Thematic fields: 11 Basic sciences
Technological platform: PT-11 Mobile and wireless communications

Director,
Dr. Ovidiu Stoican

Contact:
e-mail: [email protected]
Tel: +40 (0)722308214

Affiliation:
National Institute for Plasma, Lasers, Plasma Physics (INFLPR)
ATOMISTILOR Str. Nr. 409, P.O.Box: MG-36, 077125 BUCHAREST, ROMANIA
Tel:  +40 1 457 44 89
Fax: +40 1 457 42 43

Organizations involved:

1. National Institute for Plasma, Lasers, Plasma Physics (INFLPR) - Coordinator
2. University of Bucharest
3. University of Pitesti
4. Horia Hulubei-National Institute of Physics and Nuclear Engineering (IFIN-HH)

The study of the energy transfer between a low temperature electrical gas discharge plasma and an external electromagnetic field.

Schedule:

01.08.2006-15.12.2006  Realization of the experimental setup aimed to study interaction the RF field-glow discharge plasma .
01.01.2006-30.07.2007  Systematic experimental studies on the interaction plasma- RF and microwave field.
01.07.2007-30.10.2007  Theoretical and experimental studies regarding  the effect of the excitation electromagnetic field on plasma.
01.11.2007-01.03.2008  Studies on the global efficiency of the plasma-RF field energy transfer.
02.03.2008-30.07.2008    Studies regarding effect of the RF field  frequency and geometry, respectively, on the energy transfer phenomena.
01.08.2008- 30.09.2008   Theoretical studies on the plasma-RF  field interaction. Systematization of the experimental data. Conclusions.

