M. Tichý

An experimental study of plasma density determination by a cylindrical Langmuir probe at different pressures and magnetic fields in a cylindrical magnetron discharge in heavy rare gases

This article presents an experimental study that contributes to the problem of interpretation of cylindrical Langmuir probe data obtained in a non-isothermal low-temperature plasma in magnetic field. A discussion on the influence of positive ion - neutral collisions on the charged particle density estimation is also given and the effect is demonstrated on the experimental data. A Maxwellian electron energy distribution is assumed throughout the present study. The Langmuir probe data are obtained in a cylindrical magnetron discharge in argon at pressures from 1.5 to 6 Pa and magnetic fields between 100 and 500 G. The radially movable Langmuir probe was made of either m or m diameter tungsten wire in order to investigate the effect of the probe dimensions on the estimated plasma density. The electron density is calculated from the electron current at the space potential (used as a reference) and from the OML collisionless theory. The ion density is calculated by using ABR - Chen theory without and with the correction due to the collisions of positive ions in the probe sheath. Also, the recent collisional positive-ion-collection-theory is used for comparison. The resulting numerical values of plasma density are compared over more than one order of magnitude change in the plasma density given by its radial dependence in the cylindrical magnetron discharge. Optical measurements were made in order to quantitatively assess the neutral gas temperature in the discharge and the density of particles in excited states that could induce secondary electron emission from the probe surface and thus apparently enhance the positive-ion density estimated from the probe positive-ion current. The effect of secondary electron emission from the probe surface on the probe data interpretation has been found small compared to the experimental error limits and consequently not substantial for our experimental conditions. In the range of our experimental conditions the ABR - Chen theory with the collisional correction gives the best agreement of the estimated numerical values of ion and electron densities in the whole range of its investigated change. Also from our results it follows that the effect of the magnetic field on the thinner-probe-electron-current at the space potential and hence on the reference-electron-density-estimation is negligible within the experimental uncertainties up to a magnetic field strength of 500 G which was the maximum used in our experimental study.

Formation of TiOx films produced by high-power pulsed magnetron sputtering

Formation of thin TiOx films produced by pulsed planar magnetron sputtering deposition is reported in this paper. The formation process and layer growth were controlled by (i) the ratio of reactive O2 in Ar/O2 working gas mixture and (ii) the pressure in the vacuum chamber. The magnetron, operated in a high-power pulse mode with a low repetition frequency of 250 Hz, reached maxima peak current Ip ~ 50 A and magnetron current density peaks at ip ~ 1 A cm−2. Particular spectral lines (Ar = 420.07 nm, Ar+ = 487.98 nm, Ti = 518.96 nm) emitted by the discharge were investigated using time-resolved photon counting measurements. The phases of deposited TiOx films were determined by grazing incidence x-ray diffractometry and thickness and density were calculated from x-ray reflectometry measurements; in addition composition and chemical bounds were revealed by x-ray photoelectron spectroscopy. The film diagnostics survey the existence of different crystalline phases in the Ti–O system and their formation. Discharge properties for example, deposition rate and time evolution of discharge current are also discussed.

Germination of Chenopodium Album in Response to Microwave Plasma Treatment

The seeds of Lambs Quarters (Chenopodium album agg.) were stimulated by low-pressure discharge. The tested seeds were exposed to plasma discharge for different time durations (from 6 minutes to 48 minutes). Germination tests were performed under specified laboratory conditions during seven days in five identical and completely independent experiments. Significant differences between the control and plasma-treated seeds were observed. The treated seeds showed structural changes on the surface of the seat coat. They germinated faster and their sprout accretion on the first day of seed germination was longer. Germination rate for the untreated seeds was 15% while it increased approximately three times (max 55%) for seeds treated by plasma from 12 minutes to 48 minutes.

A contribution to the assessment of the influence of collisions on the measurements with Langmuir probes in the thick sheath working regime

In the present work we investigate theoretically and experimentally the influence of elastic collisions in a probe sheath on a cylindrical Langmuir probe. The analysed probe working regime covers conditions under which the following probe characteristic parameters are comparable: the probe radius, the Debye length and both the ion and electron mean free paths.The preliminary investigations under almost collisionless conditions show good agreement between theoretical and experimental values of the ion saturation currents and of the floating potentials only when the ion currents for the studied working regime of the cylindrical Langmuir probe are calculated according to the theory of Chen (Plasma Phys.7 (1965)47). These collisionless currents form the basis for the calculation of the collision-corrected probe characteristics according to the presented procedure by Talbot and Chou (Rarefied Gas Dynamics, Academic Press, New York, 1966, p. 1723).The applied theoretical analysis covers the influence of the collisions on the electron and ion current of the single probe characteristic and on the estimation of the space potential. The results of the calculations are presented in graphical overviews for the series of cases of practical importance. The other working regimes can be covered using the calculating procedure presented.For comparison of the calculated collision-corrected characteristics with those from an experiment we used the positive column plasma of the He glow discharge where the electron density is known and the space potential can be experimentally estimated from the lowest excitation potential of He. The comparison was carried out for the ion and electron currents, the floating potential and the zero-cross of the probe characteristic second derivative.The estimation of the secondary electron current contribution to the total probe current shows that it limits the applicability of the collision-corrected probe characteristic to the plasma diagnostic in the transition to the collision-determined working regime.

