Table of contents

The Second International Workshop & Summer School on Plasma Physics (IWSSPP'06) organized by St. Kliment Ohridsky University of Sofia, The Union of the Physicists in Bulgaria, the Bulgarian Academy of Sciences and the Bulgarian Nuclear Society, was held in Kiten, Bulgaria, on the Black Sea Coast, from 3–9 July 2006. As with the first of these scientific meetings (IWSSPP'05 Journal of Physics: Conference Series44 (2006)), its aim was to stimulate the creation and support of a new generation of young scientists for further development of plasma physics fundamentals and applications, as well as to ensure an interdisciplinary exchange of views and initiate possible collaborations by bringing together scientists from various branches of plasma physics.

This volume of Journal of Physics: Conference Series includes 33 papers (invited lectures, contributed talks and posters) devoted to various branches of plasma physics, among them fusion plasma research, dc and microwave discharge modelling, transport phenomena in gas discharge plasmas, plasma diagnostics, cross sections and rate constants of elementary processes, material processing, plasma-chemistry and technology. Some of these papers were presented by internationally known and recognized specialists in their fields; others are MSc or PhD students' first steps in science. In both cases, we believe they will raise readers' interest.

We would like to thank the members of both the International Advisory Committee and the Local Organizing Committee, the participants who sent their manuscripts and passed through the (sometimes heavy and troublesome) refereeing and editing procedure and our referees for their patience and considerable effort to improve the manuscripts. We greatly appreciate the financial support from the sponsors: the Department for Language Teaching and International Students at the University of Sofia and Natsionalna Elektricheska Kompania EAD. 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.

Zdravko Neichev, a PhD student at University of Sofia and a member of the Local Organising Committee of the International Workshop and Summer School on Plasma Physics, died September 22, 2006 at the age of 27 in a tragic car accident.

He was close to finishing his PhD thesis working thoroughly in the field of Plasma Physics. Being also an excellent programmer he produced a number of perfect programs for numerical modelling of the coaxial discharge properties.

He was a smart, friendly person, always ready to help. His colleagues and friends will never forget his radiant smile.

Understanding of underlying physics in the edge plasma of tokamaks requires knowledge of the plasma density, potential, electron and ion temperature, ion flows and their fluctuations with a high spatial and temporal resolution. A family of electric probes, which have been designed and tested for this purpose in the CASTOR tokamak, is reviewed and examples of their performance are given. In particular, we focus on description of the 1D and 2D arrays of Langmuir probes for spatially resolved measurements of the edge turbulence, the Ball pen and emissive probes for direct measurements of the plasma potential, the optimized Gundestrup probe for measurements of parallel and perpendicular ion flow, and the tunnel probe for fast measurement of electron and ion temperatures. Additional information on individual diagnostics is available in the listed references. PACS 52.70.Ds

Langmuir probes are known for their ability to provide local measurements of very important plasma parameters, namely the plasma potential, the density of the charged particles and the electron energy distribution function (EEDF). The correctness of using the probes under adverse conditions, such as presence of magnetic fields or high plasma temperature is still being questioned. In this paper we report the application of the first-derivative Langmuir probe method for processing the electron part of the volt-ampere (IV) characteristics measured in the CASTOR tokamak plasma. First results of EEDFs at different radial positions in the edge plasma are presented and the values of the plasma potential, electron temperature and electron densities are estimated. The applicability of the first-derivative Langmuir probe method in strongly magnetized, high temperature tokamak plasmas is discussed.

