Care and Feeding of Plasma Antennas
Transcript of Care and Feeding of Plasma Antennas
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The Care and Feeding of
Plasma Antennas28 Nov 01
Joseph Lee Cox
Graduate Student
Electrical Engineering Department
Graduate Engineering Research Center
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Outline
Basics of plasma physics
How is plasma defined?
Characteristics of plasma
Wave propagation in plasma media
Reflection and refraction
Topics of interest in wave propagation
Plasma Antennas
Confined space plasma antennas
Unconfined plasma antennas
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Outline
Basics of plasma physics
How is plasma defined?
Characteristics of plasma
Wave propagation in plasma media
Reflection and refraction
Topics of interest in wave propagation
Plasma Antennas
Confined space plasma antennas
Unconfined plasma antennas
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Plasma Defined
How is it created?
Ionization, of course!
Thermal ionization, RF and optical excitation
Electric fields (not magnetic)
General types of plasma
Electrons and protons, heavier ions, antimatter?
Thermal plasma and cold plasma
Isotropic/anisotropic,homogeneous/inhomogeneous
dont worry, just like any other dielectric
2
2
)(),(
ln
EZlnE
HZ
p. 52, Plasma Formulary
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Characteristics of Plasma
Max Says
Maxwells equations govern the fields
Lorentz equation governs the particles
Particles in motion generate fields that generate
motionElectric and magnetic fields areintertwined in the plasma
Key characteristic is the plasma frequency,
)( BvEqdt
vdm
D
t
BEBJ
t
DH
0
p. 19, Plasma Formulary
p
e
ep
m
qN 24
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Characteristics of Plasma
Particle approach to understanding plasma
Lorentz law determines motion of a charge in a field
Motion of plasma particles under applied E
Motion of plasma under applied B
Motion under crossed electric and magnetic fields
Motion under external forces
)(Eqdt
vdm
Different charges will move in opposite directionsgenerating an electric field that opposes the theimpressed field and a net current results.
)( Bvqdt
vdm
With nonzero velocity tangential to magnetic field,charges will spiral in opposite directions whilemaintaining the tangential velocity. Net current is zerowhile induced magnetic field opposes the applied field.
2
)(
B
BEvd
The drift velocity is perpendicular to both E and Bfields with zero net current (both charges traveling inthe same direction).
2
)(
B
Bg
q
m
vd
The drift velocity is similar to the above, yet charge isimportant. Positive and negative charges will travel in
opposite directions producing a net current in themedia opposite to the curl.
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Characteristics of Plasma
Particle approach (cont.) Statistical solution sometimes useful
Boltzman transport equation
a includes the effects of all noncollisional forcesand is the single particle distribution function
Numerically intensive for all but minute distr.
PIC, particle-in-a-cell, a mathematical model
Calculates the fields and potential of severalcells and in discrete time intervals approximatesplasma flow
collision
s
svs
s
t
ffafv
t
f
sf
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Characteristics of Plasma
Fluid approach to understanding plasma
Is plasma a fluid? Can it be contained?
Pressure is important, thermal (gas) and magnetic
Magneto-hydrodynamic approximation
Maxwell displacement current neglected
Models of plasma generally seek to understand
observations rather than predict new ones
BBvvtv
vt
1
0
BBBBB
2
1
2
1)(
1 2
The first equation is continuity, the secondNewton equation of motion with mechanicalpressure force density and magnetic forcedensity.
The magnetic force can then be written as thesum of the gradient of the magnetic pressure anda secondary tension.
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Characteristics of Plasma
Plasma containers, no Tupperware brands here
The ionosphere may be considered unconstrained
No physical container, but Earths fields are culprit
True unconstrained plasmas diffuse away
Magnetic bottles are used in fusion experiments
Other fusion reactors, stars, use gravity
What about ordinary fluorescent tubes?
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Characteristics of Plasma
Cutting edge uses of plasmas, an aside
Plasma deposition in semiconductor fabrication
Thermonuclear fusion, a never ending hallway
Will we care about the Middle East then?
Ion propulsion, slow acceleration wicked top speed
Laser power sources
The best use of plasma is
Manipulation of electromagnetic waves!
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Outline
Basics of plasma physics
How is plasma defined?
Characteristics of plasma
Wave propagation in plasma media
Reflection and refraction
Topics of interest in wave propagation
Plasma Antennas
Confined space plasma antennas
Unconfined plasma antennas
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Wave Propagation in Plasma
Development of the dispersion relationis the key!
Collision less, cold plasma with no B field
Note that nc, group velocity
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Wave Propagation in Plasma
Collisional, cold plasma (dense gas)
Effective frictional force on the electrons
Hard sphere approximation
Maxwell-Boltzman thermal velocity
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Wave Propagation in Plasma
Reflection and refraction
With index of refraction in hand, apply Snells Law
Refraction is the same as in a dielectric
For a cold plasma the dielectric has the form
Reflection occurs when , zero p gv
p
x
x
i
i
00
0
0
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Wave Propagation in Plasma
A linearly varying plasma density will produce aparabolic wave trajectory
Apex of parabola is where p
Lockwood, p. 31
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Wave Propagation in Plasma
Topics of interest in wave propagation
Space physicists use propagation to measure the depthand plasma densities of the Ionosphere
Ham radio operators frequently bounce
transmissions off of the Ionosphere and occasionallyduct through layers of plasma
Space shuttle loses radio connectivity during re-entryas the shuttle is cloaked in a plasma sheath
Hypersonic aircraft (under studyno Aurora)
Similar concerns as with the shuttle
Plasma aerodynamics may be holy grail to HST
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Outline
Basics of plasma physics
How is plasma defined?
