Igor Alexeff and Ted Anderson University of Tennessee. Haleakala Research and Development Inc *. Work supported by Phase 2 SBIR Grants from

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1 Plasma Antennas Igor Alexeff and Ted Anderson 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 FA C- 0068).

2 The NPSS Distinguished Lecturers Program Provides high quality scientific and technical lectures on a broad range of topics in the nuclear and pas plasma a sce sciencesces Sponsors the presentation of lectures to NPSSaffiliated chapters, IEEE sections, and IEEE student chapters Makes lectures available to other IEEE entities as well as to non-ieee organizations, including colleges and universities

3 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 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 frequency signals. 3. The plasma antenna can under special circumstances be made operational in microseconds. 4. The plasma antenna under special circumstances has less thermal noise than a metal antenna. 5. Other applications include plasma lenses and plasma prisms.

4 Copy of first plasma antenna.

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

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

9 Right -plasma antenna installed in an electrical l 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 by the metal pillbox, and focused in the second dimension by either the plasma antenna or a metal twin

10 Radiation Pattern Previous Work

11 Demonstration of high frequency cut off. Receiving horn Plasma Tubes 8 Ghz Transmitter Panoramic Receiver 1.7 GHz Transmitter

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

13 Advances 1. We have produced a computer controlled intelligent plasma antenna. 2. We have demonstrated that plasma windows can open in microseconds. 3. We have found that plasma thermal noise can be less than in a metal antenna.

14 Computer controlled intelligent plasma antenna

15 Opening a plasma window in microseconds. The boundary condition at a vacuum- plasma interface is given as follows: E r = 1 i β ( ) E 1+ iβ Where is the incident id electric field, is the reflected electric field, and. 0 2 p = ω β ω 2 ω 1

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

17 Normal signal cut-off

18 Signal cut-off showing resonance transmission

19 Thermal Noise People claim that thermal noise obviously must be excessive in plasma antennas. The 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.

20 The conventional equation for thermal noise in a resistor is given below. This is the Nyquest formula. π 2 RKT However, in a plasma antenna, the equation is modified as shown below. Here the electron collision rate is comparable to the applied frequency. π 1 ( 2 RKT 2 πυ (1 + ( ) 2 2 υ c ) ) The result is that at high frequencies, the plasma antenna has less thermal noise than a metal antenna,

21 Use of Ramsauer Gases The Ramsauer gases, argon, krypton and xenon, are often used in plasma tubes. The primary reason for use is that they are chemically inert, and do not attack the 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 volts. This is a quantum mechanical effect. The electrons are diffracted around the 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.

22 Use of Pulsing Drive We have found that if we supply the electrical current in microsecond pulses instead of DC, we can obtain much higher h 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% 9% of the time, the plasma tubes do not have the opportunity of generating current induced instabilities. The result is less plasma noise.

23 Plasma Tubes (12 Total) Spark Gap 1.5 * 10 6 ohm Fan to extinguish arc 10-9 F

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27 Advances 1. We have produced a computer controlled intelligent plasma antenna. 2. We have demonstrated that plasma windows can open in microseconds. 3. We have found that plasma thermal noise can be less than in a metal antenna in certain frequency regions.

28 References Plasma Antennas G.G. G Borg et. Al., Phys. Plasmas 7, 2198, (2000).; I. Alexeff et. Al., IEEE Trans. Plasma Sci., vol. 34, no. 2, pp , April 2006; Igor Alexeff et. Al., Phys. Plasmas 15, 1, 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 , April 2007.

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