Eric P. Pradeep

Experimental and theoretical results with plasma antennas

This report is a summary of an extensive research program on plasma antennas. We have found that plasma antennas are just as effective as metal antennas. In addition, they can transmit, receive, and reflect lower frequency signals while being transparent to higher frequency signals. When de-energized, they electrically disappear. Plasma noise does not appear to be a problem.

An Operating Intelligent Plasma Antenna

Summary form only given. We have developed and demonstrated a prototype for an intelligent plasma antenna. The object is to have an antenna observe a designated transmitter, while disregarding unwanted signals coming in from other azimuth angles. In this way, both unwanted background noise and multi-path reception clutter are reduced. The advantage of a plasma antenna over a scanning mechanical antenna is that the plasma antenna is electrically reconfigurable, can operate at high speeds and has no moving parts. The unit operates at about 2.5 GHz, near the operating frequencies of many cell phones. A ring of plasma tubes operating beyond microwave cut-off surrounds a metal transmitting antenna. A computer de-energizes a plasma tube, causing a lobe of microwave radiation to be emitted. Sequentially de-energizing the plasma tubes causes the radiation lobe to scan in azimuth. When a receiving antenna is detected, the computer ceases scanning, and locks onto the receiving antenna. When the receiving antenna is disconnected, the computer recommences scanning, looking for another receiving antenna.

Biological Decontamination Using an Atmospheric Pressure Resistive Barrier Plasma Discharge

The following paper discusses the observations of the surface plasma treatment conducted on Escherichia coli, Pseudomonas fluorescens (5RL), and Wild type standard Lambdaphage. The tests were aimed to determine the sterilization efficiency of the dc steady state atmospheric pressure with a water cooled resistive barrier discharge apparatus. The results achieved were encouraging and 100% decontamination was observed within the first ten minutes of plasma exposure. The decontamination efficiency was however raised significantly through the introduction of hydrogen peroxide in tandem with the plasma treatment. The combination resulted in a 100% kill rate of E. coli in under a minute and Lambdaphage in less than five minutes. Further tests involving electrostatic filters and nitrogen gas were conducted to isolate and probe the effects of ozone and charged ingredients in the decontamination process. Some interesting preliminary plasma treatment test data were also obtained on spores (Bacillus stearothermophilus).

Sterilization of Spores Using a Direct Current Steady State Atmospheric Pressure Plasma Discharge Apparatus

This paper investigates the efficacy of the atmospheric pressure plasma discharge apparatus in sterilization of spores. The plasma sterilizer was tested on a highly resistant spore producing bacterial strain, Geobacillis stearothermophillus. Tests were conduced with a resistive barrier atmospheric pressure barrier discharge made conducting with a 30 % hydrogen peroxide solution. The results showed a 90–100 % kill rate (CFU counts) within the range of 3–5 minute exposure intervals. The experimental findings are in good agreement with earlier tests conducted on Escherichia coli and Wild type standard Lambdaphage using the same device.

Experimental results of plasma antennas

Summary form only given. Considerable progress has been made on plasma antennas of which the major advances are: operation at higher plasma densities in the steady state, considerable reduction of power consumption and reduction of noise from the electrical current, which generates the plasma. We have performed experiments concerning transmission and reception, stealth, reconfigurability, shielding, protection from electronic warfare, mechanical robustness, mechanical reconfigurability, plasma waveguides and noise reduction of plasma antennas. In the past, our plasma tubes were ionized by steady state DC current. If the tubes are ionized by extremely short bursts of DC current, we find that the plasma is produced in an extremely short time of about 2 microseconds. However, the plasma persists for a much longer time of about 1/100 second. This is the reason why fluorescent lamps can operate on 60 or 50 Hz electric power. In the new mode of operation, we observe that the plasma density produced by the pulsed power technique is considerably higher than the plasma density produced by the same power supplied in the steady state, which produces two beneficial results: we can operate at much higher plasma densities and at several giga Hertz. In addition, we can operate during the long, non-current carrying phase, which should not have noise generated by current-driven instabilities. We have also operated our plasma antennas at several megawatts using a spark-gap-driven separate RF power supply. We find that even at very high power levels, the plasma antenna operates as efficiently as a metal antenna. We also find that with the proper operating mode, the plasma antenna will not ignite even in the presence of a megawatt RF field. In conclusion, our recent inclusion of a pulsed power supply for our plasma tubes provides reduced noise, higher steady state DC plasma density and reduced power consumption. There are possibly minor problems because of a slight plasma density fluctuation during the pulsing cycle, which will be addressed in the future work

Plasma Tubes Intercept Micorwave Radiation Independent of Polarization

Summary form only given. We have been designing a plasma shield intended to protect sensitive microwave equipment from intense electronic warfare signals. A layer of plasma tubes is used as a microwave reflector. The plasma tubes work extremely well in intercepting microwave radiation when the incident wave electric field is parallel to the tubes. However, if the electric field is perpendicular to the tubes, the normally induced plasma current cannot flow, and the plasma effects are not expected to appear. To our surprise, when the plasma tubes were experimentally tested with the electric field perpendicular to the tubes, the plasma tubes not only intercepted the microwave signal, but the observed cut-off with a pulsed plasma lasted about twice as long. The effect appears to be due to an electrostatic resonance, and preliminary calculations suggest that a normally ignored term in Maxwells equations is responsible. nabla2E - nabla(nablaldrE) = mu0epsiv0(omegap 2 + part2E/partt2). The term responsible appears to be nabla(nablaldrE).The frequencies involved are in the range of 2.5 GHz. The plasma tubes used are conventional fluorescent lamps. In any event, regardless of the theory, the experimental results demonstrate that shielding a sensitive receiving antenna by a layer of parallel plasma tubes is considerably more effective than had been expected.