Characteristics of plasma
Wave propagation in plasma media
Reflection and refraction
Topics of interest in wave propagation
Plasma Antennas
Confined space plasma antennas
Unconfined plasma antennas
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Plasma Antennas
Confined space plasma antennas are the focus here
Requirements for a plasma antenna
Must develop a sufficient plasma density
May provide a stimulation for plasma gyro-rotation
Must drive the antenna with a radiant source
Some antennas are simply lenses while othersactively contribute to the radiation
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HOW DO WE DRIVE A PLASMA ANTENNA?
Essentially similar to a conventional antennabut use a capacitive coupler
Additional power is needed to maintain the plasma
Plasma Antennas
PLASMA ANTENNAS: A Novel AntennaPARADIGM FOR Telecommunications and Radar
G.G. Borg, J.H. Harris, D.G. Miljakand I.V. Kamenski
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Plasma Antennas
Surface wave driven antennas rely on a uniqueproperty of plasma/dielectric interface
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PLASMA SURFACE WAVES ARE RADIALLYEVANESCENT
Comparison between a plasma and a metal
r = 1 - pe2/( + i) r = 1 - i0j = -i0(r-1)E j = E
The effect of plasma is similar to a metal. The wave
is guided with low penetration into the plasma.
Wave fields vary as ~ f(r) exp i(z-t)
Dispersion obeys: r T0 I1(Tpa) K0(T0a) + Tp K1(T0a) I0(Tpa) = 0
where Tp2 = 2 - r k02 andT02= 2 - k02
PLASMAANTENNAS:A
Nove
lAn
tenna
PARADIGM
FORTe
leco
mmun
ica
tionsan
dRa
dar
G.G.
Borg,
J.H.
Harr
is,D
.G.
Miljak
an
dI.V
.Kamens
ki
Plasma Antennas
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Plasma Antennas
Surface wave driven plasma antenna
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Plasma Antennas
Semiconductor driven plasma antenna
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Plasma Antennas
Electro-optic modulatedplasma antenna
As the modulated laser passesthrough the plasma it producespotential gradients the force the
plasma to oscillate at themodulated frequency. Thisantenna, therefore, can be usedto receive ELF and VLF signals
and is being considered fordeployment on submarines.
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Plasma Antennas
Advantages of a plasma antenna over metal antenna Non-conductor when turned off (stealth and EMI)
Quick turn on/turn off times (~msec)
Only use as needed Rapidly reconfigurable in wavelength and aperture
RF energy is not stored in antenna
Eliminates ringing when turned on/off
Disadvantages of a plasma antenna
Efficiency plasma attenuates wave energy
Expect ~50% efficiency, compared to 90+%
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Plasma Antennas
Can we apply antenna theory to design a plasmaantenna? Maybe
First determine desired angular spectrum
Inverse Fourier transform to determine aperture
Design plasma antenna to match aperture
Select E, plasma density throughout aperture
A field programmable plasma antenna
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Plasma Antennas
Unconfined plasma antennas
Some are accidental, and just plain nuisances
Space Shuttle returning to Earth
Engulfed in a plasma sheath, radio silence
Some are purposeful, and ingenious
Refraction of radar waves aroundaircraft
Psuedo-random reflectors (D. Kalluri, U Mass Lowell)
S
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Summary
We dusted off the plasma physics texts,
discovered some interesting wave phenomena
and revealed the glory of plasma antennas.
Plasma antennas are showing significant promise andreceiving the attention of commercial industries andthe military
A k l d
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Acknowledgements
Several sources were used (abused) for this seminar Lecture notes from AFIT courses
Plasma Physics, Dr. Bill Bailey
Ionospheric Electrodynamics, Major Devon Della-Rose
Introduction to Space Environment, Major Devon Della-Rose
Several excellent texts
Introduction to Ionospheric Physics, Rishbeth and Garriot
Plasma Physics, Sturrock
Classical Electrodynamics, Jackson
Electromagnetics of Complex Media, Kalluri
An enormous internet cache
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Backup Slides
Velocity parallel to magnetic field
Velocity normal to magnetic field
Separating above into drift and gyro
Rewriting normal velocity equation
Time-independent terms
Time-dependent terms
Final equation for crossed fields
p
pE
m
q
dt
dv
BvEm
q
dt
dv
)(tvvv gd
BvBvEm
q
dt
dvgd
g
0 BvE d
Bvmqdtvd
gg
2B
BEvd
Particle Motion in crossed electric and magnetic fields
B k Slid
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Equation of motion for uniform fields
Nearly identical to crossed E and B
Substitute above with effective E
Drift component of motion
)( Bvm
qg
dt
vd
BvEm
q
dt
dv
gqmEeff
2B
Bg
q
mvd
Particle motion in gravitational and magnetic fields
B k Slid
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Polarization drift
10 20 30 40
x
-10
-8
-6
-4
-2
y
B k Slid
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Transverse gradient drift
-2 -1.5 -1 -0.5
x
2
4
6
8
10
y