One of the important issues related with the cyclotron resonance heating (CRH) and current drive of fusion plasmas in thermonuclear reactors (tokamaks and stellarators) is the development of multi-megawatt class gyrotrons. There are generally three stages of the implementation of that task, notably (i) elaborating a novel generation of software tools for the physical modelling and simulation of such kind of gyrotrons, (ii) their computer aided design (CAD) and construction on the basis of the simulation's results, and finally, (iii) gyrotrons' testing in real experimental conditions. This tutorial paper concerns the first item-the development of software tools. In co-operation with the Institute for Pulsed Power and Microwave Technology at the Forschungszentrum Karlsruhe, Germany, and Centre de Recherches en Physique des Plasmas at École Polytechnique Fédérale de Lausanne, Switzerland, we work on the conceptual design of the software tools under development. The basic conclusions are that the numerical codes for gyrotrons' modelling should possess the following essential characteristics: (a) portability, (b) extensibility, (c) to be oriented toward the solution of practical problems (i.e., elaborating of computer programs that can be used in the design process), (d) to be based on self-consistent 3D physical models, which take into account the departure from axial symmetry, and (e) ability to simulate time dependent processes (electrostatic PIC simulation) alongside with a trajectory analysis (ray tracing simulation). Here, we discuss how various existing numerical codes have to be improved and implemented via the advanced programming technologies for state-of-the-art computer systems including clusters, grid, parallel platforms, and supercomputers.

The magnetic beach, minority and second harmonic ion cyclotron heating schemes are addressed. Their specific features in application to mirror devices and the results of computations are discussed.

An instrumentation approach to electron energy distribution function measurements using Langmuir probes is presented. The noise and error limitations of the most common differentiation techniques are analysed, it is shown how instrumental accuracy can be improved or how acquisition time can be drastically decreased, and a pertinent performance comparison of the harmonic vs. the numerical differentiation schemes is made. In addition, we stress the nasty effects of pink and of coherent noise, and we show how they can be minimised.

This review is on modelling of inductive discharges at low gas pressures. Both the gas-discharge part and the electrodynamical part of the models are stressed as completing a self-consistent description of the discharge structure. The gas-discharge description covers the two regimes of the discharge maintenance at low gas pressures: ambipolar-diffusion controlled discharges and discharges in a free-fall regime. The electrodynamical description of the discharge starts with analysis of the types of the skin of the transverse high-frequency bulk waves which produce the inductive discharges. Normal collisional skin providing conditions for local conductivity and anomalous skin related to nonlocal conductivity are discussed as resulting in the different types of the power deposition in the different regimes of low-pressure discharges: Joule heating in the ambipolar-controlled discharges and stochastic heating in the free-fall regime of the discharge maintenance.

This work extends our previous analysis of the nitrogen pink afterglow, by comparing our model predictions with the recently reported measurements of metastable N(²P) atoms and N₂(a¹Π_g) molecules. It is shown that both species reveal the presence of a characteristic maximum on their populations, occurring downstream from the discharge after an initial stage of decrease. Such behavior is a consequence of the V-V pumping-up mechanism taking place during the relaxation in the afterglow, which is followed by V-E transfers that create locally N₂(A³Σ⁺_u) and N₂(a' ¹Σ⁻_u) metastables. The model predictions significantly overestimate the density of the N₂(a¹Π_g) state, revealing a problem in the description of the singlet kinetics. As singlet N₂(a' ₁Σ⁻_u) metastables play a crucial role in nitrogen ionization, the new results imply that the ionization mechanisms in the afterglow may have to be reviewed.

For investigation of Al atoms transport properties a pulse-periodic longitudinal hollow cathode gas discharge in cylindrical geometry is used. The cathode represents a 20 cm in length and 1.8 cm in inner diameter hollow aluminium cylinder. During the discharge Al atoms are sputtered from the wall into the cathode volume, while in the afterglow phase their concentration decreases due to diffusive losses back to the wall surface. The diffusive loss rates at different buffer gas pressures are measured by using a double-channel time-resolved optical absorption technique with gated photon counting. By relating the experimental loss rates with a diffusion model based on the experimental geometry and a boundary condition of the third kind, the diffusion coefficient of ground-state Al atoms in argon, as well as the reflection coefficient of these atoms from the cathode surface are obtained. The experimental value for the diffusion coefficient at 313 K temperature and 133 Pa argon pressure is D₀ = 160 ± 30 cm²s⁻¹. The reflection coefficient was found to be in the range: 0.2 < ρ < 0.82. The result for the diffusion coefficient is compared with theoretical results based on 12-6 Lenard-Jones and rigid-sphere inter-atomic interaction model.

