Table of contents

The First International Workshop and Summer School on Plasma Physics (IWSSPP'05) organized by The Faculty of Physics, University of Sofia and the Foundation `Theoretical and Computational Physics and Astrophysics' was dedicated to the World Year of Physics 2005 and held in Kiten, Bulgaria, on the Black Sea Coast, from 8--12 June 2005. The aim of the workshop was to bring together scientists from various branches of plasma physics in order to ensure an interdisciplinary exchange of views and initiate possible collaborations. Another important task was to stimulate the creation and support of a new generation of young scientists for the further development of plasma physics fundamentals and applications.

This volume of Journal of Physics: Conference Series includes 31 papers (invited lectures, contributed talks and posters) devoted to various branches of plasma physics, among them fusion research, kinetics and transport phenomena in gas discharge plasmas, MHD waves and instabilities in the solar atmosphere, dc and microwave discharge modelling, plasma diagnostics, cross sections and rate constants of elementary processes, material processing, plasma-chemistry and technology. Some of them have been presented by internationally known and recognized specialists in their fields; others are Masters or PhD students' first steps in science. In both cases, we believe they will stimulate readers' interest.

We would like to thank the members of both the International Advisory Committee and the Local Organizing Committee. We greatly appreciate the financial support from the sponsors: the Department for Language Teaching and International Students at Sofia University, Dr Ivan Bogorov Publishing house, and Artgraph2 Publishing house. We would like to express our gratitude to the invited lecturers who were willing to pay the participation fee. In this way, in addition to the intellectual support they provided by means of their excellent lectures, they also supported the school financially.

The EURATOM fusion research and development programme is a well integrated and coordinated programme. It has the objective of ''developing the technology for a safe, sustainable, environmentally responsible and economically viable energy source.'' The programme is focussed on the magnetic confinement approach and supports 23 Associations which involve research entities (many with experimental and technology facilities) each having a bilateral contractual relationship with the European Commission. The paper will describe fusion reactions and present their potential advantages as an energy source. Further, it will describe the EURATOM programme and how it is organised and implemented. The success of the European programme and that of other national programmes, have provided the basis for the international ITER Project, which is the next logical step in the development of fusion energy. The paper will describe ITER, its aims, its design, and the supporting manufacture of prototype components. The European contribution to ITER, the exploitation of the Joint European Torus (JET), and the long-term reactor technology R&D are carried out under the multilateral European Fusion Development Agreement (EFDA).

We discuss the effect of shearing on transport and the formation of transport barriers. The focus is primarily on laboratory plasmas where the formation of transport barrier (the L-H transition or the formation of internal transport barrier) is thought to originate from turbulence regulation by shearing by (coherent) mean E × B flows and (random) zonal flows. We provide quantitative discussion on the reduction of turbulent transport by these flows and elucidate their roles in the barrier formation.

Nonlinearity is a direct consequence of large scale dynamics in the solar plasmas. When nonlinear steepening of waves is balanced by dispersion, solitary waves are generated. In the vicinity of resonances, waves can steepen into nonlinear waves influencing the efficiency of energy deposition. Here we review recent theoretical breakthroughs that have lead to a greater understanding of many aspects of nonlinear waves arising in homogeneous and inhomogeneous solar plasmas.

The processes of plasma self-organization and formation of plasma structures at reduced pressures is considered with respect to the electrode microwave (2.45 GHz) discharge (EMD). The term EMD is attributed to discharge maintained near the ending of powered electrode/antenna if plasma region is less than characteristic dimensions of discharge chamber. EMD is generated at gas pressures ranged between 0.5 and 400 Torr, plasma gases are Ne, Ar, H₂, N₂, O₂, air and mixtures containing CH₄ C₂H₂. Electrodes/antennas of different shapes were used to ignite the discharge: solid and tube cylindrical antennas with diameter of 1-6 mm, needle, spiral and trident antennas etc. Plasma absorbed power is less than 30 W. Phenomenology of EMD is presented. Results of emission spectroscopy and probe measurements together with results of EMD modelling are described. The concept of ''selfsustained - non-selfsustained'' mechanism of EMD is concluded from analysis of publications on microwave discharges and results of study of EMD. Areas of possible EMD application are discussed.

