New concepts in traveling wave tubes based on multiple...
Transcript of New concepts in traveling wave tubes based on multiple...
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New concepts in traveling wave tubes based on multiple transmission lines
Mohamed Othman1, Filippo Capolino1, Alex Figotin2
August 1, 2014
1Department of Electrical Engineering and Computer Science 2Department of Mathematics
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Outline
Multiple Transmission Line (MTL) concept and extension of
Pierce model
• Uniform MTL – e-beam dispersion relation and modes
Possible routes for gain enhancement in uniform finite
traveling wave tubes (TWT)
• Pierce parameters
• Gain
Periodic MTL TWT
• High-pass-type circuit
• Dispersion relation with wide band interaction
• Frozen mode regime in finite MTL TWT
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The goal is to determine whether an MTL can enhance the gain of TWTs
Part I
Part II
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Multiple transmission line (MTL) is a generic concept that can model a
variety of coupled EM guiding structure
The purpose of utilizing MTL is to enhance wave/matter interaction, and
introduce peculiar dispersion characteristics that cannot be attained
using single transmission line circuits
MTL involves rigorous mathematical formulation, since it deals with
matrices that are not necessarily diagonalizable
Multiple transmission line concept
We address two cases
Uniform
MTL TWT
Periodic
MTL TWT
Coupled cavity TWT
Examples
Coupled Helices
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Pierce theory: review
Pierce theory: simple, analytical theory still in widespread use today
Ideal representation of electron beam as a fluid.
Complex slow wave structure idealized as a simple one-
dimensional transmission line (TL) described by two parameters:
inductance (L) and capacitance (C).
4 J. R. Pierce, Traveling Wave Tubes. Princeton, NJ: Van Nostrand, 1950.
C
L
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,z z sV j LI I j CV i
0 0 0 0 0 0,b b bI I I u u v
2 0 0
0 00
z z z b zu v E k I j Eu
Equation of motion & continuity equation
Linearization
Transmission line equation
230
2 2 20
1 2 0( ) ( )
c
c
kC
k k
4 solutions for k: 2 complex, 1 forward,1 backward
Synchronism
0k
Dispersion: k is the modal propagating constant
J. R. Pierce, Traveling Wave Tubes. Princeton, NJ: Van Nostrand, 1950. 5
Bunching, space charge waves
Pierce theory : analysis
9.54 47.3G CN
/N H
H
Gain
j t jkze e
Varying as j t jkze e
Waves with
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Extension of Pierce theory to MTL
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Need for more detailed formulation to extend Pierce theory to MTL.
Complex slow wave structures and real waveguides are better
represented by Multiple transmission Lines (MTL) since most SWS or
real waveguides naturally have multiple modes.
Coupled helix waveguide traveling wave tube. This is an example of
two coupled slow-wave structures interacting with a single electron beam.
MTL formulation
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1
( ) ( ) ( )N
t n n
n
V z
E r e ρ
1
( ) ( ) ( )N
t n n
n
I z
H r h ρ
Consider a waveguide system with a uniform cross-section
that is able to support fields in the form
{Transverse
fields
We allow coupling between the modes, and we define a state vector
1 2 1 2( )T
N b N bz V V V V I I I IΨ
The state vector defines the evolution of the EM waves/space charge
waves along the z-direction
Felsen and Marcuvitz. Radiation and scattering of waves. Vol. 31. John Wiley & Sons, 1994.
Cross section z
E-beam
Defining equivalent space charge voltage and currents 0 0 ,b b bI u u 0 bb
u vV
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MTL development
ˆ ˆx y ρ x y
Modal functions
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Notations
• MTL described by inductance and capacitance matrices L and C
• Assume that L and C are symmetric and positive-definite
• V and I vectors on MTL: and
• Strength of each shunt current generator can be scaled
• Assume Ez of nth TL is related only to its voltage:
• Describe how each TL affects beam dynamics:
• We assume s = a is the cold structure modal wavenumber
• This needs to be revisited in future (possibility of exotic dispersions)
• Assume solutions varying as
• k is complex propagation constant and ω is radian frequency
1 2[ , ,.., ]TNs s ss
1 2[ , ,.., ]TNV V VV 1 2[ , ,.., ]TNI I II
,z n n z nE a V
1 2[ , ,.., ]T
Na a aa
j t jkze e
c
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Analysis of MTL TWT
Basic formulation involving evolution equations for the system
Extraction of modal characteristics
dispersion relation k – ω where k is the modal wavenumber
Evaluation of coupling parameters
Coupling impedance, Pierce parameters
Calculation of gain for finite size TWT
z
z s
j
j
V LI
I V iC e-beam is seen as a current generator Provides power to the TL
Objectives
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Tamma, Capolino. "Extension of the Pierce Model to Multiple Transmission Lines Interacting With an Electron Beam." IEEE Trans. Plasma Science, vol. 42, no. 4, pp. 899-910, (2014)
Evolution of space charge waves along the z-direction
• Assume beam induces current in every TL (shunt current generator)
• Use linearized beam equations like in Pierce theory
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Uniform MTL TWT
,z z sj j V LI I CV i
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Uniform MTL TWT: Modal properties
Tamma, Capolino. " IEEE Trans. Plasma Science, vol. 42, no. 4, pp. 899-910, (2014)
. . . .