A Plasma Microwave Barrier That Opens in Microseconds

Summary form only given. Plasma barriers are used to protect sensitive microwave apparatus from potentially damaging electronic warfare signals. Unfortunately, the characteristic decay time of the plasma after power turn-of is typically many milliseconds, so the opening time of such a barrier generally is predicted also to be many milliseconds. However, we have found both experimentally and theoretically that we can open such a barrier on a time scale of microseconds. We do this by increasing the plasma density rather than waiting for it to decay. We have two layers of plasma. We produce a standing wave between the two layers that results in microwave transmission, analogous to the transmission found in an optical Fabry-Perot Resonator. The secret lies in the boundary layer behavior of the plasma. Once microwave cutoff occurs, one would expect the plasma behavior to be static. What actually occurs is that at microwave cut-off, the reflection is in phase with the incident wave, in analogy to an open coaxial line. (The electron and displacement currents are equal, but out-of-phase.) As the plasma density further increases, the reflection smoothly changes from in-phase to 180 degrees out-of-phase, in analogy to a shorted coaxial line. (The electron current is much greater than the displacement current.).

Recent diagonostic results for the steady state - DC atmospheric pressure plasma discharge

Summary form only given. We have made the following advances for our DC, steady state atmospheric pressure plasma discharge in air. We have measured the free electron concentration in our discharge by converting the 1/2 cm. gap, parallel-plate discharge into a parallel-plate transmission line. The frequency used was 6 GHz, corresponding to a free-space wavelength of 15 cm. The path length for the microwaves in the plasma was about 15 cm. The discharge was pulsed to high current (about 50 mA) using a capacitor bank and a spark gap. Under these conditions, a collisional attenuation of 20% was observed, yielding a free electron concentration of about 1 exp 12 electrons per cc. Extrapolating to the usual steady-state discharge operating conditions of 10 mA, we conclude that the normal operating free electron density is about 2 exp 11 electrons per cc, agreeing with earlier probe estimates. Note that at atmospheric pressure and the frequency used, the plasma is resistive, not reactive. We note that the attenuation decays in time on the order of 10 microseconds after each pulse. We attribute this decay of the attenuation to the recombination of the free electrons to form negative ions. Previous probe experiments reveal negative-positive ion plasma of 1 exp 12 ions per cc. We have replaced the water in our water-loaded conducting ceramic electrode with a 30% hydrogen peroxide solution. The net result is that a multisecond second exposure completely eliminates all E-coli bacteria on our test Petri dishes, although we have increased the initial bacteria concentration by a factor of one million (over one hundred million initial bacteria per plate). We have found that the sterilizing effect penetrates through normally sealed mailing envelopes. In conclusion, we have found that the free electron density in DC atmospheric pressure air plasma can be measured easily using a transmission line technique. Also using hydrogen peroxide instead of water in the moistened ceramic electrode can enhance the sterilizing effect of the DC atmospheric pressure discharge

Advances in biological decontamination using a direct current steady state atmospheric pressure plasma discharge apparatus

Summary form only given. Recent advances in bioelectronics have made optimal reusable heat sensitive tools and smart materials, a reality in surgical instruments and catheters. Traditional sterilization methods involving high temperature, pressure and long down times renders it ineffective and unsuitable for such newer breed of instrumentation. The paper illustrates the test results of an effective alternative solution, involving the usage of a non thermal atmospheric pressure resistive barrier plasma discharge for biological decontamination. The discharge apparatus was found to be rapid, effective and portable. Its simple design eliminates the need for any operational expertise in plasma or high voltage sciences. The sterilizer was tested extensively with Escherichia coli and the results indicated 100% decontamination within the first four minutes of plasma exposure. The requisite exposure period was dramatically reduced further by 75% to one minute when the water in the resistive barrier electrodes was replaced with a solution of 30% hydrogen peroxide. Simple tests involving electrostatic filters also revealed evidence of presence of charged/activated ingredients in the generated plasma and plasma could also diffuse through sealed business envelopes to decontaminate infected samples. The plasma discharge was found to be effective for reasonable periods even after the electric current was turned off, thereby significantly lowering power consumption levels. Thus the plasma sterilizer was profiled and tested, while adding further value through important findings such as the presence of charged/activated ingredients and effectiveness of the hydrogen peroxide

Ball lightning project

Summary form only given. In the following up of our initial work on ball lightning, we have developed a special camera for photographing the initial stroke of a lightning discharge. The object is to obtain time resolved photographs of the closed current loops observed in sparkovers in the Holifield 25 MV, DC accelerator at the Oak Ridge National Laboratory. The apparatus comprises a commercial lightning photographic trigger that opens a camera shutter on reception of a light pulse from a lightning discharge. In the commercial configuration, the inherent time delay in the cameras shutter mechanism causes the camera to miss the initial lightning stroke. However, in nature, lightning in general has several strokes, and so the camera captures the second and subsequent strokes. In our modification, the lightning image is focused on a semi-transparent disc coated with a phosphor having a time constant of about a second. This disc provides a temporary image storage mechanism while the camera shutter is being activated. Test photographs of pulsed light sources have been quite successful. We anticipate on having experimental lightning results at the forthcoming ICOPS meeting