Plasma AntennasPlasma Antennas

Igor Alexeff and Ted Anderson gUniversity of Tennessee

Haleakala Research and Development Inc*. Work supported by Phase 2 SBIR Grants from

1. the US Army (contract number W15QKN-06-C-0081)

2. US Air Force (contract number FA9453-06-C-0068).

Haleakala R&D, Inc and University of Tennessee

The NPSS Distinguished Lecturers ProgramThe NPSS Distinguished Lecturers Program

• Provides high quality scientific and technical lectures on a broad range of topics in the nuclear and plasma sciencesa d p as a sc e ces

• Sponsors the presentation of lectures to NPSS-affiliated chapters, IEEE sections, and IEEE student chapters

• Makes lectures available to other IEEE entities as well as to non-IEEE organizations, including

ll d i iticolleges and universities

Haleakala R&D, Inc and University of Tennessee

What is a plasma antenna?

A plasma antenna is a column of ionized gas in which the free electrons emit, absorb and reflect radio signals just as the free electrons in a metal antenna.

Why use a plasma antenna?

1 The plasma antenna can be made to appear and disappear in milliseconds1. The plasma antenna can be made to appear and disappear in milliseconds.

2. The plasma antenna has an adjustable high-frequency cut – off. It can transmit and receive low – frequency signals while not interacting with high –transmit and receive low frequency signals while not interacting with high frequency signals.

3. The plasma antenna can under special circumstances be made operational in3. The plasma antenna can under special circumstances be made operational in microseconds.

4. The plasma antenna under special circumstances has less thermal noise p pthan a metal antenna.

5. Other applications include plasma lenses and plasma prisms.

Haleakala R&D, Inc and University of Tennessee

Copy of first plasma antenna.

A prototype plasma antenna.

The radio receives music only when the plasma antenna is on.

Haleakala R&D, Inc and University of Tennessee

Haleakala R&D, Inc and University of Tennessee

Haleakala R&D, Inc and University of Tennessee

Original project-plasma waveguide (closing switch) with PhD student Weng Lock Kangwith PhD student Weng Lock Kang.

Haleakala R&D, Inc and University of Tennessee

Ri ht l t i t ll d i l t i l h i h b• Right - plasma antenna installed in an electrical anechoic chamber• Left - metal antenna designed to be an identical twin to the plasma antenna• The microwaves are generated by a line antenna, focused in one dimension

Haleakala R&D, Inc and University of Tennessee

g yby the metal pillbox, and focused in the second dimension by either the plasma antenna or a metal twin

Radiation Pattern – Previous Work

Haleakala R&D, Inc and University of Tennessee

Demonstration of high frequency cut off.

Receiving Plasma Tubes

horn

8 Ghz Transmitter

Panoramic Receiver

1.7 GHz Transmitter

Haleakala R&D, Inc and University of Tennessee

On the upper trace, a high frequency penetrates a plasma barrier, while a lower frequency signal is cut off.

Haleakala R&D, Inc and University of Tennessee

AdvancesAdvances

1. We have produced a computer controlled intelligentplasma antenna.

2. We have demonstrated that plasma windows can openin microseconds.

3. We have found that plasma thermal noise can be lesspthan in a metal antenna.

Haleakala R&D, Inc and University of Tennessee

Computer controlled intelligent plasma antenna

Haleakala R&D, Inc and University of Tennessee

Opening a plasma window in microseconds.

• The boundary condition at a vacuum-plasma interface is given as follows:plasma interface is given as follows:

)1( EiE β−

Wh i th i id t l t i fi ld i th

0)1

( Ei

Er ββ

+=

• Where is the incident electric field, is the reflected electric field, and.

2ω12 −=

ωω

β p

Haleakala R&D, Inc and University of Tennessee

ω

When the plasma ring is completely energized, it becomes a cavity resonator.