bY
,s Ni
,1si
,2si
bjkI+
-
+
-
+
-
Y
1V
2V
NV
11: a
1: Na
21: a
MTL
Transverse resonance condition
Dispersion relation
0b Y Y V
2 22 2 0 0
2
0
det 0Tkk
k
1 LC Lsa
det 0b Y Y
Need to find complex propagation constant k of the resonant modes
Establish transverse resonance condition in terms of beam impedance
Yb and MTL impedance Y
Transverse resonance
condition from circuit point of
view for s = a
0 ,1 ,2 ,max , ,...,c c c N
Condition for growing waves
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Uniform MTL: Growing waves
• Natural propagation constants of MTL:
• Growing wave solutions always exists if electron propagation
constant satisfies
Illustrative example: 2-TL system coupled to beam with
,1 ,2 ,, ,...,c c c N
0 ,1 ,2 ,max , ,...,c c c N
β0 [rad/m]
Re(
k) [
m-1
]
Im(k
) [m
-1]
β0 [rad/m]
0 ,1c 0 ,2c
Real part of k Imaginary
part of k
,2 ,1c c
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1 21 2 1 2
0 0 0 0
, ,(4 / ) (4 / )
T
p p p p pC C C CV I V I
C
,1 ,2 0c c
Small signal gain [dB]
0 1 1 2 2dB p pG C C L
Gain can be enhanced using MTL!
Represent the coupling between TLs
and the e-beam
are the coupling
impedances to the
lines
1 2and
(0) 0bI
(0) 0bV No pre-modulation
Boundary conditions
No reflection from boundaries
Gain can be represented in terms of
Pierce parameters
2
dB
( )10log
(0)
V LG
V
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We assume 0k
Gain for 2 TL: Pierce model
As in Pierce model
0 1 2, , are constants
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Coupling parameters
1pC
Cp2
,1 0/c
,2 0/c
0 0 0Beam parameters : 6 KV, I 3 A, 0.15V u c
11 22 11 22TL parameters : 80 pF, 7μHC C L L
Condition for synchronism Trade off between Gain and Synchronism
Better gain synchronism
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• The dispersion relation is written as a polynomial
0 ,1 ,2( , , , , ) 0c cF k
k has only one pair of complex roots
Theorem: For uniform MTL described by L and C, there exists only one
unique growing wave mode*
Uniform MTL TWT: remarks
*Figotin and Reyes. "Multi-transmission-line-beam interactive system." Journal of Mathematical Physics, vol. 54 no. 11, pp. 111901 (2013)
Is it possible to have more than one growing mode ?
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Summary for uniform MTL
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Uniform MTL TWT concept:
Developing the Pierce theory for more than one transmission line
Finding the modes – dispersion characteristics:
Calculating the gain for finite size structures: possible gain
enhancement is observed
Assessing the coupling parameters, especially impedance (matrix!):
provide intuition on how to properly design the TWT
Following steps:
Developing concrete boundary conditions for finite MTL TWT
Designing real structures that support 2 modes such as coupled helixes
or coupled ladder circuits
Next topic Periodic circuit
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Periodic TWT circuit In contrast with helix TWTs , CCTWTs operate at higher-average power
levels and higher frequencies but with smaller bandwidths than helix TWTs.
The interaction structure in a CCTWT is composed of a series of cavities
that are connected via slots and a beam tunnel
The voltage across the gaps modulates the electron beam velocity
Future designs will allow interaction with more than one mode
Curnow, IEEE T-MTT (1965) Gittins, Power travelling-wave tubes. New York (1965).