In this contribution we estimate the performance of various Poisson equation solvers applied to the Particle-In-Cell plasma models. The solvers determine the practical usability of complex PIC models, especially in three dimensions. The performance is measured on 2D models with grids of various sizes, the methods studied are SOR, conjugate gradients, LU decomposition, FACR and multigrid methods. The results confirm the efficiency of the direct methods tested, namely the LU decomposition method and FACR. The advantages of using LU decomposition as a part of the multigrid method on larger grids are discussed as well.

One of problems important in the present plasma science is the surface treatment of materials at higher pressures, including the atmospheric pressure plasma. The theoretical analysis of processes in such plasmas is difficult, because the theories derived for collisionless or slightly collisional plasma lose their validity at medium and high pressures, therefore the methods of computational physics are being widely used. There are two basic ways, how to model the physical processes taking place during the interaction of plasma with immersed solids. The first technique is the particle approach, the second one is called the fluid modelling. Both these approaches have their limitations-small efficiency of particle modelling and limited accuracy of fluid models. In computer modelling is endeavoured to use advantages by combination of these two approaches, this combination is named hybrid modelling. In our work one-dimensional hybrid model of plasma-solid interaction has been developed for an electropositive plasma at higher pressures. We have used hybrid model for this problem only as the test for our next applications, e.g. pulsed discharge, RF discharge, etc. The hybrid model consists of a combined molecular dynamics-Monte Carlo model for fast electrons and fluid model for slow electrons and positive argon ions. The latter model also contains Poisson's equation, to obtain a self-consistent electric field distribution. The derived results include the spatial distributions of electric potential, concentrations and fluxes of individual charged species near the substrate for various pressures and for various probe voltage bias.

We investigate the issue of applicability of the solid phase microextraction (SPME) in the analysis of volatile organic compounds (VOCs) destruction products in the gliding arc discharge. Our research is focused on the measurements with the simple one stage gliding arc reactor, applied voltage was varied in the range of 3.5-4 kV. As a carrier gas, the dry air and its mixtures with nitrogen and oxygen, enriched by toluene, with flow rate of 1000-3500 ml/min was used. Total decomposition of toluene of 97 % was achieved at the oxygen content in carrier gas of 60 %. For measurements with air as a carrier gas, the highest efficiency was 95 %. We also tested the SPME technique suitability for the quantitative analysis of exhausts gases and if this technique can be used efficiently in the field to extract byproducts. Carbowax/divinylbenzene and Carboxen/polydimethylsiloxane/divinylbenzene fibres were chosen for sampling. Tens of various high-molecular substances were observed, especially a large number of oxygenous compounds and further several nitrogenous and C_xH_y compounds. The concentrations of various generated compounds strongly depend on the oxygen content in gas mixture composition. The results showed that the fiber coated by Carbowax/divinylbenzene can extract more products independently on the used VOC compound. The Carboxen/polydimethylsiloxane/divinylbenzene fiber is useful for the analysis of oxygenous compounds and its use will be recommended especially when the destruction is done in the oxygen rich atmosphere. With the higher ratio of oxygen in the carrier gas a distinctive decline of C_xH_y compounds amount have been observed. We also tried to describe the significant production of some compounds like benzyl alcohol, benzeneacetaldehyde, even in oxygen content is proximate 0 %. Experimental data demonstrated that it is necessary to use several SPME fibres for full-scale high-molecular products analysis.

Calculations of broadening of nine Ar I spectral lines due to collisions with neutral argon atoms are presented. The semi-classical impact broadening theory has been used. The interaction of an excited Ar atom with Ar atom in the ground state has been treated using the Kaulakys potential and a comparison with other potentials has been performed. The influence of the Maxwellian averaging and the scattering length on the broadening cross section has been studied.