Haleakala R&D, Inc and University of Tennessee

Normal signal cut-off

Haleakala R&D, Inc and University of Tennessee

Signal cut-off showing resonance transmission

Haleakala R&D, Inc and University of Tennessee

Thermal Noise

People claim that thermal noise obviously must be excessive in plasma antennas.p y pThe Nyquest formula states that the noise power is proportional to temperature and plasmas are obviously much hotter than metals.

However, the Nyquest formula is an approximation, and assumes that the electron collision rate is much higher than the applied frequency. This is not always true in a plasma.

If the collision rate is smaller that the applied frequency, the noise in this frequency range is greatly reduced.

Haleakala R&D, Inc and University of Tennessee

The conventional equation for thermal noise in a resistor is given below.This is the Nyquest formula

RKTπThis is the Nyquest formula.

RKT2

However, in a plasma antenna, the equation is modified as shown below.Here the electron collision rate is comparable to the applied frequency.

)21(

2 2RKT

πυπ

))2(1(2 2

cυπυ

+

The result is that at high frequencies, the plasma antenna has less thermal noise than a metal antenna,

Haleakala R&D, Inc and University of Tennessee

Use of Ramsauer GasesUse of Ramsauer Gases

The Ramsauer gases, argon, krypton and xenon, are often used in plasma t btubes.

The primary reason for use is that they are chemically inert, and do not attack the plasma tube and the electrodesthe plasma tube and the electrodes.

However, the Ramsauer gases also have an abnormally small electron scattering cross section in the region of a few electron voltsscattering cross section in the region of a few electron volts.

This is a quantum mechanical effect. The electrons are diffracted around the atoms.atoms.

Therefore, use of the Ramsauer gases in plasma antennas helps to reduce the thermal noise.

Argon (with a little mercury vapor) is the gas used in fluorescent lamps.

Haleakala R&D, Inc and University of Tennessee

Use of Pulsing DriveUse of Pulsing Drive

We have found that if we supply the electrical current in microsecond pulses i t d f DC bt i h hi h l d it t thinstead of DC, we can obtain much higher plasma density at the same average Input power. This allows us to operate at much higher plasma densities without thermally overloading the plasma tubes.

This results in additional noise reduction, since the plasma tubes carry current for microseconds, but the plasma survives for milliseconds. Hence, For 99 9% of the time the plasma tubes do not have the opportunity ofFor 99.9% of the time, the plasma tubes do not have the opportunity of generating current induced instabilities.

The result is less plasma noise.

Haleakala R&D, Inc and University of Tennessee

Plasma Tubes (12 Total)Total)

Spark Gap 1.5 * 106 ohm

10-9FFan to extinguish arc

Haleakala R&D, Inc and University of Tennessee

Haleakala R&D, Inc and University of Tennessee

Haleakala R&D, Inc and University of Tennessee

Haleakala R&D, Inc and University of Tennessee

AdvancesAdvances

1. We have produced a computer controlled intelligentplasma antenna.

2. We have demonstrated that plasma windows can openin microseconds.

3. We have found that plasma thermal noise can be lessthan in a metal antenna in certain frequency regions.

Haleakala R&D, Inc and University of Tennessee

ReferencesReferences

Pl A t G G B t Al Ph Pl 7 2198 (2000) IPlasma Antennas – G.G. Borg et. Al., Phys. Plasmas 7, 2198, (2000).; I. Alexeff et. Al., IEEE Trans. Plasma Sci., vol. 34, no. 2, pp 166-172, April 2006; Igor Alexeff et. Al., Phys. Plasmas 15, 1, 2008.

Plasma Lenses - P. Linardakis, Borg., G. and Martin, N. Electron. Lett. 42, 444 (2006).

Plasma Frequency Selective Surfaces – I. Alexeff et al., IEEE Trans. Plasma Sci., vol. 35, no. 2, pp 407-415, April 2007.April 2007.

Haleakala R&D, Inc and University of Tennessee

Haleakala R&D, Inc and University of Tennessee

Haleakala R&D, Inc and University of Tennessee