care and feeding of plasma antennas

8/7/2019 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







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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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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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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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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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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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







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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Collisional, cold plasma (dense gas)

Effective frictional force on the electrons

Hard sphere approximation

Maxwell-Boltzman thermal velocity


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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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A linearly varying plasma density will produce aparabolic wave trajectory

Apex of parabola is where p

Lockwood, p. 31


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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







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


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

B k Slid


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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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Backup Slides

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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Backup Slides

Polarization drift

10 20 30 40

x

-10

-8

-6

-4

-2

y

B k Slid


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Backup Slides

Transverse gradient drift

-2 -1.5 -1 -0.5

x

2

4

6

8

10

y

care and feeding of plasma antennas

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