The study of the competition between equilibrium disturbing and equilibrium restoring mechanisms, reveals that the various equilibrium departures as found in different plasmas, have much in common. They can be seen as disturbances of bilateral relation; relations effectuated by forward and corresponding backward processes in the sense of detailed balancing. The deviations from the equilibrium form of the atomic state distribution function and the electron energy distribution function of atomic plasmas, which are the result of the escape of photons and electron-ion pairs, can be given in a simple equation in which the escape per balance time plays a leading role. The same idea can be used to construct a characterisation method that relates external control parameters to average values of internal plasma properties such as the electron density, electron temperature and the gas temperature. The global discharge model presented here includes gas heating and is applicable to atomic plasmas for which convection can be neglected.

Disturbed Bilateral Relations offer a way of categorizing plasma processes by their deviation from equilibrium. This method can be used to create a simple plasma model that solves key plasma parameters, namely the electron temperature, electron density and heavy particle temperature. An implementation of this method is presented, and the results validated against a detailed plasma simulation, for a wide range of parameters.

The second-derivative Langmuir probe method for precise determination of the plasma potential, the electron energy distribution function (respectively the electron temperature,) and the electron density of gas discharge plasma at intermediate pressures (100-1000 Pa) is reviewed. Results of applying the procedure proposed to different kinds of gas discharges are presented. Factors affecting the accuracy of the plasma characteristics evaluated are discussed.

The use of the Stark broadening of Balmer lines spontaneously emitted by atmospheric-pressure plasmas as a method to determine both the electron density and temperature in high-pressure plasmas is discussed in this paper. This method is applied to argon and helium plasmas produced in microwave discharges. Especially for Ar plasmas, valuable and reliable results are obtained.

When an electric discharge is fired in a gas consisting of atoms or molecules with long-lived excited states there are several inelastic processes releasing fast electrons in the afterglow plasma after the end of the current pulse. Most common among them are the chemiionization and de-excitation electron-atom collisions. Here these processes are investigated by measurements of the reactant particle densities at the entrance and at the exit channels of the reactions. In this way the rate coefficients of the ionising collisions of the couples Ne³P₂-Ne³P₂, Ar³P₂-Ar³P₂, Kr³P₂-Kr³P₂, Xe³P₂-Xe³P₂ as well as the rate coefficients for chemi-ionization of the ground state mercury atoms by collision with Ne 3³P₂ and Ar 4³P_{2, 1, 0} atoms were obtained. By similar measurements the rate coefficients for super elastic collisions between electrons and excited heavy rare gases atoms as well as mercury atoms in the electron temperature range 500 - 2000 K were obtained. By monitoring the decay of the excited atom's density and electron's density and temperature the rate coefficients of electron impact induced transfer of excitation between the levels of configuration np⁵(n + 1) s in Ne, Kr and Xe were determined. In the same way were derived the rate coefficients of the transfer of excitation between Cd 6³P_{2, 1, 0} levels induced by collisions with ground state cadmium and neon atoms. The diffusion coefficients of magnesium atoms in rare gases and their reflection coefficients from the vessel metallic wall are determined. Besides, the diffusion coefficients of metastable Cd 6³P_{2, 0} atoms in neon are also determined.

The method of disturbed Bilateral Relations (dBR) is used to characterize highly transient plasmas that are used for the generation of Extreme Ultra Violet (EUV), i.e. radiation with a wavelength around 13.5 nm. This dBR method relates equilibrium disturbing to equilibrium restoring processes and follows the degree of equilibrium departure from the global down to the elementary plasma-level. The study gives global values of the electron density and electron temperature. Moreover, it gives a method to construct the atomic state distribution function (ASDF). This ASDF, which is responsible for the spectrum generated by the discharge, is found to be far from equilibrium. There are two reasons for this: first, systems with high charge numbers radiate strongly, second the highly transient behaviour makes that the distribution over the various ionization stages lags behind the temperature evolution.

Computer aided design (CAD) based on numerical experiments performed by using adequate physical models and efficient simulation codes is an indispensable tool for development, investigation, and optimization of gyrotrons used as radiation sources for electron cyclotron resonance heating (ECRH) of fusion plasmas. In this paper, we review briefly the state-of-the-art in the field of modelling and simulation of intense, relativistic, helical electron beams formed in the electron-optical systems (EOS) of powerful gyrotrons. We discuss both the limitations of the known computer codes and the requirements for increasing their capabilities for solution of various design problems that are being envisaged in the development of the next generation gyrotrons for ECRH. Moreover, we present the concept followed by us in an attempt to unite the advantages of the modern programming techniques with self-consistent, first-principles 3D physical models in the creation of a new highly efficient and versatile software package for simulation of powerful gyrotrons.