The main scientific results obtained in the frame of INFLPR are related to the plasma antenna technology and to the study of plasma-RF electromagnetic field applied to the plasma diagnose. Plasma antenna technology represents a novel approach in the field of the wireless communications. Conventional antennas employ the various structures of metallic conductors in order to transmit or to receive RF signals. In the case of the plasma antennas the metallic conductors are replaced by the ionized gas columns. One of the advantages of a plasma antenna is related to the operation in the pulsed regime. Because the gas can be ionized only during the signal transmission, certain transient effects (e.g. “ringing) owing to rf signal switch are avoided. This feature is interesting for various applications, especially in the areas of the radar technology or digital communication. Such studies are important in order to perform an appropriate design for the plasma antenna based communication systems. Two discharge tube geometries, equivalent of a whip antenna and a loop antenna, respectively, have been studied. Especially, the dc discharges have been studied. The dc electrical discharges have advantage to be easy obtaining and switching on/off. The researches regarding plasma antennas had three main objectives, namely, RF pulses forming, study of RF radiation due to the discharge switch and the effect of the excitation circuit on the plasma properties. Additionally, variations of the gain and VSWR for various experimental conditions have been measured. A simple, cheap and reliable method to obtain RF pulses is by inserting the discharge tube in a relaxation oscillator circuit. Thus, discharge tube operates simultaneously as plasma antenna, dc voltage switch and RF switch. Consequently, the RF current pulses carried by dc electrical discharge are generated without to use high voltage rated semiconductor devices. In the frame of the project, in order to improve RF pulses control, several relaxation oscillator topologies have been considered. Certain applications require the existence of two antennas, one operating as transmitter while the other operating as receiver for RF pulses. During transmission the receiver antenna operation must be inhibited. For this kind of applications a circuit topology based on relaxation oscillator principle have been developed. The circuit contains two discharge tubes acting as transmitter and receiver antenna, respectively, being equivalent to an astable circuit. One disadvantage is that the shape of RF pulse is very difficult to be controlled. The method is useful for the applications where the shape of the RF pulses is not critical. Another way to obtain RF pulses is to supply electrical discharge tube using a self oscillating flyback converter. The operating point of a flyback converter is self-adjusting so that the output voltage is matched to the operating regime of the discharge and no ballast resistor is necessary. This device can be also used in order to detect the discharge changes. It is known that electrical parameters of a self oscillating flyback converter (supply current, duty cycle, operating frequency) depend on the electrical characteristics of the load. By monitoring these parameters the variation of the discharge electrical characteristics can be detected. Another effect studied was the influence of the excitation RF field on the dc discharge properties. It was investigated the drift of a discharge tube relaxation oscillator period placed in a low intensity microwave field. A significant variation of the relaxation oscillator period has been observed. The experiments led to the conclusion that even RF fields much smaller than dc field can modify the discharge breakdown voltage. A simple method to detect the microwave fields using discharge tube represents a side application of this work. To be equal to performance the ionized gas acting as plasma antenna must exhibit the same properties as a metallic conductor, e.g. an uniform electrical conductivity. To obtain homogenous plasma it is necessary to employ special electrodes configurations. On the other hand it is known that in certain conditions electrical discharge plasma can radiate RF energy. The experimental study on the RF field characteristics due to the short current pulses flowing through a hollow cathode discharge tube has been performed. It has been demonstrated that such as electrical discharge act as RF signal sources. Experiments shown that RF field consists of a train of RF pulses associated to each current pulse through the discharge tube. The waveform of the voltage corresponding to these pulses is similar to that of an underdamped harmonic oscillation in a coupled inductive circuit. Several frequency peaks were found in the spectrum of the perturbation generated by the electrical discharge. The spectrum of such kind of perturbation has been acquired. As a rule only the region of the dc electrical discharge called positive column is appropriate to be used as a conductive element in a plasma antenna. The positive column is characterized by a high and spatial homogeneous electrical conductivity and also by a space charge equal to zero, similar to the metallic conductors. Most of experimental arrangements employ metallic sleeves in order to perform RF excitation of a plasma antenna. These sleeves slide along the discharge tube in order to be positioned over the positive column. Usually, to obtain a good coupling between RF excitation signal and plasma column, the two sleeves have a large surface being similar to a capacitor armature. Because at low frequencies the RF excitation circuit is equivalent to a short cut to the ground, it was evaluated the RF excitation circuit effect on the characteristics of the discharge plasma. The modifications of the plasma properties when the external electrode is connected to the ground are evidenced. The effect was detected using a relaxation oscillator circuit. The oscillations frequency for various operating conditions has been measured. The external electrode induces a significant drift of the oscillation frequency. Based on this experimental observation a method to switch a plasma antenna has been proposed. The study of the RF field-plasma interaction represents an important tool for plasma diagnosis. A simple and practical method aimed to investigate the spatial distribution of the RF field absorption by a plasma column in a wide frequency band has been developed. The method is based on the measurement of the amplitude variation of the RF field harmonics components due to the presence of a plasma volume. As a signal source is used a noise generator which generates a broadband RF field. The effect of the plasma on the RF field amplitude at different frequencies for various experimental conditions is observed by means of a spectrum analyzer. Compared to a RF sweep generator the noise generator is able to deliver a signal containing simultaneously all harmonics components. The RF sweep generator output has a well defined frequency, varying in time. This method has been tested using a dc electrical discharge as a plasma source. A modified version of the experimental setup intended to investigate the excitation of the RF harmonics in plasmas was performed. The major experimental results are summarized as follows: observation of the presence of RF absorption dips, variations of RF absorption dips as a function on discharge region and effect of the external supply circuit on the RF absorption dips magnitude.
During project progress it was necessary to build certain equipments and devices related to the applied electronics field which cannot be acquired from commercial providers. The most important were: various gate dip meter and autodyne circuits, system for the RF pulses frequency measurement, system based on optical and laser beams used to isolate galvanically the various equipment stages, high voltage power supplies. In order to explain some of experimental results, the team belonging with IFIN-HH performed theoretical studies regarding wave propagation through inhomogeneous media. An approximate method was devised for estimating scalar waves diffracted on various obstacles. The method can be applied to a semi-infinite circular pipe, a small aperture in an infinite screen and a semi-infinite plane screen. The method is based upon approximating the effect of the boundary conditions with surface distribution of sources. A theoretical model based on the investigation of an analogy between magnetic field and fluids equations has been also achieved. The researches focused on the Helmholtz vorticial solution which has been expressed using a given vorticial the analogy between the electromagnetism equations which provide magnetic field distribution as a function of currents distributions (Biot-Savart and Ampere laws) and vorticial motions of a fluid has been deeply studied. At University of Bucharest experimental studies on the plasma-RF field using a transmission line have been performed. The studies were aimed to evaluate the possibility to modify transmission line parameters by changing ionized region parameters placed in the vicinity of the transmission line. The effect of the plasma on the transmission line has been detected by measuring the variations of VSWR as function of the discharge parameters.
At University of Piteşti an experimental instalations intended to study interaction beetween the plasma and microwave field has been achieved. The aim of the studies was to evaluate the possibility of stimulating the emission of electromagnetic waves in the microwave range within a plasma column coupled with a pulsating double layer of electric charge. The experiments have shown the stimulation by the double layer of a longitudinal iono-acoustic stationary wave and occurrence of a microwave field in the centimetric domain. A phenomenological scenarios concerning the generation of complexity in the material media that are far from the state of thermodynamic equilibrium has been also proposed.
 

 O. S. Stoican, Study of a dc glow discharge supplied by a flyback converter, 6^th International Conference of the Balkan Physical Union-BPU-6, August 22-26, 2006, Istanbul, Turkey, Book of Abstracts, p.937 (8-P-022)