Staggered slots Interaction gaps
Beam tunnel
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j t jkze e
z
Periodic MTL – e-beam interaction
A. Figotin and I. Vitebsky, Frozen light in photonic crystals with degenerate band edge,
Phys. Rev. E, 2006
C. Locker, K. Sertel and J. L. Volakis, “Emulation of propagation in layered anisotropic
media with equivalent coupled microstrip lines,” IEEE Microw. Wireless Compon. Lett.,
Dec. 2006.
A B C
d
AdBd Cd
AC
. . . . . .
ABX BCXCAX
zBeam
j t jkze e All a.c. quantities: and k is the Block wavenumber
2 TL
z
amplification k j ,i jX model interfaces between different
waveguide cross-sections
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MTL high pass type CC-TWT circuit
High pass type periodic circuit z
11L11cC
11C
22L11cC 22C
21L21C
One segment of MTL
DBE
RBE
Beam line
Cold structure dispersion + electron beam
(0) (0)jkdeT Ψ ΨDispersion relation is found
Transfer matrix of one unit cell
The dispersion may develop a regular band edge (RBE) Or a degenerate band edge (DBE)
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k : Bloch wave-
number 4k
2k
Unit cell composed of THREE MTL segments with different coupling
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Dispersion of coupled system
Wide band interaction
Interaction with two modes at two different frequencies creates a wide band
region where amplification is possible
More than one growing wave
Unlike the uniform MTL, here it is possible to have two
growing waves simultaneously!
Two possible growing waves!
Wid
e b
and
amp
lification
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Blue: exponential growing mode Green: exponential growing mode Black: exponential decaying mode Red: real k mode
Only forward modes With positive real k
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Interaction near a degenerate band edge
Growing waves can be observed in the vicinity of DBE
We are investigating the backward wave excitation at that condition.
Growing waves
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Interaction near the edge of transmission band
Time-Domain analysis!
Green: uncoupled system
DBE
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Frozen wave amplifying regime
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Field distribution at DBE resonance
The unique transmission properties of Fabry-Perot resonators with DBE can be utilized*
DBE
A. Figotin, and I. Vitebskiy, "Gigantic transmission band-edge resonance in periodic stacks of anisotropic layers.” Phys. Rev. E 72(3) (2005)
Gigantic gain (very narrow frequency)!
sV
N = 32 unit cells
z
u
Electron super bunching as theymove along z-axis with velocity u
d
Frozen DBE mode magnitude profile
p(z)
TWT with super amplification via the DBE mode. A, B, and C are three different waveguide sections with distinct transverse anisotropy.
Injected electron beam
Frozen mode axial electric field phase profile
+- -
A B C A B C
RBE mode magnitude profile
outV
Will be implemented using waveguides with rotated elliptical cross sections
very low energy electron beam
(1)V (2)V (3)V ( 1)V N
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Conclusions
Complex slow wave structures and real waveguides are better
represented by MTLs
MTL brings a promise in enhancing the gain of TWT, through optimizing
the coupling parameters
Periodic MTL offers phenomenological paradigm change since all modes
can be amplified
Future Research Directions Investigate wave amplification in finite uniform MTL TWT
Investigate finite periodic TWTs, excitations, and termination
Develop the theory of interaction near a regular band edge (RBE) and a
degenerate band edge (DBE)
Analyze TWT MTL in time domain including nonlinear effects (software
development) 23
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Future Research Directions (continued)
0
0
z
b bt
b b tz t z t
b bt z t z z
b bt
t
uu
A D B V L R IA B V DV L R I
C G V E F I H IE H F I C G V
1 2 3
1 2 3 1
, , ,......, , , [ ,0]
, , ,......, , , [ , ...... ,0]
T T
N b b
T T
N b b 2 N
V V V V V
I I I I I s s , s
V a a
I s
0
0
0
0
, , , ,1 0 0 0
, , , ,1 0 0
T
b b b
b b
u
uu
I 0 0 0 L 0 R 0 I 0A B L R D a
0 0 0 0 0
C 0I 0 0 0 G 0 0 s
E F G C H00 0 0 0
Considering the following definitions
Time dependent equations can be written as
where
Finite Difference Time Domain (FDTD) analysis of MTL interaction with e-beam
* Collaboration with Mehdi Veysi, UC Irvine
Objective: investigate evolution of MTL TWT and study oscillations
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Thank you
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