practical design considerations for high power...
TRANSCRIPT
Practical Design Considerations for High Power TWT
Muhammed Zuboraj, Ushemadzoro Chipengo, Niru K. Nahar John. L Volakis
1
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
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
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
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
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)
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
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
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
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
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
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
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
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
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