Quasi-optical (QO) mode converters are used to transform electromagnetic waves of complex structure and polarization generated in gyrotron cavities into a linearly polarized, Gaussian-like beam suitable for transmission. The efficiency of this conversion as well as the maintenance of low level of diffraction losses are crucial for the implementation of powerful gyrotrons as radiation sources for electron-cyclotron-resonance heating of fusion plasmas. The use of adequate physical models, efficient numerical schemes and up-to-date computer codes may provide the high accuracy necessary for the design and analysis of these devices. In this review, we briefly sketch the most commonly used QO converters, the mathematical base they have been treated on and the basic features of the numerical schemes used. Further on, we discuss the applicability of several commercially available and free software packages, their advantages and drawbacks, for solving QO related problems.

In this work we present a model of argon microwave sustained discharge at high pressure (1 atm), which includes two self-consistently linked parts - electrodynamic and kinetic ones. The model is based on a steady-state Boltzmann equation in an effective field approximation coupled with a collisional-radiative model for high-pressure argon discharge numerically solved together with Maxwell's equation for an azimuthally symmetric TM surface wave and wave energy balance equation. It is applied for the purpose of theoretical description of the discharge in a stationary state. The phase diagram, the electron energy distribution function as well as the dependences of the electron and heavy particles densities and the mean input power per electron on the electron number density and wave number are presented.

Penning collisions determine the creation of population inversion and the output parameters in a number of ion metal vapor lasers. Some different gaseous additives to the buffer gas also improve metal vapor laser operation via Penning impacts. Assuming that the process takes place by electron exchange between metastable and target atom, cross-sections and rate constants for Penning ionization into ion ground state are calculated for the metastable atoms He and Ne, and 22 target metal atoms, which are active particles in the metal vapour lasers. Cross-sections and rate constants for Penning ionization into excited ion states of the copper and bromine atoms are also calculated.

Trifluoromethane (CHF₃) is widely used in semiconductor processing. However, electron collision cross section set in CHF₃ is not finally completed and verified yet. Cross section sets developed before were tested only in pure CHF₃ and calculations with these sets do not describe swarm parameters in CHF₃ diluted by Ar. In this work a numerical code based on the Monte Carlo method has been developed and used to calculate swarm parameters in pure CHF₃ and Ar/CHF₃ mixtures. Calculations in CHF₃ diluted in Ar allowed us to validate the absolute value of vibration cross sections. The values and the shapes of momentum transfer, attachment and dissociation cross sections in CHF₃ were renormalized on the base of the thorough analysis of all available experimental data and quantum-mechanical calculations. As a result physically well-grounded cross section set for CHF₃ was developed and verified on various swarm experiments in CHF₃ and CHF₃/Ar mixtures.

The phenomenon of energy conversion in the streams of directed electrons and ions into electromagnetic radiation was found out experimentally and proved theoretically. The direct proofs of the domain mechanism of the charged particles acceleration and mechanism of ball lightning generation were obtained and the theoretical calculations were refined.

The paper discusses a new configuration of the surface-wave sustained plasma - ''the coaxial structure''. The coaxial structure is investigated on the base of one-dimensional axial fluid model. That model is adequate enough for low pressure plasma, when the main process for charged particles production is the direct ionization from the ground state and the loss of electrons is due to diffusion to the wall. The role of the geometric factors is evaluated and discussed, varying the discharge conditions in the theoretical model. The main equations of the model - the local dispersion relation and the wave energy balance equation are obtained from Maxwell's equations with appropriate boundary conditions. The phase diagrams, the radial profiles of the electric field and the axial profiles of dimensionless electron number density, wave number, wave power are obtained at various plasma radii and dielectric tube thickness. The results are compared with those for the typical cylindrical plasma column at similar conditions. For the purpose of modelling at low pressure of a coaxial discharge sustained by a travelling electromagnetic wave, some important characteristics of the propagation of surface waves have been investigated experimentally. The axial profiles of the propagation coefficient and radial profiles of the electric field at different experimental conditions have been obtained and discussed.