Physical properties of homogeneous TiO2 films prepared by high power impulse magnetron sputtering as a function of crystallographic phase and nanostructure

Optical, photo-electrochemical, crystallographic and morphological properties of TiO2 thin films prepared by high power impulse magnetron sputtering at low substrate temperatures (<65 °C) without post-deposition thermal annealing are studied. The film composition—anatase, rutile or amorphous TiO2—is adjusted by the pressure (p ~ 0.75–15 Pa) in the deposition chamber. The different crystallographic phases were determined with grazing incidence x-ray diffractometry. The surface morphology and size of TiO2 grains/clusters were imaged with atomic force microscopy. Basic plasma parameters were determined by means of the time-resolved Langmuir probe technique. The power density influx on the substrate was estimated from calorimetric probe measurement. The data from calorimetric probe measurements and time-resolved Langmuir probe served as input parameters for the calculation of influx contributions of particular species. The band-gap energy Eg depends on the film composition and crystallographic phase. Optical parameters (refractive index n + ik, transmittance T, reflectance R and absorbance A) are measured as functions of photon energy in the UV–Vis range by spectroscopic ellipsometry. For the rutile and anatase films agreement with the respective bulk phase is found. Incident photon-current conversion efficiency determined by photo-electrochemical measurements reached the highest values (0.312) for the anatase film.

A novel approach to direct measurement of the plasma potential

A novel probe and approach to the direct measurements of the plasma potential in a strong magnetic field is suggested. The principle of this method is to reduce the electron saturation current to the same magnitude as that of the ion saturation current. In this case, the floating potential of the probe becomes indentical to the plasma potential. This goal is attained by a shield, which screens off an adjustable part of the electron current from the probe collector due to the much smaller gyro-radius of the electrons. First systematic measurements have been perfomred in the CASTOR tokamak.

The recombination of H3+ ions with electrons: dependence on partial pressure of H2

Abstract We observed an increase in the effective recombination coefficient ( α eff ) of the recombination of H 3 + ions with electrons with a number density of H 2 , n( H 2 ) . With increasing n (H 2 ) from 1×10 11 to 2×10 12 cm −3 the α eff increased from 1.3×10 −8 to 1.5×10 −7 cm 3 s −1 . The dependence of α eff on n (H 2 ) indicates that the recombination of H 3 + ions in an afterglow plasma is a complex process in which collisions with H 2 take place. It is stressed that at conditions corresponding to plasma in interstellar space the dissociative recombination of H 3 + ions is very slow with a rate coefficient α⩽1.3×10 −8 cm 3 s −1 .

Emissive probe measurements of plasma potential fluctuations in the edge plasma regions of tokamaks

The plasma potential Φpl and its fluctuations Φpl were measured by electron emissive probes in the edge plasma regions of two fusion experiments: the Instituto Superior Tecnico Tokamak (ISTTOK) (Lisbon, Portugal), and the Czech Academy of Sciences Torus (CASTOR) tokamak (Prague, Czech Republic). Into ISTTOK, three emissive probes were inserted outside the last closed flux surface (LCFS) on different minor radii. In CASTOR, two emissive probes, poloidally separated, and two cold cylindrical probes, mounted on the same shaft, were used, which could be radially shifted outside and inside the LCFS. The advantages of a sufficiently emissive probe are that in principle Φpl and Φpl can be measured directly, without being affected by electron temperature fluctuations or drifting electrons.

Radial behaviour of the electron energy distribution function in the cylindrical magnetron discharge in argon

The cylindrical magnetron consists of a coaxial inner cathode and an outer anode. The magnetic field is applied in the axial direction and is almost homogeneous in the whole magnetron volume. The electric field has radial direction and therefore the charged particles in the cylindrical magnetron discharge move under the influence of the × field. Due to its comparatively simple geometry, the cylindrical magnetron represents a suitable experimental tool that can be used to confirm theoretical results of modelling and theoretical studies of magnetrons in general. We studied experimentally the radial behaviour of the electron velocity distribution function (EVDF) in a cylindrical magnetron discharge in argon. We checked experimentally the anisotropy of the EVDF due to the influence of the magnetic field. For the assessment of the anisotropy of the EVDF we used a planar probe, whose collecting surface was adjustable at different angles to the direction of the magnetic field in the plane perpendicular to the electric field, as well as being movable in the radial direction. We found that in the measurable range of electron energies (energies greater than approximately 2 eV) and at magnetic fields up to 40 mT the anisotropy of the EVDF is not detectable within the experimental error limits. Therefore, for the study of the radial behaviour of the EVDF we used the thin (42 µm in diameter) tungsten cylindrical probe that was movable in the radial direction by a precise screw. For the theoretical determination of the EVDF in the cylindrical magnetron discharge we solved numerically the Boltzmann equation in a crossed × field, assuming the usual simplifications. The results of the calculation and the experiment in argon are compared and discussed.

The high pressure torch discharge plasma source

We present a plasma source which works on the principle of the arc torch discharge. The powered electrode of the arc torch discharge was made from a thin pipe that simultaneously acts as the nozzle through which the working gas flows to the discharge region. The flow of the working gas stabilizes the arc torch discharge and a well defined plasma channel is created. The advantage of this system is that it is able to work at high pressure of working gas up to atmospheric pressure inside the plasma-chemical reactor and also in free space.