The production of negative hydrogen ions by volume processes depends on the electron energy in the expansion region of a plasma source. The study presents experimental results for the electron energy distribution function (EEDF) obtained by probe diagnostics in this region. Measurements are performed in axial and radial directions at low gas pressures. The results for the EEDF show that its shape changes along the chamber axis because the plasma density and the electron temperature decrease to a nearly constant value in the main part of the expansion chamber. The EEDF is Maxwellian-like with tail up to 40 eV close to the driver. The nonlocal formation of the EEDF is experimentally obtained in expanded plasma.

Results of research work of a pulsed plasma accelerator, designed as diagnostic and material science stands in SRIETP are presented. We present results on the development of electric and magnetic probes used for measurement of plasma parameters. The physical properties and changes in structure of vanadium alloy, common quality carbon and stainless steels have been investigated as well.

The impact of the introduction of small amounts (up to 3.6 %) of H₂ in Ar plasmas produced by a surfatron was studied. This was done in view of futures plans to measure the electron density via H_β-broadening. The H₂ concentration was varied between 1.2 and 3.6 % while the pressure was in range from 5 to 20 mbar. While changing the H₂ concentration we studied the length of the plasma, the plasma wall heating and the electron temperature. The latter was measured using the method introduced in a previous study based on a combination of absolute line intensity measurements and a collisional radiative model. It was found that, for the mixture ratios we used, the electron temperature does not change but that the H₂-addition has a substantial impact on the gas temperature.

Quasistatic model of wakefield generation in ultrahigh intensity laser-plasma interaction is presented. The model implies that the plasma wake is slowly changed in the laser pulse frame. 2D axially symmetrical quasistatic PIC code based on this model is developed. The code provides stable numerical computation in ultrahigh intensity regime and much shorter computational time than that of standard PIC codes. The numerical modeling of the plasma wakefield generated by high power laser pulse is presented. A comparison with simulation results obtained by fully 3D electromagnetic PIC code is done.

Cross-sections and rate constants for thermal energy charge transfer into some Ag⁺, I⁺, and Cu⁺ excited states are theoretically and experimentally obtained for a gas discharge in the He-CuBr, Ne-CuBr, He-AgI, and Ne-AgI mixtures. Besides the pumping process the formation of the inversion population is determined by the radiative transitions, which populate or depopulate the upper and lower laser levels.

A group of galaxies emits strong permitted and forbidden lines from the central part where a massive black hole is supposed to be located. The cores of this type of galaxies, so called Active Galactic Nuclei (AGN) are known as a most powerful sources of radiation in the Universe. Here we discuss the emitting plasma conditions in emitting line regions of AGN as well as methods which are in usage to determine the physical properties of the emitting plasma around super massive black holes.

In the presented contribution two groups of techniques of computational physics-fluid modelling and non self-consistent particle technique were used to study plasma-solid interaction in argon plasma. We focused both on the physical processes taking place in the sheath at various pressures and on the problems of computational physics. The attention was given to preparation of two-dimensional fluid models with realistic assumptions about physical processes taking place in plasma during the plasma-solid interaction, further to improvement of the non self-consistent technique of particle modelling, where the external electric field was obtained either from the fluid model or directly from the trajectories of charged particles and finally to efficiency of individual algorithms.

The study is on nonlocal conductivity and stochastic heating in low-pressure inductive discharges with cylindrical coils. The rf current density derived by solving the Boltzmann equation in cylindrical coordinates, with accounting for both the high-frequency heating field and the dc field in the discharge, is coupled with the wave equation resulting into the spatial distribution-under the conditions of anomalous skin-of the electromagnetic field components, of the rf current density and the power deposition in the free-fall regime of maintenance of hydrogen discharges.

Operation of magnetic filters is studied experimentally regarding their use for electron cooling in the sources of the negative hydrogen ion beams. Axial profiles of the plasma parameters-electron temperature and density-are measured by probe diagnostics in the expansion plasma region of an inductively driven tandem type of a plasma source. The obtained drop of the electron temperature down to values of about and below 1 eV required for efficient production of negative hydrogen ions is the result proving the proper operation of the filter. The mechanism of electron cooling is discussed based on thermal conductivity effects.

