copper Guide

HFSS-Copper Guide with InnerSemiconductor Tube (∆r=0.05 mm, ρ/ρ

Cu=70,000)

HFSS-Resistive Guide ρ/ρ

Cu=70,000)

rw

=2.01 mm

Measurement

Measurement versus HFSS simulation

• 90 dB loss is measured at 93 GHz

• Loss lowers the gain but this can becompensated by increasing the circuitlength to just below the critical length

9

Experimental Design and Setup

• Single Anode MIG

• High Voltage Modulator

• RF Couplers

• Interaction Circuit

• Vacuum System

• Superconducting Magnet System

• RF Drive Sources

• RF Diagnostics

10

Single Anode MIG

Glowing Cathode Emission Ring

Activated MIG

Assembled MIG

Cathode Stalk

CathodeEmission Ring

• Designed MIG beam parametersBeam voltage 100 kV Beam current 5 AVelocity ratio (v⊥/ vz) 1.0Velocity spread 2% Cathode radius 5.1 mmGuiding center radius 0.9 mm

EGUN simulation of electron trajectoryand magnetic field profile

11

RF Couplers

Cross section of the Fabricated Coax Coupler

TE10 →TE51 → TE01 Coax CouplerDesigned with HFSSAll Modes are Matched

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

Rectangular Input waveguide

(TE10 )Coaxial Cavity

(TE51 )

Interaction Circuit(TE01 )

HFSS cross sectional view of electromagnetic field intensity

12

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

Input coupler

Scalar measurement

Vector measurement

Output coupler

Scalar measurement

Vector measurement

13

Interaction Circuit• Interaction region is heavily loaded with ‘Aquadag’, a carbon colloid

with ρ/ρcu= 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

Beam TunnelInteraction Region

(13.6cm)

Output Coupler

InputCoupler

Load Collector

Coated withAquadag Uncoated

30cm ruler

14

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%)

Hughes 94 GHz, 100 W Folded Waveguide TWT

SLAC-UC Davis W-Band Modulator

15

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

Highpower load

CirculatorInputdriverGyro-TWT

IN

Frequencymeter

Directionalcoupler

1

3

2OUT

Variableattenuator

scope

Cross guide coupler

Crystaldetector

Ka-Band overmoded waveguide

3

2

Fabry-Perotinterferometer

16

Magnet System

• Refrigerated Superconducting Magnet

Superconducting magnet

Coil power supply Axial position (cm)M

agne

tic F

ield

(kG

)

• Magnetic field profile for 4 coils

- 50 kG ± 0.1% over 50 cm - 4 compensated independent coils- 6” large bore

17

Integrated Gyro-TWT System

Superconducting Magnet

MIG MainVacuum Pump

RF Input

RF Output

Gun Vacuum Pump

Collector

Beam Tunnel

Axial Position of Superconducing Magnet (cm)

Mag

netic

Fie

ld (k

G)

18

Experimental Progress Flowchart

1st version Gyro-TWT - Employed MIG ∆vz/vz=5% (predicted)- Small signal gain=34dB, BW=2%- Performance hampered by misaligned MIG(∆vz/vz=10% inferenced by nonlinear code)

2nd version Gyro-TWT

- Employed realigned MIG ∆vz/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

4th 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

- Employed well matched output section and wellaligned MIG

- Currently under hot test

19

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

20

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

21

Frequency Identification using Fabry-Perot Interferometer

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

crystal detector

horn antenna

micrometer

metalmesh

22

Mode Competition Identification23

2nd version Gyro-TWT 3rd version Gyro-TWT

TE02 mode oscillation (170 GHz)

EliminatedShorten circuit length

TE01 mode drift tube oscillation (85 GHz)

Eliminated

Reduced drift tube radius

TE01 mode cutoff oscillation (91 GHz)

Higher start oscillation current

Shorten circuit length

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

24

2nd version 3rd version

60 kV

85 kV

85 kV

60 kV

Drift Tube Oscillation- In 2nd version, oscillation has been measured at 85 GHz at the drift tubeusing Fabry-Perot interferometer

- TE01 mode at the drift tube has been identified to be the source of oscillationdrift tube radius reduced in 3rd version and oscillation eliminated

• Cyclotron and cutoff frequency vs. axial position of beam tunnel region

TE11 cutoffTM01 cutoffTE21 cutoff

TE01 cutoffcyclotronFrequency (100 kV)

cyclotronfrequency (61 kV)

2nd version Gyro-TWT

25

Mode Competition- 2nd version Gyro-TWT performance limited to lower voltage due to modecompetition

- 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

26

Measured Absolute Instability- In 2nd version, oscillations near cutoff frequency (~91 GHz) have beenobserved at higher voltages than > 70 kV

- The cutoff oscillation degrades the amplified signal

• Vb=72 kV, Ib=5.3 A, Bo=34.1 kG•Vb=80 kV, Ib=5.1 A, Bo=34.8 kG

2nd version Gyro-TWT

27

Measured Bandwidth- 3rd version gyro-TWT performance limited due to the excessive return loss at the output end (verified by simulation)

3rd version Gyro-TWT

28

• Different return loss assumed in simulation• Effect of return loss on bandwidth and comparison with measurement

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)

29

Summary• UCD 94 GHz TE01 Gyro-TWT has been constructed with predicted capability

of 110 kW with ∆ω/ω=5% and η=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).

30