The modification of some biopolymers and amino-acids by the Electron-Beam Plasma was studied experimentally. The plasma was generated by injecting the continuous electron beam in gaseous or vapor media. The powders of the substances under consideration were found to change their physical-chemical and biological properties due to the treatment. The aggregation degree of human blood platelets in vitro was chosen as the quantitative characteristics of the biological effect. The untreated compounds were not dissolvable in distilled water at room temperature and did not inhibit the human platelet aggregation. The modified by the Electron-Beam Plasma synthetic derivative of 2-aminopropanoic acid (alanine) was proved to acquire the anti-aggregation activity for platelets. Products of the plasma modified fibrin-monomer were found to be soluble in water at room temperature and reduced the aggregation degree up to ≈ 33-35 % in vitro, treatment in the water EBP being more effective than the treatment in helium.

The surface tension of the polymer materials depends on functional groups over its surface. As a result from the plasma treatment the kind and concentration of the functional groups can be changed. In the present work, the possible kinetic reactions are defined. They describe the interaction between the plasma and the polymer surface of polyethylene terephthalate (PET). Basing on these reactions, the systems of differential kinetic equations are suggested. The solutions are obtained analytically for the system kinetic equations at defined circumstances.

In this paper, the solutions of kinetic model are qualitatively analyzed. The model describes treatment of polymer surfaces in RF-discharge (12.56 MHz). The treating of polymer surfaces in different gases leads to alteration of surface tension of the material. The experimental bearings for wetting contact angle and work of adhesion during the time of treating are compared with theoretical results obtained from the model. As a pattern polymer material, polyethylene terephthalate (PET) is used in plasmacreating gases: argon (Ar), oxygen (O₂), Freon 14 (CF₄) and Freon 12 (CF₂C_l2).

We present a mathematical model describing the elimination of volatile organic compounds from gases by plasmachemical methods, without specifying particular chemical reactions. The proposed model of plasmachemical processes description is universal. Conditions of effective removal of impurity are formulated. Comparison of the proposed theory to experimental data on conversion of impurity in ionized air shows the adequacy of the description. The description allows to choose methods of electrophysical actions on removed impurity, to allocate key parameters which act on efficiency of the process of VOCs conversion, i.e. to plan experiment.

The laser photodetachment technique was applied to investigate the negative ion dynamics in the positive column of a pure hydrogen dc glow discharge at pressures P = 0.1 - 3 Torr. Upon the discharge current modulation, the negative ion H^- concentration decayed with the characteristic loss time of hydrogen H(1s²S) atom concentration. The evolution of H atom concentration was investigated by the time-resolved actinometry. The axial electric field was measured by double probe technique. The analysis of the experimental data, i.e. the dependencies of H^- and H concentrations as well as reduced electric field on the discharge parameters (discharge current and pressure), allowed us to determine the rate constant of H^- production in the discharge, where vibrational excitation and reactions with atoms H greatly influence upon H^- production and loss processes. The main processes, which contribute to H^- production, are discussed in detail. The vibrational excitation of H₂ is shown to increase the rate of dissociative attachment essentially not only via the excitation of the well-known lowthreshold (e_th≈3 eV) resonance of H₂^-(X²∑_u⁺), but also via the excitation of the high-threshold resonances of H₂^-(²∑_g⁺ (e_th≈6 eV), ²Π_g, ²Π_u (e_th≈8-13 eV) etc). Detailed analysis of dissociative attachment via high-threshold resonances allowed us to describe experimental data in the whole range of E/N being investigated, whereas taking any other mechanisms into account (attachment to Rydberg states of H₂ etc) failed to do that.

In a d.c. discharge tube with sectional cathodes and a common grid anode, second derivative Langmuir probe measurements were performed in the Faraday dark space in argon gas discharge at intermediate pressures. Experimental results for different radial probe positions and different distances from the cathode in axial direction are presented. It is shown that the electron energy distribution function is bi-Maxwellian. Taking into account the electron depletion caused by their sinking on the probe surface, an extension of the Druyvesteyn formula is applied for more accurate determination of the electron temperature value, T, the electron density, n, and the plasma potential, U_pl, from the experimental results acquired.

The visible spectra of Cu I and Cu II species of the late (t > 1µs after the pinch) plasma focus have been used to determine the electron temperature and density of the decaying plasma. The intensity ratio of Cu I 4530A and Cu I 4539A lines was used for the determination of the electron temperature. The preliminary results showed a decrease of this parameter from T_e = 1.9 eV, as was estimated near the maximum pinch, to T_e = 1.3 eV after about 50 µs. A modified Saha equation, using the ion-to-atom (Cu I 5105A and Cu II 4953A) line intensity ratio was applied to determine the electron density. The method gave a decrease of this parameter from 10¹⁹ cm⁻³ to 10¹⁷ cm⁻³.