In the present work an estimation shows that the photo effect on the cathode near the insulator has significant role in the breakdown of the Plasma Focus device. An analysis is made to establish the number of the emitted photons from the exited molecules and respectively the number of ejected electrons from the cathode due to the photo effect.

In the typical experiments of surface wave sustained plasma columns at atmospheric pressure the ratio of collision to wave frequency (ν/ω) is much greater than unity. Therefore, one might expect that the usual analysis of the wave dispersion relation, performed under the assumption ν/ω = 0, cannot give adequate description of the wave propagation characteristics. In order to study these characteristics we have analyzed the wave dispersion relationship for arbitrary ν/ω. Our analysis includes phase and wave dispersion curves, attenuation coefficient, and wave phase and group velocities. The numerical results show that a turning back point appears in the phase diagram, after which a region of backward wave propagation exists. The experimentally observed plasma column is only in a region where wave propagation coefficient is higher than the attenuation coefficient. At the plasma column end the electron density is much higher than that corresponding to the turning back point and the resonance.

The purpose of this work is to investigate phase diagrams and electric field radial distribution of coaxial discharges, sustained by a traveling electromagnetic wave, assuming finite and infinite thickness of the discharge chamber in the model. The calculations are made for azimuthally symmetric and dipolar wave modes. The phase diagrams and the radial profiles of the electric field at various thicknesses of the outer dielectric tube of the chamber and different discharge conditions are obtained. For the purpose of low pressure coaxial plasma modelling, radial profiles of the electric field at different discharge conditions have been investigated experimentally and compared with the theoretical results.

The formation of spatial structures in high-frequency and microwave discharges has been known for several decades. Nevertheless it still raises increased interest, probably due to the variety of the observed phenomena and the lack of adequate and systematic theoretical interpretation. In this paper we present preliminary results on observation of spatial structures appearing along a surface wave sustained plasma column. The experiments have been performed in noble gases (xenon and neon) at low to intermediate pressure and the surface wave has been launched by a surfatron. Under these conditions we have observed and documented: i) appearance of stationary plasma rings; ii) formation of standing-wave striationlike patterns; iii) contraction of the plasma column; iv) plasma column transition into moving plasma balls and filaments. Some of the existing theoretical considerations of these phenomena are reviewed and discussed.

Mode operation of inductively driven plasmas in argon gas at low pressures is studied by optical emission spectroscopy. The plasma source is a tandem type source with a driver and an expansion plasma region. The driver region of the discharge is in the classical form of a cylindrically shaped inductive discharge, with a coil positioned over a gas discharge tube. The inductively coupled plasma discharge is maintained at high frequency f = 27 MHz, applied power varied in the limits P = (20–700) W and gas pressure in the range p = (8–260) mTorr. The Ar line intensity dependencies on the applied power injected into the discharge is analysed. With the power increase a mode transition of the discharge regime is not observed. The investigations on the discharge mode operation are supported by theoretical calculations based on kinetic modelling of main processes contributing to the line intensity behaviour.

Previously developed theoretical approach to model asymmetrical capacitively coupled RF discharge is applied in the case of RF magnetron discharge. The method is based on deriving theoretical dependence of the FR power dissipated in the discharge on the DC selfbias voltage. These are easily measurable quantities and this experimental data is fitted with the theoretical model. Latter is based on rather broad assumptions and is applicable in large variety of cases. Experiments with Ar and O₂ at low pressure are conducted. The data obtained fits perfectly with the model. The dependence of the total ion current, bombarding the target on the discharge conditions (applied power) is derived. The results show that independently on the similarity in the initial data, a substantial difference in the plasma sustaining power in both cases can be distinguished. This fact can be attributed to the presence of a number of vibrational rotational levels in O₂ molecules, absent in Ar. The additional power needed to sustain O₂ plasma goes for excitation of the vibrational rotational levels.

