practical design considerations for high power...

15
Practical Design Considerations for High Power TWT Muhammed Zuboraj, Ushemadzoro Chipengo, Niru K. Nahar John. L Volakis 1

Upload: others

Post on 26-Apr-2020

2 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

Practical Design Considerations for High Power TWT

Muhammed Zuboraj, Ushemadzoro Chipengo, Niru K. Nahar John. L Volakis

1

Page 2: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

2

Part-1:

Objectives Review of previous work Practical considerations Design with support layout Excitation of cylindrical TM01 mode Coupling and S-paramters Future Directions and Remarks

Part-2: Slow wave structures for high power BWO Mode control in BWO Design objective for mode control in BWO

Outlines

Page 3: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

Design a SWS for TWTA compatible with relativistic beam

Practical design layout considerations for cold test

Perform cold test at ElectroScience Laboratory

3

Objective

Beam specification at UNM

• Cathode voltage of 500KeV • Beam current of 100KA • Minimum beam radius is 2mm

Part-1

Page 4: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

2 2.5 3 3.5 440

45

50

55

60

65

70

Frequency(GHz)K

0( Ω)

2 2.5 3 3.5 40.7

0.75

0.8

0.85

υ p/c

Frequency(GHz)

4

Review: Curved Ring-Bar

Dimensions: a=4mm b=30mm p=20mm w=δ=1mm h1=8mm h2=4.8mm

Interaction impedance

Normalized Phase velocity

2

0 22 g

EzKWvβ

=

Normalized Group velocity

Deign goals: Vp>0.8c K0>50Ω

Inside TWT

2 2.5 3 3.5 40.2

0.25

0.3

0.35

0.4

υg/c

Frequency(GHz)

E-field Profile(V/m)

0.75c<Vp<0.81c Minimum K0>48Ω Strong E-field at center suitable for bunching No support layout considered

Page 5: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

Challenges in Practical design considerations

5

• Design of support layout Challenges:

• Reduces RF wave phase velocity → Low output power

• Reduces interaction impedance →Poor coupling to electron beam

• Discharges at very high power considerations

Solution Approaches:

• Choose a low permittivity with high melting point materials (e.g Teflon, Quartz, Beryllia )

• Choose suitable support rods

Typical support Layout1

Dielectric rod

Ring-Bar SWS

Metal Vane

Seshadri, R., S. Ghosh, A. Bhansiwal, S. Kamath, and P. K. Jain, "A simple analysis of helical slow-wave structure loaded by dielectric embedded metal segments for wideband traveling-wave tubes," Progress In Electromagnetics Research B, Vol. 20, 303-320, 2010.

Features: • Provides stable support for SWS inside TWT • Segmented dielectric rods create effective dielectric medium • Metal Vanes improves dispersion • Metal Vanes reduce interaction impedance drastically

Typical low εr materials

Material Permittivity(εr) Melting point(˚C)

Quartz 2.4 ~1700

Teflon 2.1 327

Beryllia (BeO) 6.8 2507

Page 6: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

2 2.5 3 3.5 4

0.65

0.7

0.75

0.8

υ p/c

Frequency(GHz)

2 2.5 3 3.5 40

20

40

60

80

100

Frequency(GHz)

K0( Ω

)

Analysis of Curved Ring-Bar with Support Rods

6

Dimensions: a = 4mm b = 70mm p = 20mm w = δ = 1mm h1= 8mm h2 = 4.8mm Td = 6mm

Inside TWT

εr = 2.1(Teflon)

Interaction impedance

Normalized Phase velocity

Normalized Group velocity

Deign goals: Vp>0.7c K0>50Ω

2 2.5 3 3.5 40.1

0.15

0.2

0.25

0.3

0.35

υ g/c

Frequency(GHz)

0.7c<Vp<0.78c (2-3.3GHz) Average K0=55Ω Strong E-field at center

suitable for bunching

E-field Profile(V/m)

Page 7: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

Excitation of TM01 mode in Cylindrical Waveguide

7

Challenges: 1. Fundamental mode is not TM01 2. Circular field symmetry to support

TM01 mode 3. Feed technique is crucial for

coupling. 4. Hybrid modes can cause group

delay

Typical Mode profile (cross-section)

Ez, Eρ, Hφ Eφ, Hz, Hρ

Feeding Techniques Probe feeding Aperture feeding

Advantages: • Easy to implement • Portable • Minimal or no dispersion

Disadvantages : • Cannot operate at high power • Narrow bandwidth • TM01 mode is difficult to excite

Advantages : • High power handling • Strong E-field coupling • Easier to excite a TM01 mode • Easier to match to an horn antenna

Disadvantages : • Not portable • Higher dispersion

Page 8: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

Excitation of TM01 mode in Cylindrical Waveguide

8

Objectives: 1. TM mode is desired 2. Minimum reflections 3. Strong Ez-field at the center

X-Y plane

Typical feeding method for exciting TM mode in cylindrical waveguide

Y-Z Plane

X-Y Plane

Z

Y

X

Features: • Perfect TM01 mode excited • E-field is strong at the center • Probe must be at the beam-line

(Ez , Eρ)

Probe feeding

λg

Page 9: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

RF Port-1

Z Y

X

X-Y plane

Advantages: 1. Handles large power. 2. No interference in beam line 3. Rectangular apertures can provide simple

cut-off wavelength prediction

Disadvantages: 1. Coupling is polarization sensitive 2. Higher order modes at excitation junction 3. Sensitivity to field orientation 4. Efficient power divider is required