We present preliminary results of an experimental study of a hydrogen plasma flame produced using a microwave axial injection torch (torche à injection axiale or TIA). This device usually runs in argon or helium at atmospheric pressure providing a stable discharge at high HF power levels and a two-temperature plasma. In the current work, hydrogen plasma is launched in a helium filled chamber to prevent hydrogen-air explosive reactions. Seven spectroscopic hydrogen lines have been detected in the visible spectrum (the Balmer series of hydrogen). Using a Boltzmannplot representation it is shown that the plasma is far from equilibrium, but the electron temperature still can be obtained from a modified plot via a p^−x-correction, which shows that the lower energy levels of Balmer series of the hydrogen system is ruled by the excitation-saturation balance (ESB).

A hydrogen plasma flame produced by an axial injection torch powered at the microwave frequency of 2.45 GHz is studied using the method of disturbed Bilateral Relations (dBR). The application of this method which relates the influence of equilibrium disturbing, as produced by transport, to equilibrium restoring processes, reveals that the most dominant excitation balance is the Excitation Saturation Balance. Moreover, a global discharge model leads to an electron density of n_e = 4.10²⁰m⁻³ and an electron temperature of T_e = 0.86 eV. The gas temperature was estimated to be T_h = 0.3 eV. The values of n_e and T_e were found to be in good agreement with the value obtained with the modified Boltzmann-plot and the crossing method of Stark broadening. The dBR method points out that the first level in partial local Saha equilibrium will be the level with principal quantum number p = 10 which is in fair agreement with experimental results.

A computerized experimental set-up for second derivative Langmuir probe measurements is presented. As a signal generator for the differentiating signal a computer stereo sound card is used. The probe signal is detected using a selective amplifier - the lock-in amplifier combination, whose output is measured by an ADC card inside a computer. Special drivers working under Microsoft Windows98 Second Edition were created in Visual C++ 6.0 to control the operation of the probe system.

An analysis of the general energy level structure of the singly ionised Al is made. It is found that in the UV spectrum of Al II there are many intensive lines starting from levels which can be populated selectively via charge transfer collisions with either helium or neon ions. The emission spectrum of aluminium in the 200-400 nm spectral range is measured in a cylindrical Al hollow cathode at discharge conditions typical for laser oscillation. An enhancement of the spontaneous emission intensity on a number of Al II lines in the UV spectral range in Ne compared to He discharges is observed. The Al atom density in the cathode for different values of current density and voltage are calculated.

We present three simple techniques that enable restoring time-averaged Langmuir probe characteristics measured under time-varying plasmas, in the sense that plasma parameters (electron density and average energy, electron energy distribution function, and plasma potential) will be deduced more accurately than what otherwise can be retrieved from averaged data.

The work is dedicated to investigations of the diffusive transport of sputtered ground-state tungsten atoms in buffer gas krypton at ambient temperatures. For sputtering of the metal, a pulse-periodic longitudinal hollow cathode gas discharge in cylindrical geometry is used. The cathode represents a 20 cm in length and 2.6 cm in diameter hollow copper cylinder, the inner surface of which is coated with a thin tungsten layer. During the discharge W atoms are sputtered from that layer into the cathode volume, while in the afterglow phase their density decreases due to diffusive losses back to the surface. The diffusive loss rates are measured by using a time-resolved double-channel optical absorption technique. By relating the experimental loss rates with a diffusion model based on the experimental geometry and a boundary condition of the third kind, the following value for the diffusion coefficient of W atoms in krypton is obtained: D₀ = 130⁺⁴⁰₋₃₀ cm²s⁻¹ at 133 Pa buffer gas pressure and 315 K temperature. The reflection coefficient of these atoms at the cathode surface is obtained within the range 0 ⩽ ρ ⩽ 0.8. The experimental results for the diffusion coefficient are compared with theoretical calculations based on model interaction potentials.

Using PIC-MCC method for simulation of the breakdown of plasma focus device we obtained electron energy distribution function, spatial distributions of potential and charged particle densities. Studied is the first phase of the development of ionization process at pressures 133 - 931 Pa. Satisfactory agreement with experiment of the PF-1000 operation is observed for pressures in the range 665 - 931 Pa.