The dependence of the laser output power and the small signal gain for the 780.8 nm copper ion transition as a function of the cathode segments length in a sputtering longitudinal hollow cathode discharge are measured. The optimal cathode length in regard of maximum laser power is determined. From one and the same active volume at equal input power a considerable increase of laser output power is observed using the optimal length cathode segments. The results are in good agreement with the previously performed calculations and measurements of axial current and plasma characteristics, showing that the plasma is most intense near the anode ends of the cathodes. The measurements confirm that the highest laser power and excitation efficiency is achieved when the laser active volume comprises a series of anodes and cathodes, each cathode 2 cm long. This report is a part of a series of investigations aimed at optimization of the longitudinal hollow cathode discharge used as excitation medium of cathode sputtered metal ion lasers.

A CCD-line module is built to upgrade conventional spectrograph to the CCD ability of advanced data acquisition as well as a cost-effective solution to the plasma diagnostic spectroscopy measurement needs. The CCD module is adapted to the ISP-51 (Lomo-Russia) spectrograph, used for light dispersion and as a wavelength pre-selector when a Fabry-Perot interferometer is positioned into the parallel optical path of the spectrograph. A high-resolution spectrometer system is developed both computerized and easy to maintain. The main advantage over conventional method of spectral line profile registration is achievement of single-shot capability. Thus the line profile distortions caused by intensity fluctuations, which occur during the scanning process are eliminated. The capability of the designed spectrometer are presented with the measurements of the resolved superfine structure of the Cd I 480 nm spectral line profile as well as analyzing Ar I 738.4 nm spectral line profile broadening resulting with the gas temperature determination of argon inductively coupled plasma at low pressure and applied power.

A differential measurement technique has been proposed in order to reduce noise level and stray capacitance leakage usually affecting Langmuir probe data. The technique employs two identically designed and biased Langmuir probes, connected to an instrumentation amplifier. Both probes are immersed in plasma of approximately the same space potential, one of them being plasma current collecting probe, and the second one being isolated from plasma and serving as a pick-up probe, detecting leakage currents from parasitic capacitive coupling and noise. Avoiding averaging of probe current data is the main advantage of the proposed differential technique. Experiments in the plasma expansion region of inductively driven RF source are shown to achieve lower electron temperature and higher electron density as measured by conventional single Langmuir probe. Obtaining more sharpness of the "knee" on the characteristic, thus lowering the uncertainty in plasma potential is another true merit of the differential Langmuir probe technique.

A process of plasma polymerization of dimethylaniline and acrylic acid vapours on the surface of poly(ethylene terephthalate) track membranes has been investigated. The surface and hydrodynamic properties of the composite membranes produced in this case have been studied. It is shown that the water permeability of the obtained polymeric membranes can be controlled by changing the filtrate pH. Membranes with such properties can be used for controllable drug delivery and in sensor control.

A process of extraction of the low-molecular products of the synthesis from the poly(ethylene terephthalate) track membranes modified by plasma has been investigated. It is shown that the deposition of a thin polymeric hydrocarbon film by cyclohexane plasma on the membrane surface with preliminary treatment in a plasma of non-polymerizing gases, for example oxygen, allows one to produce membranes possessing a high productivity. Their advantages are much better hydrodynamic properties and a small amount of the low-molecular products of the synthesis extracted by organic solvents.

The modification of some biomaterials by the Electron-Beam Plasma was studied experimentally. The powder of the fibrin-monomer (the natural protein which is contained in the blood of mammalians) was treated in the plasma generated by injecting the continuous electron beam in gaseous or vapor media. The fibrin-monomer was found to change its physical-chemical and biological properties due to the treatment. In particular, being modified the fibrin-monomer reduced the human platelet aggregation degree in vitro to ≈ 33–35 %, whereas the untreated compound did not inhibit the aggregation. Both experiments and computer simulation were carried out to separate plasmachemical effects from other factors acting on the sample during thee treatment (β-irradiation, UV- and X-ray radiation, heating). The study showed the fibrin-monomer to acquire the anti-aggregation activity due to combined action of all factors mentioned above but the plasmachemical processes are predominant.