Input Ports

RF Port-2

RF RF Port-1 Port-2

Excitation of TM01 mode in Cylindrical Waveguide

Aperture feeding1

Feeding method: 1. Excite TE01 mode at RF input ports 2. Couple rectangular guide TE01 Ez

field to a TM01 mode in a cylindrical waveguide

3. Match components 4. Must provide good isolation at

input ports

Output Port

Horn Antenna

Power divider

Port Field Polarization RF Port-1 RF Port-2

1. D. Shiffler , J. A. Nation and G. S. Kerslick "A high-power, travelling wave tube amplifier", IEEE Trans. Plasma Sci., vol. 18, pp.546 1990

TE01 mode

Z

9

Page 10: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

Field Profile

Y-Z Plane

Z-X Plane

Slow Wave TM mode

X-Y plane

1. Good coupling implies proper field polarization at the feeds and also inside the guide 2. Need to optimize S-parameters to maximize coupling and reduce mismatch

Excitation of TM01 mode in Cylindrical Waveguide

λg

RF Port-2

RF Port -1

RF Port -3

10

Page 11: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

RF Port-2

RF Port -1 RF Port -3

Port-4

Transmission Co-efficient

2 2.2 2.4 2.6 2.8 3

-12

-10

-8

-6

-4

-2

Frequency in GHz

S-p

aram

ters

in d

B

|S31|=|S32||S12|=|S21|

Issues: • Ports not matched • Poor isolation between input ports(|S12|

or |S21|)

Solution Approaches: • Ports should be λ0 apart • Cylinder radius can be increased further • Matching needs to improved

Coupling of TM mode

Mode coupling comparison • Average E-field strength (at 2.5GHz)

At the center of port-1 Ez = 1.2 KV/m (TE01) At the center of port-2 Ez = 1.2 KV/m (TE01) At center of tube Ezavg = 90 V/m ( cylindrical TM01) At center of tube Eρavg = 95 V/m (cylindrical TE01) At center of tube Eφavg = 1.75e-3V/m (cylindrical TE11)

Z

Y

X

• TE11 mode is almost absent • 10% coupling is achieved • Mismatch is possible reason for poor coupling

-3dB line

11

Page 12: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

Application of Slow Wave Concepts to High Power Backward Wave Oscillators (S-band)

B.W.O Issues at High frequencies 1. Low Output Power: • Output power scales as 𝑷𝑷 ∝ 𝟏𝟏

𝒇𝒇𝟐𝟐 (see fig 3)

2. Mode Control : • Slow wave structures require mode control. 3. Stronger Magnetic focusing systems required : • Require stonger magnetic focusing fiels, leading to more bulky

devices.(see fig 1&4) FIG. 4: Magnetic field and electron beam current density requirements for a 100 W. T.W.T obtained by 3 different scaling of device parameters with frequencies from 5GHz to 200 GHz. [1] ------- Magnetic Field , Current Density[1]

FIG. 3: Power vs Frequency for V.E.D’s (2008)[1]

FIG 2 : BWO Schematic

FIG 1 : Experimental setup for a Backward Wave Oscillator.

[1] John H. Booske, “Plasma physics and related challenges of millimeter wave to terahertz and high power microwave generation,” Physics of Plasmas 15 ,Feb 2008 12

Part-2

Page 13: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

Mode Control In BWO SWS

FIG. 5 : Cylindrical, corrugated Slow wave structure[2]

Benefits of Mode Control 1. Reduction in size, power consumption and weight • Mode control reduces requirement of large magnetic field 2. Increasing of output power • Internal R.F. breakdown is prevented and maximum

power handling enhanced.

FIG. 6 : Resonant reflector cavity.[3]

[2] H. Zhang,J.Wang and C. Tong,” Progress in Theoretical Design and numerical simulation of High power terahertz Backward Wave oscillator,” Piers Online Vol 4 2008. [3] Z. Li and Yu Qi ,”Mode Control in an oversized backward wave oscillator,” Physics of Plasmas 15 ,2008.

Mode Control How to achieve it: 1. Suppress excitation of higher order modes by

creating wider stop-bands between modes. 2. Suppress higher order modes.

13

Page 14: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

NOVEL BWO SWS FOR EFFECTIVE MODE CONTROL

Design Goals • Miniaturization • High output power (500KW-1MW) . Performance Parameters: • Power output. • Power conversion efficiency. • Bandwidth. • Pulse duration.

Electron Beam D

Resonant Reflector

Drift Tube

Slow Wave Structure

Fig 7 .Dominant TM01 mode in different unit cell designs ,Corrugated Waveguide and Helical Corrugated Waveguide

14

Page 15: Practical Design Considerations for High Power TWTece-research.unm.edu/FY12MURI/pdf_Files/MURI_2nd_May_osu.pdf · 2015-06-19 · Practical Design Considerations for High Power TWT

Concluding Remarks

15

• Designed a Curved Ring-Bar that operates with ve->0.7c

– Support Layout included – Aperture coupling applied – Almost flat impedance profile over S-band

• Next Steps:

– S-parameters must be optimized – Perform cold test at ElectroScience Lab. – Perform Hot test in UNM

Curved Bar

BWO

• Design study of Backward Wave Oscillators • Next Steps:

– Application of Slow wave concept in BWO – Effective mode control – Cold test simulations and evaluation